The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable unit of memory in many computer architectures. To disambiguate arbitrarily sized bytes from the common 8-bit definition, network protocol documents such as the Internet Protocol (RFC791) refer to an 8-bit byte as an octet. Those bits in an octet are usually counted with numbering from 0 to 7 or 7 to 0 depending on the bit endianness.
The size of the byte has historically been hardware-dependent and no definitive standards existed that mandated the size. Sizes from 1 to 48 bits have been used. The six-bit character code
was an often-used implementation in early encoding systems, and
computers using six-bit and nine-bit bytes were common in the 1960s.
These systems often had memory words
of 12, 18, 24, 30, 36, 48, or 60 bits, corresponding to 2, 3, 4, 5, 6,
8, or 10 six-bit bytes, and persisted, in legacy systems, into the
twenty-first century. In this era, bit groupings in the instruction
stream were often referred to as syllables or slab, before the term byte became common.
The modern de facto standard of eight bits, as documented in ISO/IEC 2382-1:1993, is a convenient power of two permitting the binary-encoded values 0 through 255 for one byte, as 2 to the power of 8 is 256. The international standard IEC 80000-13
codified this common meaning. Many types of applications use
information representable in eight or fewer bits and processor designers
commonly optimize for this usage. The popularity of major commercial
computing architectures has aided in the ubiquitous acceptance of the
8-bit byte. Modern architectures typically use 32- or 64-bit words, built of four or eight bytes, respectively.
The unit symbol for the byte was designated as the upper-case letter B by the International Electrotechnical Commission (IEC) and Institute of Electrical and Electronics Engineers (IEEE). Internationally, the unit octet explicitly defines a sequence of eight bits, eliminating the potential ambiguity of the term "byte". The symbol for octet, 'o', also conveniently eliminates the ambiguity in the symbol 'B' between byte and bel.
Etymology and history
The term byte was coined by Werner Buchholz in June 1956, during the early design phase for the IBM Stretch
computer, which had addressing to the bit and variable field length
(VFL) instructions with a byte size encoded in the instruction. It is a deliberate respelling of bite to avoid accidental mutation to bit.
Another origin of byte for bit groups smaller than a computer's word size, and in particular groups of four bits, is on record by Louis G. Dooley, who claimed he coined the term while working with Jules Schwartz and Dick Beeler on an air defense system called SAGE at MIT Lincoln Laboratory in 1956 or 1957, which was jointly developed by Rand, MIT, and IBM. Later on, Schwartz's language JOVIAL actually used the term, but the author recalled vaguely that it was derived from AN/FSQ-31.
Early computers used a variety of four-bit binary-coded decimal (BCD) representations and the six-bit codes for printable graphic patterns common in the U.S. Army (FIELDATA) and Navy.
These representations included alphanumeric characters and special
graphical symbols. These sets were expanded in 1963 to seven bits of
coding, called the American Standard Code for Information Interchange (ASCII) as the Federal Information Processing Standard,
which replaced the incompatible teleprinter codes in use by different
branches of the U.S. government and universities during the 1960s. ASCII
included the distinction of upper- and lowercase alphabets and a set of
control characters
to facilitate the transmission of written language as well as printing
device functions, such as page advance and line feed, and the physical
or logical control of data flow over the transmission media. During the early 1960s, while also active in ASCII standardization, IBM simultaneously introduced in its product line of System/360 the eight-bit Extended Binary Coded Decimal Interchange Code (EBCDIC), an expansion of their six-bit binary-coded decimal (BCDIC) representations used in earlier card punches.
The prominence of the System/360 led to the ubiquitous adoption of the eight-bit storage size, while in detail the EBCDIC and ASCII encoding schemes are different.
In the early 1960s, AT&T introduced digital telephony on long-distance trunk lines. These used the eight-bit μ-law encoding. This large investment promised to reduce transmission costs for eight-bit data.
In Volume 1 of The Art of Computer Programming (first published in 1968), Donald Knuth uses byte in his hypothetical MIX computer to denote a unit which "contains an unspecified amount of information ... capable of holding at least 64 distinct values ... at most 100 distinct values. On a binary computer a byte must therefore be composed of six bits". He notes that "Since 1975 or so, the word byte
has come to mean a sequence of precisely eight binary digits...When we
speak of bytes in connection with MIX we shall confine ourselves to the
former sense of the word, harking back to the days when bytes were not
yet standardized."
The development of eight-bitmicroprocessors in the 1970s popularized this storage size. Microprocessors such as the Intel 8080, the direct predecessor of the 8086, could also perform a small number of operations on the four-bit pairs in a byte, such as the decimal-add-adjust (DAA) instruction. A four-bit quantity is often called a nibble, also nybble, which is conveniently represented by a single hexadecimal digit.
The term octet unambiguously specifies a size of eight bits. It is used extensively in protocol definitions.
Historically, the term octad or octade was used to denote eight bits as well at least in Western Europe;
however, this usage is no longer common. The exact origin of the term
is unclear, but it can be found in British, Dutch, and German sources of
the 1960s and 1970s, and throughout the documentation of Philips mainframe computers.
In the International System of Quantities (ISQ), B is also the symbol of the bel, a unit of logarithmic power ratio named after Alexander Graham Bell,
creating a conflict with the IEC specification. However, little danger
of confusion exists, because the bel is a rarely used unit. It is used
primarily in its decadic fraction, the decibel (dB), for signal strength and sound pressure level
measurements, while a unit for one-tenth of a byte, the decibyte, and
other fractions, are only used in derived units, such as transmission
rates.
The lowercase letter o for octet is defined as the symbol for octet in IEC 80000-13 and is commonly used in languages such as French and Romanian, and is also combined with metric prefixes for multiples, for example ko and Mo.
More than one system exists to define unit multiples based on the byte. Some systems are based on powers of 10, following the International System of Units (SI), which defines for example the prefix kilo as 1000 (103); other systems are based on powers of two. Nomenclature for these systems has led to confusion. Systems based on powers of 10 use standard SI prefixes (kilo, mega, giga, ...) and their corresponding symbols (k, M, G, ...). Systems based on powers of 2, however, might use binary prefixes (kibi, mebi, gibi, ...)
and their corresponding symbols (Ki, Mi, Gi, ...) or they might use the
prefixes K, M, and G, creating ambiguity when the prefixes M or G are
used.
While the difference between the decimal and binary
interpretations is relatively small for the kilobyte (about 2% smaller
than the kibibyte), the systems deviate increasingly as units grow
larger (the relative deviation grows by 2.4% for each three orders of
magnitude). For example, a power-of-10-based terabyte is about 9%
smaller than power-of-2-based tebibyte.
Units based on powers of 10
Definition of prefixes using powers of 10—in which 1 kilobyte (symbol kB) is defined to equal 1,000 bytes—is recommended by the International Electrotechnical Commission (IEC). The IEC standard defines eight such multiples, up to 1 yottabyte (YB), equal to 10008 bytes. The additional prefixes ronna- for 10009 and quetta- for 100010 were adopted by the International Bureau of Weights and Measures (BIPM) in 2022.
Prior art, the IBM System 360 and the related tape systems set the byte at 8 bits.Early 5.25" disks used decimal even though they used 128 byte and 256 byte sectors.Hard disks used mostly 256 byte and then 512 byte before 4096 byte blocks became standard.RAM was always sold in powers of 2.
Units based on powers of 2
A system of units based on powers of 2 in which 1 kibibyte (KiB) is equal to 1,024 (i.e., 210) bytes is defined by international standard IEC 80000-13 and is supported by national and international standards bodies (BIPM, IEC, NIST). The IEC standard defines eight such multiples, up to 1 yobibyte (YiB), equal to 10248 bytes. The natural binary counterparts to ronna- and quetta-
were given in a consultation paper of the International Committee for
Weights and Measures' Consultative Committee for Units (CCU) as robi- (Ri, 10249) and quebi- (Qi, 102410), but have not yet been adopted by the IEC or ISO.
An alternative system of nomenclature for the same units (referred to here as the customary convention), in which 1 kilobyte (KB) is equal to 1,024 bytes, 1 megabyte (MB) is equal to 10242 bytes and 1 gigabyte (GB) is equal to 10243 bytes is mentioned by a 1990s JEDEC
standard. Only the first three multiples (up to GB) are mentioned by
the JEDEC standard, which makes no mention of TB and larger. While
confusing and incorrect, the customary convention is used by the Microsoft Windows operating system and random-access memory capacity, such as main memory and CPU cache size, and in marketing and billing by telecommunication companies, such as Vodafone, AT&T, Orange and Telstra.
For storage
capacity, the customary convention was used by macOS and iOS through
Mac OS X 10.5 Leopard and iOS 10, after which they switched to units
based on powers of 10.
Parochial units
Various
computer vendors have coined terms for data of various sizes, sometimes
with different sizes for the same term even within a single vendor.
These terms include double word, half word, long word, quad word, slab, superword and syllable. There are also informal terms. e.g., half byte and nybble for 4 bits, octal K for 10008.
History of the conflicting definitions
Percentage difference between decimal and binary interpretations of the unit prefixes grows with increasing storage size
Contemporary computer memory has a binary architecture making a definition of memory units based on powers of 2 most practical. The use of the metric prefix kilo for binary multiples arose as a convenience, because 1024 is approximately 1000. This definition was popular in early decades of personal computing, with products like the Tandon 51⁄4-inch DD floppy format (holding 368640 bytes) being advertised as "360 KB", following the 1024-byte convention. It was not universal, however. The Shugart SA-400 51⁄4-inch floppy disk held 109,375 bytes unformatted, and was advertised as "110 Kbyte", using the 1000 convention. Likewise, the 8-inch DEC RX01 floppy (1975) held 256256 bytes formatted, and was advertised as "256k". Some devices were advertised using a mixture of the two definitions: most notably, floppy disks advertised as "1.44 MB" have an actual capacity of 1440 KiB, the equivalent of 1.47 MB or 1.41 MiB.
In 1995, the International Union of Pure and Applied Chemistry's (IUPAC) Interdivisional Committee on Nomenclature and Symbols attempted to resolve this ambiguity by proposing a set of binary prefixes for the powers of 1024, including kibi (kilobinary), mebi (megabinary), and gibi (gigabinary).
In December 1998, the IEC
addressed such multiple usages and definitions by adopting the IUPAC's
proposed prefixes (kibi, mebi, gibi, etc.) to unambiguously denote
powers of 1024. Thus one kibibyte (1 KiB) is 10241 bytes = 1024 bytes, one mebibyte (1 MiB) is 10242 bytes = 1048576 bytes, and so on.
In 1999, Donald Knuth suggested calling the kibibyte a "large kilobyte" (KKB).
Modern standard definitions
The IEC adopted the IUPAC proposal and published the standard in January 1999.[54][55] The IEC prefixes are part of the International System of Quantities. The IEC further specified that the kilobyte should only be used to refer to 1000 bytes.
Lawsuits over definition
Lawsuits
arising from alleged consumer confusion over the binary and decimal
definitions of multiples of the byte have generally ended in favor of
the manufacturers, with courts holding that the legal definition of
gigabyte or GB is 1 GB = 1000000000 (109) bytes (the decimal definition), rather than the binary definition (230, i.e., 1073741824).
Specifically, the United States District Court for the Northern
District of California held that "the U.S. Congress has deemed the
decimal definition of gigabyte to be the 'preferred' one for the
purposes of 'U.S. trade and commerce' [...] The California Legislature
has likewise adopted the decimal system for all 'transactions in this
state.'"
Earlier lawsuits had ended in settlement with no court ruling on the question, such as a lawsuit against drive manufacturer Western Digital.
Western Digital settled the challenge and added explicit disclaimers to
products that the usable capacity may differ from the advertised
capacity. Seagate was sued on similar grounds and also settled.
Practical examples
Unit
Approximate equivalent
bit
a Boolean variable indicating true (1) or false (0).
The C and C++ programming languages define byte
as an "addressable unit of data storage large enough to hold any member
of the basic character set of the execution environment" (clause 3.6 of
the C standard). The C standard requires that the integral data type unsigned char
must hold at least 256 different values, and is represented by at least
eight bits (clause 5.2.4.2.1). Various implementations of C and C++
reserve 8, 9, 16, 32, or 36 bits for the storage of a byte.
In addition, the C and C++ standards require that there be no gaps
between two bytes. This means every bit in memory is part of a byte.
Java's primitive data type byte is defined as eight bits. It is a signed data type, holding values from −128 to 127.
.NET programming languages, such as C#, define byte as an unsigned type, and the sbyte as a signed data type, holding values from 0 to 255, and −128 to 127, respectively.
In data transmission systems, the byte is used as a contiguous
sequence of bits in a serial data stream, representing the smallest
distinguished unit of data. For asynchronous communication a full transmission unit usually additionally includes a start bit, 1 or 2 stop bits, and possibly a parity bit, and thus its size may vary from seven to twelve bits for five to eight bits of actual data. For synchronous communication the error checking usually uses bytes at the end of a frame.
Plot of extinction intensity (percentage of marine genera that are present in each interval of time but do not exist in the following interval) vs time in the past.
Geological periods are annotated (by abbreviation and colour) above.
The Permian–Triassic extinction event is the most significant event for
marine genera, with just over 50% (according to this source) perishing. (source and image info)
Permian–Triassic boundary at Frazer Beach in New South Wales, with the End Permian extinction event located just above the coal layer
Approximately 251.9 million years ago, the Permian–Triassic (P–T, P–Tr) extinction event (PTME; also known as the Late Permian extinction event, the Latest Permian extinction event, the End-Permian extinction event, and colloquially as the Great Dying) forms the boundary between the Permian and Triassicgeologic periods, and with them the Paleozoic and Mesozoic eras. It is Earth's most severe known extinction event, with the extinction of 57% of biological families, 83% of genera, 81% of marinespecies and 70% of terrestrialvertebrate species. It is also the greatest known mass extinction of insects. It is the greatest of the "Big Five" mass extinctions of the Phanerozoic. There is evidence for one to three distinct pulses, or phases, of extinction.
The scientific consensus is that the main cause of the extinction was the flood basalt volcanic eruptions that created the Siberian Traps,which released sulfur dioxide and carbon dioxide, resulting in euxinia (oxygen-starved, sulfurous oceans), elevated global temperatures,
and acidified oceans.
The level of atmospheric carbon dioxide rose from around 400 ppm
to 2,500 ppm with approximately 3,900 to 12,000 gigatonnes of carbon
being added to the ocean-atmosphere system during this period. Several other contributing factors have been proposed, including the emission of carbon dioxide from the burning of oil and coal deposits ignited by the eruptions;
emissions of methane from the gasification of methane clathrates; emissions of methane by novel methanogenic microorganisms nourished by minerals dispersed in the eruptions; longer and more intense El Niño events; and an extraterrestrial impact which created the Araguainha crater and caused seismic release of methane and the destruction of the ozone layer with increased exposure to solar radiation.
Previously, it was thought that rock sequences spanning the
Permian–Triassic boundary were too few and contained too many gaps for
scientists to reliably determine its details.[46] However, it is now possible to date the extinction with millennial precision. U–Pbzircon dates from five volcanic ash beds from the Global Stratotype Section and Point for the Permian–Triassic boundary at Meishan, China,
establish a high-resolution age model for the extinction – allowing
exploration of the links between global environmental perturbation, carbon cycle disruption, mass extinction, and recovery at millennial timescales. The first appearance of the conodont Hindeodus parvus has been used to delineate the Permian-Triassic boundary.
The extinction occurred between 251.941 ± 0.037 and 251.880 ± 0.031 million years ago, a duration of 60 ± 48 thousand years. A large, abrupt global decrease in δ13C, the ratio of the stable isotopecarbon-13 to that of carbon-12, coincides with this extinction, and is sometimes used to identify the Permian–Triassic boundary and PTME in rocks that are unsuitable for radiometric dating. The negative carbon isotope excursion's magnitude was 4-7% and lasted for approximately 500 kyr, though estimating its exact value is challenging due to diagenetic alteration of many sedimentary facies spanning the boundary.
Further evidence for environmental change around the Permian-Triassic boundary suggests an 8 °C (14 °F) rise in temperature, and an increase in CO 2 levels to 2,500 ppm (for comparison, the concentration immediately before the Industrial Revolution was 280 ppm, and the amount today is about 422 ppm). There is also evidence of increased ultraviolet radiation reaching the earth, causing the mutation of plant spores.
It has been suggested that the Permian–Triassic boundary is
associated with a sharp increase in the abundance of marine and
terrestrial fungi, caused by the sharp increase in the amount of dead plants and animals fed upon by the fungi. This "fungal spike" has been used by some paleontologists to identify a lithological
sequence as being on or very close to the Permian–Triassic boundary in
rocks that are unsuitable for radiometric dating or have a lack of
suitable index fossils.
However, even the proposers of the fungal spike hypothesis pointed out
that "fungal spikes" may have been a repeating phenomenon created by the
post-extinction ecosystem during the earliest Triassic. The very idea of a fungal spike has been criticized on several grounds, including: Reduviasporonites, the most common supposed fungal spore, may be a fossilized alga; the spike did not appear worldwide; and in many places it did not fall on the Permian–Triassic boundary. The Reduviasporonites
may even represent a transition to a lake-dominated Triassic world
rather than an earliest Triassic zone of death and decay in some
terrestrial fossil beds. Newer chemical evidence agrees better with a fungal origin for Reduviasporonites, diluting these critiques.
Uncertainty exists regarding the duration of the overall
extinction and about the timing and duration of various groups'
extinctions within the greater process. Some evidence suggests that
there were multiple extinction pulses
or that the extinction was long and spread out over a few million
years, with a sharp peak in the last million years of the Permian. Statistical analyses of some highly fossiliferous strata in Meishan, Zhejiang Province in southeastern China, suggest that the main extinction was clustered around one peak, while a study of the Liangfengya section found evidence of two extinction waves, MEH-1 and MEH-2, which varied in their causes, and a study of the Shangsi section showed two extinction pulses with different causes too.
Recent research shows that different groups became extinct at different
times; for example, while difficult to date absolutely, ostracod and brachiopod extinctions were separated by around 670,000 to 1.17 million years. Palaeoenvironmental analysis of Lopingian strata in the Bowen Basin of Queensland
indicates numerous intermittent periods of marine environmental stress
from the middle to late Lopingian leading up to the end-Permian
extinction proper, supporting aspects of the gradualist hypothesis.
Additionally, the decline in marine species richness and the structural
collapse of marine ecosystems may have been decoupled as well, with the
former preceding the latter by about 61,000 years according to one
study.
Whether the terrestrial and marine extinctions were synchronous
or asynchronous is another point of controversy. Evidence from a
well-preserved sequence in east Greenland
suggests that the terrestrial and marine extinctions began
simultaneously. In this sequence, the decline of animal life is
concentrated in a period approximately 10,000 to 60,000 years long, with
plants taking an additional several hundred thousand years to show the
full impact of the event. Many sedimentary sequences from South China show synchronous terrestrial and marine extinctions.
Research in the Sydney Basin of the PTME's duration and course also
supports a synchronous occurrence of the terrestrial and marine biotic
collapses.
Other scientists believe the terrestrial mass extinction began between
60,000 and 370,000 years before the onset of the marine mass extinction. Chemostratigraphic analysis from sections in Finnmark and Trøndelag shows the terrestrial floral turnover occurred before the large negative δ13C shift during the marine extinction. Dating of the boundary between the Dicynodon and Lystrosaurus assemblage zones in the Karoo Basin indicates that the terrestrial extinction occurred earlier than the marine extinction. The Sunjiagou Formation of South China also records a terrestrial ecosystem demise predating the marine crisis. Other research still has found that the terrestrial extinction occurred after the marine extinction in the tropics.
Studies of the timing and causes of the Permian-Triassic extinction are complicated by the often-overlooked Capitanian extinction (also called the Guadalupian extinction), just one of perhaps two mass extinctions in the late Permian
that closely preceded the Permian-Triassic event. In short, when the
Permian-Triassic starts it is difficult to know whether the
end-Capitanian had finished, depending on the factor considered. Many of the extinctions once dated to the Permian-Triassic boundary have more recently been redated to the end-Capitanian.
Further, it is unclear whether some species who survived the prior
extinction(s) had recovered well enough for their final demise in the
Permian-Triassic event to be considered separate from Capitanian event. A
minority point of view considers the sequence of environmental
disasters to have effectively constituted a single, prolonged extinction
event, perhaps depending on which species is considered. This older
theory, still supported in some recent papers,
proposes that there were two major extinction pulses 9.4 million years
apart, separated by a period of extinctions that were less extensive,
but still well above the background level, and that the final extinction
killed off only about 80% of marine species alive at that time, whereas
the other losses occurred during the first pulse or the interval
between pulses. According to this theory, one of these extinction pulses
occurred at the end of the Guadalupianepoch of the Permian. For example, all dinocephalian genera died out at the end of the Guadalupian, as did the Verbeekinidae, a family of large-size fusulineforaminifera.
The impact of the end-Guadalupian extinction on marine organisms
appears to have varied between locations and between taxonomic groups –
brachiopods and corals had severe losses.
Marine invertebrates
suffered the greatest losses during the P–Tr extinction. Evidence of
this was found in samples from south China sections at the P–Tr
boundary. Here, 286 out of 329 marine invertebrate genera disappear
within the final two sedimentary zones containing conodonts from the Permian. The decrease in diversity was probably caused by a sharp increase in extinctions, rather than a decrease in speciation.
The extinction primarily affected organisms with calcium carbonate skeletons, especially those reliant on stable CO2 levels to produce their skeletons. These organisms were susceptible to the effects of the ocean acidification that resulted from increased atmospheric CO2. Organisms that relied on haemocyanin or haemoglobin for transporting oxygen were more resistant to extinction than those utilising haemerythrin or oxygen diffusion.
There is also evidence that endemism was a strong risk factor
influencing a taxon's likelihood of extinction. Bivalve taxa that were
endemic and localised to a specific region were more likely to go
extinct than cosmopolitan taxa. There was little latitudinal difference in the survival rates of taxa. Organisms that inhabited refugia less affected by global warming experienced lesser or delayed extinctions.
Among benthic organisms the extinction event multiplied background extinction rates, and therefore caused maximum species loss to taxa that had a high background extinction rate (by implication, taxa with a high turnover). The extinction rate of marine organisms was catastrophic.
Bioturbators were extremely severely affected, as evidenced by the loss
of the sedimentary mixed layer in many marine facies during the
end-Permian extinction.
Surviving marine invertebrate groups included articulate brachiopods (those with a hinge), which had undergone a slow decline in numbers since the P–Tr extinction; the Ceratitida order of ammonites; and crinoids ("sea lilies"),
which very nearly became extinct but later became abundant and diverse.
The groups with the highest survival rates generally had active control
of circulation,
elaborate gas exchange mechanisms, and light calcification; more
heavily calcified organisms with simpler breathing apparatuses suffered
the greatest loss of species diversity. In the case of the brachiopods, at least, surviving taxa were generally small, rare members of a formerly diverse community.
Conodonts were severely affected both in terms of taxonomic and
morphological diversity, although not as severely as during the
Capitanian mass extinction.
The ammonoids, which had been in a long-term decline for the 30 million years since the Roadian (middle Permian), suffered a selective extinction pulse 10 million years before the main event, at the end of the Capitanian stage. In this preliminary extinction, which greatly reduced disparity,
or the range of different ecological guilds, environmental factors were
apparently responsible. Diversity and disparity fell further until the
P–Tr boundary; the extinction here (P–Tr) was non-selective, consistent
with a catastrophic initiator. During the Triassic, diversity rose
rapidly, but disparity remained low.
The range of morphospace occupied by the ammonoids, that is, their
range of possible forms, shapes or structures, became more restricted as
the Permian progressed. A few million years into the Triassic, the
original range of ammonoid structures was once again reoccupied, but the
parameters were now shared differently among clades.
Ostracods experienced prolonged diversity perturbations during
the Changhsingian before the PTME proper, when immense proportions of
them abruptly vanished. At least 74% of ostracods died out during the PTME itself.
Bryozoans had been on a long-term decline throughout the Late
Permian epoch before they suffered even more catastrophic losses during
the PTME, being the most severely affected clade among the lophophorates.
Deep water sponges suffered a significant diversity loss and
exhibited a decrease in spicule size over the course of the PTME.
Shallow water sponges were affected much less strongly; they experienced
an increase in spicule size and much lower loss of morphological
diversity compared to their deep water counterparts.
Foraminifera suffered a severe bottleneck in diversity. Evidence from South China indicates the foraminiferal extinction had two pulses. Foraminiferal biodiversity hotspots shifted into deeper waters during the PTME.
Approximately 93% of latest Permian foraminifera became extinct, with
50% of the clade Textulariina, 92% of Lagenida, 96% of Fusulinida, and
100% of Miliolida disappearing. Foraminifera that were calcaerous suffered an extinction rate of 91%.
The reason why lagenides survived while fusulinoidean fusulinides went
completely extinct may have been due to the greater range of
environmental tolerance and greater geographic distribution of the
former compared to the latter.
Cladodontomorph sharks likely survived the extinction by
surviving in refugia in the deep oceans, a hypothesis based on the
discovery of Early Cretaceous cladodontomorphs in deep, outer shelf environments. Ichthyosaurs, which evolved immediately before the PTME, were also PTME survivors.
The Lilliput effect,
the phenomenon of dwarfing of species during and immediately following a
mass extinction event, has been observed across the Permian-Triassic
boundary, notably occurring in foraminifera, brachiopods, bivalves,and ostracods. Though gastropods that survived the cataclysm were smaller in size than those that did not, it remains debated whether the Lilliput effect truly took hold among gastropods. Some gastropod taxa, termed "Gulliver gastropods", ballooned in size during and immediately following the mass extinction, exemplifying the Lilliput effect's opposite, which has been dubbed the Brobdingnag effect.
Terrestrial invertebrates
The Permian had great diversity in insect and other invertebrate species, including the largest insects ever to have existed. The end-Permian is the largest known mass extinction of insects; according to some sources, it may well be the only mass extinction to significantly affect insect diversity.Eight or nine insect orders became extinct and ten more were greatly reduced in diversity. Palaeodictyopteroids
(insects with piercing and sucking mouthparts) began to decline during
the mid-Permian; these extinctions have been linked to a change in
flora. The greatest decline occurred in the Late Permian and was
probably not directly caused by weather-related floral transitions. However, some observed entomofaunal declines in the PTME were biogeographic changes rather than outright extinctions.
Terrestrial plants
The geological record of terrestrial plants is sparse and based mostly on pollen and spore
studies. Floral changes across the Permian-Triassic boundary are highly
variable depending on the location and preservation quality of any
given site.
Plants are relatively immune to mass extinction, with the impact of all
the major mass extinctions "insignificant" at a family level. Floral diversity losses were more superficial than those of marine animals. Even the reduction observed in species diversity (of 50%) may be mostly due to taphonomic processes.
However, a massive rearrangement of ecosystems does occur, with plant
abundances and distributions changing profoundly and all the forests
virtually disappearing. The dominant floral groups changed, with many groups of land plants entering abrupt decline, such as Cordaites (gymnosperms) and Glossopteris (seed ferns). The severity of plant extinction has been disputed.
The Glossopteris-dominated flora that characterised
high-latitude Gondwana collapsed in Australia around 370,000 years
before the Permian-Triassic boundary, with this flora's collapse being
less constrained in western Gondwana but still likely occurring a few
hundred thousand years before the boundary.
The collapse of this flora is indirectly marked by an abrupt change in
river morphology from meandering to braided river systems, signifying
the widespread demise of rooted plants.
Palynological or pollen studies from East Greenland of sedimentary rock strata laid down during the extinction period indicate dense gymnosperm woodlands before the event. At the same time that marine invertebrate macrofauna declined, these large woodlands died out and were followed by a rise in diversity of smaller herbaceous plants including Lycopodiophyta, both Selaginellales and Isoetales. Data from Kap Stosch suggest that floral species richness was not significantly affected during the PTME.
The Cordaites flora, which dominated the Angaran floristic realm corresponding to Siberia, collapsed over the course of the extinction. In the Kuznetsk Basin,
the aridity-induced extinction of the regions's humid-adapted forest
flora dominated by cordaitaleans occurred approximately 252.76 Ma,
around 820,000 years before the end-Permian extinction in South China,
suggesting that the end-Permian biotic catastrophe may have started
earlier on land and that the ecological crisis may have been more
gradual and asynchronous on land compared to its more abrupt onset in
the marine realm.
In North China, the transition between the Upper Shihhotse and
Sunjiagou Formations and their lateral equivalents marked a very large
extinction of plants in the region. Those plant genera that did not go
extinct still experienced a great reduction in their geographic range.
Following this transition, coal swamps vanished. The North Chinese
floral extinction correlates with the decline of the Gigantopteris flora of South China.
In South China, the subtropical Cathaysiangigantopterid dominated rainforests abruptly collapsed.
The floral extinction in South China is associated with bacterial
blooms in soil and nearby lacustrine ecosystems, with soil erosion
resulting from the die-off of plants being their likely cause. Wildfires too likely played a role in the fall of Gigantopteris.
A conifer flora in what is now Jordan, known from fossils near the Dead Sea, showed unusual stability over the Permian-Triassic transition, and appears to have been only minimally affected by the crisis.
Terrestrial vertebrates
The
tempo of the terrestrial vertebrate extinction is disputed. Some
evidence from the Karoo Basin indicates a protracted extinction lasting a
million years. Other evidence from the Karoo deposits suggest it took 50,000 years or less, while a study of coprolites in the Vyazniki fossil beds in Russia suggests it took only a few thousand years. Aridification induced by global warming was the chief culprit behind terrestrial vertebrate extinctions. There is enough evidence to indicate that over two thirds of terrestrial labyrinthodontamphibians, sauropsid ("reptile") and therapsid ("proto-mammal") taxa became extinct. Large herbivores suffered the heaviest losses.
All Permian anapsid reptiles died out except the procolophonids (although testudines have morphologically-anapsid skulls, they are now thought to have separately evolved from diapsid ancestors). Pelycosaurs
died out before the end of the Permian. Too few Permian diapsid fossils
have been found to support any conclusion about the effect of the
Permian extinction on diapsids (the "reptile" group from which lizards, snakes, crocodilians, and dinosaurs (including birds) evolved). Tangasaurids were largely unaffected.
Gorgonopsians are traditionally thought to have gone extinct during the
PTME, but some tentative evidence suggests they may have survived into
the Triassic.
Freshwater and euryhaline fishes, having experienced minimal diversity
losses before the PTME, were unaffected during the PTME and actually
appear to have increased in diversity across the Permian-Triassic
boundary. However, faunal turnovers in freshwater fish communities occurred in areas like the Kuznetsk Basin.
The groups that survived suffered extremely heavy losses of
species and some terrestrial vertebrate groups very nearly became
extinct at the end of the Permian. Some of the surviving groups did not
persist for long past this period, but others that barely survived went
on to produce diverse and long-lasting lineages. However, it took 30million years for the terrestrial vertebrate fauna to fully recover both numerically and ecologically.
It is difficult to analyze extinction and survival rates of land
organisms in detail because few terrestrial fossil beds span the
Permian–Triassic boundary. The best-known record of vertebrate changes across the Permian–Triassic boundary occurs in the Karoo Supergroup of South Africa, but statistical analyses have so far not produced clear conclusions.
One study of the Karoo Basin found that 69% of terrestrial vertebrates
went extinct over 300,000 years leading up to the Permian-Triassic
boundary, followed by a minor extinction pulse involving four taxa that
survived the previous extinction interval. Another study of latest Permian vertebrates in the Karoo Basin found that 54% of them went extinct due to the PTME.
In the wake of the extinction event, the ecological structure of
present-day biosphere evolved from the stock of surviving taxa. In the
sea, the "Palaeozoic evolutionary fauna" declined while the "modern
evolutionary fauna" achieved greater dominance;
the Permian-Triassic mass extinction marked a key turning point in this
ecological shift that began after the Capitanian mass extinction and culminated in the Late Jurassic. Typical taxa of shelly benthic faunas were now bivalves, snails, sea urchins and Malacostraca, whereas bony fishes and marine reptiles diversified in the pelagic zone. On land, dinosaurs and mammals arose in the course of the Triassic.
The profound change in the taxonomic composition was partly a result of
the selectivity of the extinction event, which affected some taxa
(e.g., brachiopods) more severely than others (e.g., bivalves).
However, recovery was also differential between taxa. Some survivors
became extinct some million years after the extinction event without
having rediversified (dead clade walking, e.g. the snail family Bellerophontidae), whereas others rose to dominance over geologic times (e.g., bivalves).
A cosmopolitanism event began immediately after the end-Permian extinction event. Marine post-extinction faunas were mostly species-poor and were dominated by few disaster taxa such as the bivalves Claraia, Unionites, Eumorphotis, and Promyalina, the conodonts Clarkina and Hindeodus, the inarticulate brachiopod Lingularia, and the foraminifera Earlandia and Rectocornuspira kalhori, the latter of which is sometimes classified under the genus Ammodiscus. Their guild diversity was also low.[198] Post-PTME faunas had a flat, insignificant latitudinal diversity gradient.
The speed of recovery from the extinction is disputed. Some scientists estimate that it took 10 million years (until the Middle Triassic) due to the severity of the extinction. However, studies in Bear Lake County, near Paris, Idaho, and nearby sites in Idaho and Nevada showed a relatively quick rebound in a localized Early Triassic marine ecosystem (Paris biota), taking around 1.3 million years to recover, while an unusually diverse and complex ichnobiota is known from Italy less than a million years after the end-Permian extinction. Additionally, the complex Guiyang biota found near Guiyang, China also indicates life thrived in some places just a million years after the mass extinction, as does a fossil assemblage known as the Shanggan fauna found in Shanggan, China, the Wangmo biota from the Luolou Formation of Guizhou, and a gastropod fauna from the Al Jil Formation of Oman. Regional differences in the pace of biotic recovery existed,
which suggests that the impact of the extinction may have been felt
less severely in some areas than others, with differential environmental
stress and instability being the source of the variance.
In addition, it has been proposed that although overall taxonomic
diversity rebounded rapidly, functional ecological diversity took much
longer to return to its pre-extinction levels;
one study concluded that marine ecological recovery was still ongoing
50 million years after the extinction, during the latest Triassic, even
though taxonomic diversity had rebounded in a tenth of that time.
The pace and timing of recovery also differed based on clade and
mode of life. Seafloor communities maintained a comparatively low
diversity until the end of the Early Triassic, approximately 4 million
years after the extinction event. Epifaunal benthos took longer to recover than infaunal benthos. This slow recovery stands in remarkable contrast with the quick recovery seen in nektonic organisms such as ammonoids, which exceeded pre-extinction diversities already two million years after the crisis, and conodonts, which diversified considerably over the first two million years of the Early Triassic.
Recent work suggests that the pace of recovery was intrinsically
driven by the intensity of competition among species, which drives rates
of niche differentiation and speciation.
That recovery was slow in the Early Triassic can be explained by low
levels of biological competition due to the paucity of taxonomic
diversity,
and that biotic recovery explosively accelerated in the Anisian can be
explained by niche crowding, a phenomenon that would have drastically
increased competition, becoming prevalent by the Anisian. Biodiversity rise thus behaved as a positive feedback loop enhancing itself as it took off in the Spathian and Anisian. Accordingly, low levels of interspecific competition in seafloor communities that are dominated by primary consumers correspond to slow rates of diversification and high levels of interspecific competition among nektonic secondary and tertiary consumers to high diversification rates.
Other explanations state that life was delayed in its recovery because
grim conditions returned periodically over the course of the Early
Triassic, causing further extinction events, such as the Smithian-Spathian boundary extinction.
Continual episodes of extremely hot climatic conditions during the
Early Triassic have been held responsible for the delayed recovery of
oceanic life, in particular skeletonised taxa that are most vulnerable to high carbon dioxide concentrations. The relative delay in the recovery of benthic organisms has been attributed to widespread anoxia, but high abundances of benthic species contradict this explanation.
A 2019 study attributed the dissimilarity of recovery times between
different ecological communities to differences in local environmental
stress during the biotic recovery interval, with regions experiencing
persistent environmental stress post-extinction recovering more slowly,
supporting the view that recurrent environmental calamities were
culpable for retarded biotic recovery.
Recurrent Early Triassic environmental stresses also acted as a ceiling
limiting the maximum ecological complexity of marine ecosystems until
the Spathian. Recovery biotas appear to have been ecologically uneven and unstable into the Anisian, making them vulnerable to environmental stresses.
Whereas most marine communities were fully recovered by the Middle Triassic,
global marine diversity reached pre-extinction values no earlier than
the Middle Jurassic, approximately 75 million years after the extinction
event.
Sessile filter feeders like this Carboniferous crinoid, the mushroom crinoid (Agaricocrinus americanus), were significantly less abundant after the P–Tr extinction.
Prior to the extinction, about two-thirds of marine animals were sessile
and attached to the seafloor. During the Mesozoic, only about half of
the marine animals were sessile while the rest were free-living.
Analysis of marine fossils from the period indicated a decrease in the
abundance of sessile epifaunalsuspension feeders such as brachiopods and sea lilies and an increase in more complex mobile species such as snails, sea urchins and crabs.
Before the Permian mass extinction event, both complex and simple
marine ecosystems were equally common. After the recovery from the mass
extinction, the complex communities outnumbered the simple communities
by nearly three to one, and the increase in predation pressure and durophagy led to the Mesozoic Marine Revolution.
Marine vertebrates recovered relatively quickly, with complex
predator-prey interactions with vertebrates at the top of the food web
being known from coprolites five million years after the PTME. Post-PTME hybodonts exhibited extremely rapid tooth replacement. Ichthyopterygians appear to have ballooned in size extremely rapidly following the PTME.
Bivalves rapidly recolonised many marine environments in the wake of the catastrophe.
Bivalves were fairly rare before the P–Tr extinction but became
numerous and diverse in the Triassic, taking over niches that were
filled primarily by brachiopods before the mass extinction event.
Bivalves were once thought to have outcompeted brachiopods, but this
outdated hypothesis about the brachiopod-bivalve transition has been
disproven by Bayesian analysis.
The success of bivalves in the aftermath of the extinction event may
have been a function of them possessing greater resilience to
environmental stress compared to the brachiopods that they coexisted
with, whilst other studies have emphasised the greater niche breadth of the former.
The rise of bivalves to taxonomic and ecological dominance over
brachiopods was not synchronous, however, and brachiopods retained an
outsized ecological dominance into the Middle Triassic even as bivalves
eclipsed them in taxonomic diversity.
Some researchers think the brachiopod-bivalve transition was
attributable not only to the end-Permian extinction but also the
ecological restructuring that began as a result of the Capitanian
extinction. Infaunal habits in bivalves became more common after the PTME.
Linguliform brachiopods were commonplace immediately after the
extinction event, their abundance having been essentially unaffected by
the crisis. Adaptations for oxygen-poor and warm environments, such as
increased lophophoral cavity surface, shell width/length ratio, and
shell miniaturisation, are observed in post-extinction linguliforms. The surviving brachiopod fauna was very low in diversity and exhibited no provincialism whatsoever. Brachiopods began their recovery around 250.1 ± 0.3 Ma, as marked by the appearance of the genus Meishanorhynchia, believed to be the first of the progenitor brachiopods that evolved after the mass extinction.
Major brachiopod rediversification only began in the late Spathian and
Anisian in conjunction with the decline of widespread anoxia and extreme
heat and the expansion of more habitable climatic zones.
Brachiopod taxa during the Anisian recovery interval were only
phylogenetically related to Late Permian brachiopods at a familial
taxonomic level or higher; the ecology of brachiopods had radically
changed from before in the mass extinction's aftermath.
Ostracods were extremely rare during the basalmost Early Triassic. Taxa associated with microbialites were disproportionately represented among ostracod survivors. Ostracod recovery began in the Spathian.
Despite high taxonomic turnover, the ecological life modes of Early
Triassic ostracods remained rather similar to those of pre-PTME
ostracods.
Bryozoans in the Early Triassic were restricted to the Boreal realm. They were also not diverse, represented mainly by members of Trepostomatida. During the Middle Triassic, there was a rise in bryozoan diversity, which peaked in the Carnian. However, bryozoans took until the Late Cretaceous to recover their full diversity.
Crinoids ("sea lilies") suffered a selective extinction, resulting in a decrease in the variety of their forms.
Though cladistic analyses suggest the beginning of their recovery to
have taken place in the Induan, the recovery of their diversity as
measured by fossil evidence was far less brisk, showing up in the late
Ladinian. Their adaptive radiation after the extinction event resulted in forms possessing flexible arms becoming widespread; motility, predominantly a response to predation pressure, also became far more prevalent.
Though their taxonomic diversity remained relatively low, crinoids
regained much of their ecological dominance by the Middle Triassic
epoch. Stem-group echinoids survived the PTME. The survival of miocidarid echinoids such as Eotiaris is likely attributable to their ability to thrive in a wide range of environmental conditions.
Conodonts saw a rapid recovery during the Induan, with anchignathodontids experiencing a diversity peak in the earliest Induan. Gondolellids
diversified at the end of the Griesbachian; this diversity spike was
most responsible for the overall conodont diversity peak in the
Smithian.
Segminiplanate conodonts again experienced a brief period of domination
in the early Spathian, probably related to a transient oxygenation of
deep waters. Neospathodid conodonts survived the crisis but underwent proteromorphosis.
In the PTME's aftermath, disaster taxa of benthic foraminifera
filled many of their vacant niches. The recovery of benthic foraminifera
was very slow and frequently interrupted until the Spathian.
In the Tethys, foraminiferal communities remained low in diversity into
the Middle Triassic, with the exception of a notable Ladinian fauna
from the Catalonian Basin.
Microbial reefs were common across shallow seas for a short time during the earliest Triassic,predominating in low latitudes while being rarer in higher latitudes, occurring both in anoxic and oxic waters. Polybessurus-like microfossils often dominated these earliest Triassic microbialites. Microbial-metazoan reefs appeared very early in the Early Triassic; and they dominated many surviving communities across the recovery from the mass extinction. Microbialite deposits appear to have declined in the early Griesbachian synchronously with a significant sea level drop that occurred then. Metazoan-built reefs reemerged during the Olenekian, mainly being composed of sponge biostrome and bivalve builups.
Keratose sponges were particularly noteworthy in their integral
importance to Early Triassic microbial-metazoan reef communities, and they helped to create stability in heavily damaged ecosystems during early phases of biotic recovery. "Tubiphytes"-dominated
reefs appeared at the end of the Olenekian, representing the earliest
platform-margin reefs of the Triassic, though they did not become
abundant until the late Anisian, when reefs' species richness increased.
The first scleractinian corals appear in the late Anisian as well,
although they would not become the dominant reef builders until the end
of the Triassic period. Bryozoans, after sponges, were the most numerous organisms in Tethyan reefs during the Anisian.
Metazoan reefs became common again during the Anisian because the
oceans cooled down then from their overheated state during the Early
Triassic.
Biodiversity amongst metazoan reefs did not recover until well into the
Anisian, millions of years after non-reef ecosystems recovered their
diversity. Microbially induced sedimentary structures
(MISS) from the earliest Triassic have been found to be associated with
abundant opportunistic bivalves and vertical burrows, and it is likely
that post-extinction microbial mats played a vital, indispensable role
in the survival and recovery of various bioturbating organisms.
The microbialite refuge hypothesis has been criticised as reflecting a
taphonomic bias due to the greater preservation potential of
microbialite deposits, however, rather than a genuine phenomenon.
Ichnocoenoses show that marine ecosystems recovered to pre-extinction levels of ecological complexity by the late Olenekian.
Anisian ichnocoenoses show slightly lower diversity than Spathian
ichnocoenoses, although this was likely a taphonomic consequence of
increased and deeper bioturbation erasing evidence of shallower
bioturbation.
Ichnological
evidence suggests that recovery and recolonisation of marine
environments may have taken place by way of outward dispersal from
refugia that suffered relatively mild perturbations and whose local
biotas were less strongly affected by the mass extinction compared to
the rest of the world's oceans.
Although complex bioturbation patterns were rare in the Early Triassic,
likely reflecting the inhospitability of many shallow water
environments in the extinction's wake, complex ecosystem engineering
managed to persist locally in some places, and may have spread from
there after harsh conditions across the global ocean were ameliorated
over time.
Wave-dominated shoreface settings (WDSS) are believed to have served as
refugium environments because they appear to have been unusually
diverse in the mass extinction's aftermath.
Terrestrial plants
The
proto-recovery of terrestrial floras took place from a few tens of
thousands of years after the end-Permian extinction to around 350,000
years after it, with the exact timeline varying by region.
Furthermore, severe extinction pulses continued to occur after the
Permian-Triassic boundary, causing additional floral turnovers.
Gymnosperms recovered within a few thousand years after the
Permian-Triassic boundary, but around 500,000 years after it, the
Dominant gymnosperm genera were replaced by lycophytes – extant lycophytes are recolonizers of disturbed areas – during an extinction pulse at the Griesbachian-Dienerian boundary.
The particular post-extinction dominance of lycophytes, which were well
adapted for coastal environments, can be explained in part by global
marine transgressions during the Early Triassic. The worldwide recovery of gymnosperm forests took approximately 4–5 million years.
However, this trend of prolonged lycophyte dominance during the Early
Triassic was not universal, as evidenced by the much more rapid recovery
of gymnosperms in certain regions,
and floral recovery likely did not follow a congruent, globally
universal trend but instead varied by region according to local
environmental conditions.
In East Greenland, lycophytes replaced gymnosperms as the
dominant plants. Later, other groups of gymnosperms again become
dominant but again suffered major die-offs. These cyclical flora shifts
occurred a few times over the course of the extinction period and
afterward. These fluctuations of the dominant flora between woody and herbaceous
taxa indicate chronic environmental stress resulting in a loss of most
large woodland plant species. The successions and extinctions of plant
communities do not coincide with the shift in δ13C values but occurred many years after.
In what is now the Barents Sea of the coast of Norway, the
post-extinction flora is dominated by pteridophytes and lycopods, which
were suited for primary succession and recolonisation of devastated
areas, although gymnosperms made a rapid recovery, with the lycopod
dominated flora not persisting across most of the Early Triassic as
postulated in other regions.
In Europe and North China, the interval of recovery was dominated by the lycopsid Pleuromeia,
an opportunistic pioneer plant that filled ecological vacancies until
other plants were able to expand out of refugia and recolonise the land.
Conifers became common by the early Anisian, while pteridosperms and
cycadophytes only fully recovered by the late Anisian.
During the survival phase in the terrestrial extinction's
immediate aftermath, from the latest Changhsingian to the Griesbachian,
South China was dominated by opportunistic lycophytes. Low-lying herbaceous vegetation dominated by the isoetalean Tomiostrobus
was ubiquitous following the collapse of the gigantopterid-dominated
forests of before. In contrast to the highly biodiverse gigantopterid
rainforests, the post-extinction landscape of South China was
near-barren and had vastly lower diversity. Plant survivors of the PTME in South China experienced extremely high rates of mutagenesis induced by heavy metal poisoning.
From the late Griesbachian to the Smithian, conifers and ferns began to
rediversify. After the Smithian, the opportunistic lycophyte flora
declined, as the newly radiating conifer and fern species permanently
replaced them as the dominant components of South China's flora.
In Tibet, the early Dienerian Endosporites papillatus–Pinuspollenites thoracatus
assemblages closely resemble late Changhsingian Tibetan floras,
suggesting that the widespread, dominant latest Permian flora resurged
easily after the PTME. However, in the late Dienerian, a major shift
towards assemblages dominated by cavate trilete spores took place,
heralding widespread deforestation and a rapid change to hotter, more
humid conditions. Quillworts and spike mosses dominated Tibetan flora
for about a million years after this shift.
In Pakistan, then the northern margin of Gondwana, the flora was
rich in lycopods associated with conifers and pteridosperms. Floral
turnovers continued to occur due to repeated perturbations arising from
recurrent volcanic activity until terrestrial ecosystems stabilised
around 2.1 Myr after the PTME.
In southwestern Gondwana, the post-extinction flora was dominated by bennettitaleans and cycads, with members of Peltaspermales, Ginkgoales,
and Umkomasiales being less common constituents of this flora. Around
the Induan-Olenekian boundary, as palaeocommunities recovered, a new Dicroidium
flora was established, in which Umkomasiales continued to be prominent
and in which Equisetales and Cycadales were subordinate forms. The Dicroidium
flora further diversified in the Anisian to its peak, wherein
Umkomasiales and Ginkgoales constituted most of the tree canopy and
Peltaspermales, Petriellales, Cycadales, Umkomasiales, Gnetales, Equisetales, and Dipteridaceae dominated the understory.
Coal gap
No coal
deposits are known from the Early Triassic, and those in the Middle
Triassic are thin and low-grade. This "coal gap" has been explained in
many ways. It has been suggested that new, more aggressive fungi,
insects, and vertebrates evolved and killed vast numbers of trees. These
decomposers
themselves suffered heavy losses of species during the extinction and
are not considered a likely cause of the coal gap. It could simply be
that all coal-forming plants were rendered extinct by the P–Tr
extinction and that it took 10 million years for a new suite of plants
to adapt to the moist, acid conditions of peatbogs. Abiotic factors (factors not caused by organisms), such as decreased rainfall or increased input of clastic sediments, may also be to blame.
On the other hand, the lack of coal may simply reflect the scarcity of all known sediments from the Early Triassic. Coal-producing ecosystems, rather than disappearing, may have moved to areas where we have no sedimentary record for the Early Triassic. For example, in eastern Australia a cold climate had been the norm for a long period, with a peat mire ecosystem adapted to these conditions. Approximately 95% of these peat-producing plants went locally extinct at the P–Tr boundary; coal deposits in Australia and Antarctica disappear significantly before the P–Tr boundary.
Terrestrial vertebrates
Land vertebrates took an unusually long time to recover from the P–Tr extinction; palaeontologist Michael Benton estimated the recovery was not complete until 30 million years after the extinction, i.e. not until the Late Triassic, when the first dinosaurs had risen from bipedal archosaurian ancestors and the first mammals from small cynodont ancestors.
A tetrapod gap may have existed from the Induan until the early
Spathian between ~30 °N and ~ 40 °S due to extreme heat making these low
latitudes uninhabitable for these animals. During the hottest phases of
this interval, the gap would have spanned an even greater latitudinal
range.
East-central Pangaea, with its relatively wet climate, served as a
dispersal corridor for PTME survivors during their Early Triassic
recolonisation of the supercontinent. In North China, tetrapod body and ichnofossils
are extremely rare in Induan facies, but become more abundant in the
Olenekian and Anisian, showing a biotic recovery of tetrapods
synchronous with the decreasing aridity during the Olenekian and
Anisian.
In Russia, even after 15 Myr of recovery, during which ecosystems were
rebuilt and remodelled, many terrestrial vertebrate guilds were absent,
including small insectivores, small piscivores, large herbivores, and
apex predators. Coprolitic
evidence indicates that freshwater food webs had recovered by the early
Ladinian, with a lacustrine coprolite assemblage from the Ordos Basin
of China providing evidence of a trophically multileveled ecosystem
containing at least six different trophic levels. The highest trophic
levels were filled by vertebrate predators.
Overall, terrestrial faunas after the extinction event tended to be
more variable and heterogeneous across space than those of the Late
Permian, which exhibited less provincialism, being much more
geographically homogeneous.
Synapsids
Lystrosaurus was by far the most abundant early Triassic land vertebrate.
Lystrosaurus, a pig-sized herbivorous dicynodonttherapsid,
constituted as much as 90% of some earliest Triassic land vertebrate
fauna, although some recent evidence has called into question its status
as a post-PTME disaster taxon. The dicynodont genus is often used as a biostratigraphic marker for the PTME. The evolutionary success of Lystrosaurus
in the aftermath of the PTME is believed to be attributable to the
dicynodont taxon's grouping behaviour and tolerance for extreme and
highly variable climatic conditions. Other likely factors behind the success of Lystrosaurus included extremely fast growth rate exhibited by the dicynodont genus, along with its early onset of sexual maturity.
Antarctica may have served as a refuge for dicynodonts during the PTME
from which surviving dicynodonts spread out of in its aftermath.
Ichnological evidence from the earliest Triassic of the Karoo Basin
shows dicynodonts were abundant in the immediate aftermath of the biotic
crisis. Smaller carnivorous cynodonttherapsids also survived, a group that included the ancestors of mammals. As with dicynodonts, selective pressures favoured endothermic epicynodonts. Therocephalians likewise survived; burrowing may have been a key adaptation that helped them make it through the PTME. In the Karoo region of southern Africa, the therocephaliansTetracynodon, Moschorhinus and Ictidosuchoides survived, but do not appear to have been abundant in the Triassic.
Early Triassic therocephalians were mostly survivors of the PTME rather
than newly evolved taxa that originated during the evolutionary
radiation in its aftermath. Both therocephalians and cynodonts, known collectively as eutheriodonts, decreased in body size from the Late Permian to the Early Triassic. This decrease in body size has been interpreted as an example of the Lilliput effect.
Sauropsids
Archosaurs (which included the ancestors of dinosaurs and crocodilians)
were initially rarer than therapsids, but they began to displace
therapsids in the mid-Triassic. Olenekian tooth fossil assemblages from
the Karoo Basin indicate that archosauromorphs were already highly
diverse by this point in time, though not very ecologically specialised. In the mid to late Triassic, the dinosaurs evolved from one group of archosaurs, and went on to dominate terrestrial ecosystems during the Jurassic and Cretaceous. This "Triassic Takeover" may have contributed to the evolution of mammals by forcing the surviving therapsids and their mammaliform successors to live as small, mainly nocturnalinsectivores; nocturnal life probably forced at least the mammaliforms to develop fur, better hearing and higher metabolic rates,
while losing part of the differential color-sensitive retinal receptors
reptilians and birds preserved. Archosaurs also experienced an increase
in metabolic rates over time during the Early Triassic. The archosaur dominance would end again due to the Cretaceous–Paleogene extinction event, after which both birds (only extant dinosaurs) and mammals (only extant synapsids) would diversify and share the world.
Temnospondyls
Temnospondylamphibians
made a quick recovery; the appearance in the fossil record of so many
temnospondyl clades suggests they may have been ideally suited as
pioneer species that recolonised decimated ecosystems. During the Induan, tupilakosaurids in particular thrived as disaster taxa, including Tupilakosaurus itself, though they gave way to other temnospondyls as ecosystems recovered. Temnospondyls were reduced in size during the Induan, but their body size rebounded to pre-PTME levels during the Olenekian. Mastodonsaurus and trematosaurians were the main aquatic and semiaquatic predators during most of the Triassic, some preying on tetrapods and others on fish.
Terrestrial invertebrates
Most
fossil insect groups found after the Permian–Triassic boundary differ
significantly from those before: Of Paleozoic insect groups, only the Glosselytrodea, Miomoptera, and Protorthoptera have been discovered in deposits from after the extinction. The caloneurodeans, paleodictyopteroids, protelytropterans, and protodonates
became extinct by the end of the Permian. Though Triassic insects are
very different from those of the Permian, a gap in the insect fossil
record spans approximately 15 million years from the late Permian to
early Triassic. In well-documented Late Triassic deposits, fossils
overwhelmingly consist of modern fossil insect groups.
Microbially induced sedimentary structures (MISS) dominated North Chinese terrestrial fossil assemblages in the Early Triassic. In Arctic Canada as well, MISS became a common occurrence following the Permian-Triassic extinction.
The prevalence of MISS in many Early Triassic rocks shows that
microbial mats were an important feature of post-extinction ecosystems
that were denuded of bioturbators that would have otherwise prevented
their widespread occurrence. The disappearance of MISS later in the
Early Triassic likely indicated a greater recovery of terrestrial
ecosystems and specifically a return of prevalent bioturbation.
Hypotheses about cause
Explaining
an event from 250 million years ago is inherently difficult, with much
of the evidence on land eroded or deeply buried, while the spreading seafloor is completely recycled over 200 million years, leaving no useful indications beneath the ocean.
Yet, scientists have gathered significant evidence for causes,
and several mechanisms have been proposed. The proposals include both
catastrophic and gradual processes (similar to those theorized for the Cretaceous–Paleogene extinction event, but with much less current consensus).
The catastrophic group includes one or more large bolideimpact events, increased volcanism, and sudden release of methane from the seafloor, either due to dissociation of methane hydrate deposits or metabolism of organic carbon deposits by methanogenic microbes.
The gradual group includes sea level change, increasing hypoxia, and increasing aridity.
Any hypothesis about the cause must explain the selectivity of the
event, which affected organisms with calcium carbonate skeletons most
severely; the long period (4 to 6 million years) before recovery
started, and the minimal extent of biological mineralization (despite inorganic carbonates being deposited) once the recovery began.
Volcanism
Siberian Traps
The flood basalt eruptions that produced the large igneous province of the Siberian Traps
were among the largest known volcanic events, extruding lava over
2,000,000 square kilometres (770,000 sq mi), roughly the size of Saudi
Arabia, producing a catastrophic impact.The date of the Siberian Traps eruptions matches well with the extinction event.
A study of the Norilsk and Maymecha-Kotuy regions of the northern
Siberian platform indicates that volcanic activity occurred during a few
enormous pulses of magma, as opposed to more regular flows.
The Siberian Traps caused one of the most rapid rises of atmospheric carbon dioxide levels in the geologic record, with the rate of carbon dioxide emissions estimated as five times faster than during the preceding catastrophic Capitanian mass extinction during the eruption of the Emeishan Traps. Overwhelming inorganic carbon sinks,
carbon dioxide levels might have jumped from between 500 and 4,000 ppm
prior to the extinction to around 8,000 ppm after, according to one
estimate.
Another study estimated pre-extinction carbon dioxide levels at 400
ppm, which then rose to 2,500 ppm, with 3,900 to 12,000 gigatonnes of
carbon added to the ocean-atmosphere system. Extreme temperature rise would have followed,
though some evidence suggests a lag of 12,000 to 128,000 years between
the rise in volcanic carbon dioxide emissions and global warming. Although this discrepancy could be also attributed to a incorrect biochronology. During the latest Permian before the extinction, global average surface temperatures were about 18.2 °C, which shot up to as much as 35 °C, this hyperthermal condition lasting as long as 500,000 years. Air temperatures at Gondwana's high southern latitudes experienced a warming of ~10–14 °C.
According to oxygen isotope shifts from conodont apatite in South
China, low latitude surface water temperatures surged about 8 °C.
In present-day Iran, tropical sea surface temperatures were between 27
and 33 °C during the Changhsingian but jumped to over 35 °C during the
PTME. The increased mean state temperatures also brought stronger El Nino events, heightening short-term climate variability.
These extremely high atmospheric carbon dioxide concentrations persisted over a long period. The position and alignment of Pangaea at the time made the inorganic carbon cycle very inefficient at burying carbon. In a 2020 paper, scientists reconstructed the mechanisms that led to the extinction event in a biogeochemical model, showed the consequences of the greenhouse effect on the marine environment, and concluded that the mass extinction can be traced back to volcanic CO2 emissions. Evidence also points to volcanic combustion of underground fossil fuel deposits, based on paired coronene-mercury spikes coinciding with geographically widespread mercury anomalies and the rise in isotopically light carbon. Te/Th values increase twentyfold over the PTME, further indicating it was concomitant with extreme volcanism.
A major volcanogenic influx of isotopically light zinc from the
Siberian Traps has also been recorded, further confirming that volcanism
was contemporary with the PTME.
The Siberian Traps eruptions had unusual features that made them even more dangerous. The Siberian lithosphere is rich in halogens
extremely destructive to the ozone layer, and evidence from
subcontinental lithospheric xenoliths indicates that as much as 70% of
their halogen content was released into the atmosphere. Around 18 teratonnes of hydrochloric acid were emitted, along with sulphur-rich volatiles that caused dust clouds and acid aerosols, which would have blocked out sunlight and disrupted photosynthesis on land and in the photic zone
of the ocean, causing food chains to collapse. These volcanic outbursts
of sulphur also induced brief but severe global cooling punctuating the
broader trend of rapid global warming, with glacio-eustatic sea level fall. However, the briefness of these cold events makes them unlikely to have been a significant kill mechanism.
The eruptions may also have caused acid rain as the aerosols washed out of the atmosphere. That may have killed land plants and mollusks and planktonic
organisms with calcium carbonate shells. Pure flood basalts produce
fluid, low-viscosity lava, and do not hurl debris into the atmosphere.
It appears, however, that 20% of the output of the Siberian Traps
eruptions was pyroclastic ash thrown high into the atmosphere, increasing the short-term cooling effect.
When this had washed out of the atmosphere, the excess carbon dioxide
would have remained and global warming would have proceeded unchecked.
Burning of hydrocarbon deposits may have exacerbated the
extinction. The Siberian Traps are underlain by thick sequences of
Early-Mid Paleozoic aged carbonate and evaporite deposits, as well as Carboniferous-Permian aged coal bearing clastic rocks. When heated, such as by igneous intrusions, these rocks may emit large amounts of greenhouse and toxic gases. The unique setting of the Siberian Traps over these deposits is likely the reason for the severity of the extinction. The basalt lava erupted or intruded into carbonate
rocks and sediments in the process of forming large coal beds, which
would have emitted large amounts of carbon dioxide, leading to stronger
global warming after the dust and aerosols settled.
The change of the eruptions from flood basalt to sill dominated
emplacement, liberating even more trapped hydrocarbon deposits,
coincides with the main onset of the extinction and is linked to a major negative δ13C excursion. The intermediate temperature of the Siberian Traps magmas optimised the extremely voluminous release of CO2 by way of heating of evaporites and carbonates.
Venting of coal-derived methane was accompanied by explosive combustion of coal and discharge of coal-fly ash.
A 2011 study led by Stephen E. Grasby reported evidence that volcanism
caused massive coal beds to ignite, possibly releasing more than
3 trillion tons of carbon. They found ash deposits in deep rock layers
near what is now the Buchanan Lake Formation:
"coal ash dispersed by the explosive Siberian Trap eruption would be
expected to have an associated release of toxic elements in impacted
water bodies where fly ash
slurries developed. ... Mafic megascale eruptions are long-lived events
that would allow significant build-up of global ash clouds."
Grasby said, "In addition to these volcanoes causing fires through
coal, the ash it spewed was highly toxic and was released in the land
and water, potentially contributing to the worst extinction event in
earth history."
However, some researchers propose that these supposed fly ashes were
actually the result of wildfires not related to massive coal combustion
by intrusive magmatism.
A 2013 study led by Q.Y. Yang reported that the total amounts of
important volatiles emitted from the Siberian Traps consisted of 8.5 ×
107 Tg CO2, 4.4 × 106 Tg CO, 7.0 × 106 Tg H2S, and 6.8 × 107 Tg SO2.
The sill-dominated emplacement of the Siberian Traps prolonged
their warming effects; whereas extrusive volcanism generates an
abundance of subaerial basalts that efficiently sequester carbon dioxide
via the silicate weathering process, underground sills cannot sequester atmospheric carbon dioxide and mitigate global warming.
Additionally, enhanced reverse weathering and depletion of siliceous
carbon sinks enabled extreme warmth to persist for much longer than
expected if the excess carbon dioxide was sequestered by silicate rock.
The reduction in marine primary productivity diminished emissions of dimethyl sulphate and dimethylsulphoniopropionate, enhancing warming.
Also, the decline in biological silicate deposition resulting from the
mass extinction of siliceous organisms acted as a positive feedback loop
wherein mass death of marine life exacerbated and prolonged extreme
hothouse conditions by depleting yet another siliceous carbon sink.
Mercury anomalies corresponding to the time of Siberian Traps activity have been found in many geographically disparate sites, indicating that these volcanic eruptions released significant quantities of toxic mercury into the atmosphere and ocean, causing even larger terrestrial and marine die-offs.
A series of surges raised terrestrial and marine environmental mercury
concentrations by orders of magnitude above normal background levels and
caused mercury poisoning over periods of a thousand years each.Mutagenesis in surviving plants after the PTME coeval with mercury and copper loading confirms volcanically induced heavy metal toxicity. Increased bioproductivity may have sequestered mercury and party mitigated poisoning. Immense volumes of nickel aerosols and cobalt and arsenic emisions, were also released, further contributing to metal poisoning.
The devastation wrought by the Siberian Traps did not end
following the Permian-Triassic boundary. Carbon isotope fluctuations
suggest that massive Siberian Traps activity recurred many times during
the Early Triassic, a finding corroborated by mercury spikes, causing further extinction events during the epoch.
Choiyoi Silicic Large Igneous Province
A
second flood basalt event that produced the Choiyoi Silicic Large
Igneous Province in southwestern Gondwana between around 286 Ma and 247
Ma has also been suggested as a significant additional extinction
mechanism. At 1,300,000 cubic kilometres in volume and 1,680,000 square kilometres in area, this event was 40% the size of the Siberian Traps. Specifically, this flood basalt has been implicated in the regional demise of the Gondwanan Glossopteris flora.
Indochina–South China subduction-zone volcanic arc
Mercury
anomalies preceding the end-Permian extinction have been discovered in
what was then the boundary between the South China craton and the
Indochinese plate, a subduction zone with a volcanic arc. Hafnium
isotopes from syndepositional magmatic zircons found in ash beds created
by this volcanic pulse confirm its origin in subduction-zone volcanism
rather than large igneous province activity. The enrichment of copper samples from these deposits in isotopically light copper provide additional confirmation.
This volcanism has been speculated to have caused local biotic stress
among radiolarians, sponges, and brachiopods over the 60,000 years
preceding the end-Permian marine extinction, as well as an ammonoid
crisis with decreased morphological complexity and size and increased
rate of turnover beginning in the lower C. yini biozone, around 200,000 years before the extinction.
Methane clathrates,
also known as methane hydrates, consist of molecules of methane trapped
in the crystal lattice of ice. This methane, produced by methanogen microbes, has a 13C ⁄12C isotope ratio about 6% below normal (δ13C −6.0%). At the right combination of pressure and temperature, clathrates form near the surface of permafrost and in large quantities on continental shelves and nearby seabed at water depths of at least 300 m (980 ft), buried in sediments up to 2,000 m (6,600 ft) below the sea floor.
Massive release of methane from these clathrates may have
contributed to the PTME, as scientists have found worldwide evidence of a
swift decrease of about 1% in the13C ⁄12C ratio in carbonate rocks from the end-Permian.
This is the first, largest, and fastest of a series of excursions
(decreases and increases) in the ratio, until it abruptly stabilised in
the middle Triassic, followed soon afterwards by the recovery of
calcifying shelled sealife. The seabed probably contained methane hydrate deposits, and the lava caused the deposits to dissociate, releasing vast quantities of methane.
A vast release of methane might cause significant global warming since methane is a very powerful greenhouse gas.
Strong evidence suggests the global temperatures increased by about
6 °C (10.8 °F) near the equator and therefore by more at higher
latitudes: a sharp decrease in oxygen isotope ratios (18O ⁄16O); the extinction of Glossopteris flora (Glossopteris and plants that grew in the same areas), which needed a cold climate, with its replacement by floras typical of lower paleolatitudes. It was also suggested that a large-scale release of methane and other greenhouse gases from the ocean into the atmosphere was connected to the anoxic events and euxinic (sulfidic) events at the time, with the exact mechanism compared to the 1986 Lake Nyos disaster.
The clathrate hypothesis seemed the only proposed mechanism sufficient to cause a global 1% reduction in the 13C ⁄12C ratio .
While a variety of factors may have contributed to the ratio drop, a
2002 review found most of them insufficient to account for the observed
amount:
Gases from volcanic eruptions have a13C ⁄12C ratio about 0.5 to 0.8% below standard (δ13C
−0.5 to −0.8%), but a 1995 assessment concluded that the observed 1.0%
worldwide reduction would have required eruptions massively larger than
any found. (However, this analysis addressed only CO2 produced by the magma itself, not from interactions with carbon bearing sediments, as described below.)
A reduction in organic activity would extract 12C more slowly from the environment and leave more of it to be incorporated into sediments, thus reducing the13C ⁄12C ratio. Biochemical
processes preferentially use the lighter isotopes since chemical
reactions are ultimately driven by electromagnetic forces between atoms
and lighter isotopes respond more quickly to these forces, but a study
of a smaller drop of 0.3 to 0.4% in 13C ⁄12C (δ13C −3 to −4 ‰) at the Paleocene-Eocene Thermal Maximum (PETM) concluded that even transferring all the organic carbon
(in organisms, soils, and dissolved in the ocean) into sediments would
be insufficient: Even such a large burial of material rich in 12C would not have produced the 'smaller' drop in the 13C ⁄12C ratio of the rocks around the PETM.
Buried sedimentary organic matter has a 13C ⁄12C ratio 2.0 to 2.5% below normal (δ13C −2.0 to −2.5%). Theoretically, if the sea level fell sharply, shallow marine sediments would be exposed to oxidation. But 6,500–8,400 gigatonnes (1 gigatonne = 1012
kg) of organic carbon would have to be oxidized and returned to the
ocean-atmosphere system within less than a few hundred thousand years to
reduce the 13C ⁄12C ratio by 1.0%, which is not thought to be a realistic possibility. Moreover, sea levels were rising rather than falling at the time of the extinction.
Rather than a sudden decline in sea level, intermittent periods of ocean-bottom hyperoxia and anoxia (high-oxygen and low- or zero-oxygen conditions) may have caused the 13C ⁄12C ratio fluctuations in the Early Triassic;
and global anoxia may have been responsible for the end-Permian blip.
The continents of the end-Permian and early Triassic were more clustered
in the tropics than they are now, and large tropical rivers would have
dumped sediment into smaller, partially enclosed ocean basins at low
latitudes. Such conditions favor oxic and anoxic episodes; oxic/anoxic
conditions would result in a rapid release/burial, respectively, of
large amounts of organic carbon, which has a low 13C ⁄12C ratio because biochemical processes use the lighter isotopes more.
That or another organic-based reason may have been responsible for both
that and a late Proterozoic/Cambrian pattern of fluctuating 13C ⁄12C ratios.
However, the clathrate hypothesis has also been criticized.
Carbon-cycle models which include consideration of roasting carbonate
sediments by volcanism confirm that it would have had enough effect to
produce the observed reduction.
Also, the pattern of isotope shifts expected to result from a massive
release of methane does not match the patterns seen throughout the Early
Triassic. Not only would such a cause require the release of five times
as much methane as postulated for the PETM, but would it also have to
be reburied at an unrealistically high rate to account for the rapid
increases in the 13C ⁄12C ratio (episodes of high positive δ13C) throughout the early Triassic before it was released several times again. The latest research suggests that greenhouse gas release during the extinction event was dominated by volcanic carbon dioxide,
and while methane release had to have contributed, isotopic signatures
show that thermogenic methane released from the Siberian Traps had
consistently played a larger role than methane from clathrates and any
other biogenic sources such as wetlands during the event.
Adding to the evidence against methane clathrate release as the
central driver of warming, the main rapid warming event is also
associated with marine transgression rather than regression; the former
would not normally have initiated methane release, which would have
instead required a decrease in pressure, something that would be
generated by a retreat of shallow seas.
The configuration of the world's landmasses into one supercontinent
would also mean that the global gas hydrate reservoir was lower than
today, further damaging the case for methane clathrate dissolution as a
major cause of the carbon cycle disruption.
Hypercapnia and acidification
Marine organisms are more sensitive to changes in CO2 (carbon dioxide) levels than terrestrial organisms for a variety of reasons. CO2 is 28 times more soluble in water than is oxygen. Marine animals normally function with lower concentrations of CO2 in their bodies than land animals, as the removal of CO2 in air-breathing animals is impeded by the need for the gas to pass through the respiratory system's membranes (lungs' alveolus, tracheae, and the like), even when CO2 diffuses more easily than oxygen. In marine organisms, relatively modest but sustained increases in CO2 concentrations hamper the synthesis of proteins, reduce fertilization rates, and produce deformities in calcareous hard parts. Higher concentrations of CO2 also result in decreased activity levels in many active marine animals, hindering their ability to obtain food. An analysis of marine fossils from the Permian's final Changhsingian stage found that marine organisms with a low tolerance for hypercapnia
(high concentration of carbon dioxide) had high extinction rates, and
the most tolerant organisms had very slight losses. The most vulnerable
marine organisms were those that produced calcareous hard parts (from
calcium carbonate) and had low metabolic rates and weak respiratory systems, notably calcareous sponges, rugose and tabulate corals, calcite-depositing brachiopods, bryozoans, and echinoderms; about 81% of such genera became extinct. Close relatives without calcareous hard parts suffered only minor losses, such as sea anemones,
from which modern corals evolved. Animals with high metabolic rates,
well-developed respiratory systems, and non-calcareous hard parts had
negligible losses except for conodonts, in which 33% of genera died out. This pattern is also consistent with what is known about the effects of hypoxia, a shortage but not total absence of oxygen. However, hypoxia cannot have been the only killing mechanism for marine organisms. Nearly all of the continental shelf
waters would have had to become severely hypoxic to account for the
magnitude of the extinction, but such a catastrophe would make it
difficult to explain the very selective pattern of the extinction. Mathematical models
of the Late Permian and Early Triassic atmospheres show a significant
but protracted decline in atmospheric oxygen levels, with no
acceleration near the P–Tr boundary. Minimum atmospheric oxygen levels
in the Early Triassic are never less than present-day levels and so the
decline in oxygen levels does not match the temporal pattern of the
extinction.
In addition, an increase in CO2 concentration is inevitably linked to ocean acidification, consistent with the preferential extinction of heavily calcified taxa and other signals in the rock record that suggest a more acidic ocean, such as a carbonate production crisis that occurred a few thousand years after volcanic greenhouse gas emissions began. The decrease in ocean pH is calculated to be up to 0.7 units. An extreme aragonite sea formed.
Ocean acidification was most extreme at mid-latitudes, and the major
marine transgression associated with the end-Permian extinction is
believed to have devastated shallow shelf communities in conjunction
with anoxia. Evidence from paralic facies spanning the Permian-Triassic boundary in western Guizhou and eastern Yunnan, however, shows a local marine transgression
dominated by carbonate deposition, suggesting that ocean acidification
did not occur across the entire globe and was likely limited to certain
regions of the world's oceans. One study, published in Scientific Reports,
concluded that widespread ocean acidification, if it did occur, was not
intense enough to impede calcification and only occurred during the
beginning of the extinction event.
The relative success of many marine organisms that were very vulnerable
to acidification has further been used to argue that acidification was
not a major extinction contributor.
The persistence of highly elevated carbon dioxide concentrations in the
atmosphere and ocean during the Early Triassic would have impeded the
recovery of biocalcifying organisms after the PTME.
Acidity generated by increased carbon dioxide concentrations in
soil and sulphur dioxide dissolution in rainwater was also a kill
mechanism on land.
The increasing acidification of rainwater caused increased soil erosion
as a result of the increased acidity of forest soils, evidenced by the
increased influx of terrestrially derived organic sediments found in
marine sedimentary deposits during the end-Permian extinction. Further evidence of an increase in soil acidity comes from elevated Ba/Sr ratios in earliest Triassic soils. A positive feedback loop further enhancing and prolonging soil acidification
may have resulted from the decline of infaunal invertebrates like
tubificids and chironomids, which remove acid metabolites from the soil.
The increased abundance of vermiculitic clays in Shansi, South China
coinciding with the Permian-Triassic boundary strongly suggests a sharp
drop in soil pH causally related to volcanogenic emissions of carbon
dioxide and sulphur dioxide. Hopane anomalies have also been interpreted as evidence of acidic soils and peats.
As with many other environmental stressors, acidity on land
episodically persisted well into the Triassic, stunting the recovery of
terrestrial ecosystems.
Evidence for widespread ocean anoxia (severe deficiency of oxygen) and euxinia (presence of hydrogen sulfide) is found from the Late Permian to the Early Triassic.Throughout most of the Tethys and Panthalassic Oceans, evidence for anoxia appears at the extinction event, including small pyrite framboids, negative δ238U excursions, negative δ15N excursions, positive δ82/78Se isotope excursions, relatively positive δ13C ratios in polycyclic aromatic hydrocarbons, high Th/U ratios, positive Ce/Ce* anomalies, depletions of molybdenum, uranium, and vanadium from seawater, and fine laminations in sediments. However, evidence for anoxia precedes the extinction at some other sites, including Spiti, India, Shangsi, China, Meishan, China, Opal Creek, Alberta, and Kap Stosch, Greenland. Biogeochemical evidence also points to the presence of euxinia during the PTME. Biomarkers for green sulfur bacteria, such as isorenieratane, the diagenetic product of isorenieratene, are widely used as indicators of photic zone euxinia because green sulfur bacteria
require both sunlight and hydrogen sulfide to survive. Their abundance
in sediments from the P–T boundary indicates euxinic conditions were
present even in the shallow waters of the photic zone. Negative mercury isotope excursions further bolster evidence for extensive euxinia during the PTME.
The disproportionate extinction of high-latitude marine species
provides further evidence for oxygen depletion as a killing mechanism;
low-latitude species living in warmer, less oxygenated waters are
naturally better adapted to lower levels of oxygen and are able to
migrate to higher latitudes during periods of global warming, whereas
high-latitude organisms are unable to escape from warming, hypoxic
waters at the poles.
Evidence of a lag between volcanic mercury inputs and biotic turnovers
provides further support for anoxia and euxinia as the key killing
mechanism, because extinctions would be expected to be synchronous with
volcanic mercury discharge if volcanism and hypercapnia was the primary
driver of extinction.
The sequence of extinctions in some sections, with deep water organisms
being affected first followed by shallow water and then by bottom water
organisms, is believed to reflect the migration of oxygen minimum
zones. Models of ocean chemistry suggest that anoxia and euxinia were closely associated with hypercapnia. This suggests that poisoning from hydrogen sulfide,
anoxia, and hypercapnia acted together as a killing mechanism.
Hypercapnia best explains the selectivity of the extinction, but anoxia
and euxinia probably contributed to the high mortality of the event.
The sequence of events leading to anoxic oceans may have been
triggered by carbon dioxide emissions from the eruption of the Siberian
Traps.
In that scenario, warming from the enhanced greenhouse effect would
reduce the solubility of oxygen in seawater, causing the concentration
of oxygen to decline. Increased coastal evaporation would have caused
the formation of warm saline bottom water (WSBW) depleted in oxygen and
nutrients, which spread across the world through the deep oceans. The
influx of WSBW caused thermal expansion of water that raised sea levels,
bringing anoxic waters onto shallow shelfs and enhancing the formation
of WSBW in a positive feedback loop.
The flux of terrigeneous material into the oceans increased as a result
of soil erosion, which would have facilitated increased eutrophication; marine regression likewise enhanced terrigeneous material inputs. Increased chemical weathering of the continents due to warming and the acceleration of the water cycle would increase the riverine flux of nutrients to the ocean.
Additionally, the Siberian Traps directly fertilised the oceans with
iron and phosphorus as well, triggering bioblooms and marine snowstorms.
Increased phosphate levels would have supported greater primary productivity in the surface oceans.
The increase in organic matter production would have caused more
organic matter to sink into the deep ocean, where its respiration would
further decrease oxygen concentrations. Once anoxia became established,
it would have been sustained by a positive feedback loop because deep water anoxia tends to increase the recycling efficiency of phosphate, leading to even higher productivity.
Along the Panthalassan coast of South China, oxygen decline was also
driven by large-scale upwelling of deep water enriched in various
nutrients, causing this region of the ocean to be hit by especially
severe anoxia. Convective overturn helped facilitate the expansion of anoxia throughout the water column. A severe anoxic event at the end of the Permian would have allowed sulfate-reducing bacteria to thrive, causing the production of large amounts of hydrogen sulfide in the anoxic ocean, turning it euxinic. In some regions, anoxia briefly disappeared when transient cold snaps resulting from volcanic sulphur emissions occurred.
The persistence of anoxia through the Early Triassic may explain
the slow recovery of marine life and low levels of biodiversity after
the extinction, particularly that of benthic organisms. Anoxia disappeared from shallow waters more rapidly than the deep ocean.
Reexpansions of oxygen-minimum zones did not cease until the late
Spathian, periodically setting back and restarting the biotic recovery
process.
The decline in continental weathering towards the end of the Spathian
at last began ameliorating marine life from recurrent anoxia. In some regions of Panthalassa, pelagic zone anoxia continued to recur as late as the Anisian, probably due to increased productivity and a return of aeolian upwelling.
Some sections show a rather quick return to oxic water column
conditions, however, so for how long anoxia persisted remains debated. The volatility of the Early Triassic sulphur cycle suggests marine life continued to face returns of euxinia as well.
Some scientists have challenged the anoxia hypothesis on the
grounds that long-lasting anoxic conditions could not have been
supported if Late Permian thermohaline ocean circulation conformed to
the "thermal mode" characterised by cooling at high latitudes. Anoxia
may have persisted under a "haline mode" in which circulation was driven
by subtropical evaporation, although the "haline mode" is highly
unstable and was unlikely to have represented Late Permian oceanic
circulation.
Oxygen depletion via extensive microbial blooms also played a
role in the biological collapse of not just marine ecosystems but
freshwater ones as well. Persistent lack of oxygen after the extinction
event itself helped delay biotic recovery for much of the Early Triassic
epoch.
Aridification
Increasing
continental aridity, a trend well underway even before the PTME as a
result of the coalescence of the supercontinent Pangaea, was drastically
exacerbated by terminal Permian volcanism and global warming. The combination of global warming and drying generated an increased incidence of wildfires.
Tropical coastal swamp floras such as those in South China may have
been very detrimentally impacted by the increase in wildfires, though it is ultimately unclear if an increase in wildfires played a role in driving taxa to extinction.
Aridification trends varied widely in their tempo and regional
impact. Analysis of the fossil river deposits of the floodplains of the
Karoo Basin indicate a shift from meandering to braided river patterns, indicating a very abrupt drying of the climate. The climate change may have taken as little as 100,000 years, prompting the extinction of the unique Glossopteris flora and its associated herbivores, followed by the carnivorous guild.
A pattern of aridity-induced extinctions that progressively ascended up
the food chain has been deduced from Karoo Basin biostratigraphy.
Evidence for aridification in the Karoo across the Permian-Triassic
boundary is not, however, universal, as some palaeosol evidence
indicates a wettening of the local climate during the transition between
the two geologic periods. Evidence from the Sydney Basin
of eastern Australia, on the other hand, suggests that the expansion of
semi-arid and arid climatic belts across Pangaea was not immediate but
was instead a gradual, prolonged process. Apart from the disappearance
of peatlands, there was little evidence of significant sedimentological changes in depositional style across the Permian-Triassic boundary.
Instead, a modest shift to amplified seasonality and hotter summers is
suggested by palaeoclimatological models based on weathering proxies
from the region's Late Permian and Early Triassic deposits. In the Kuznetsk Basin of southwestern Siberia, an increase in aridity led to the demise of the humid-adapted Cordaites
forests in the region a few hundred thousand years before the
Permian-Triassic boundary. Drying of this basin has been attributed to a
broader poleward shift of drier, more arid climates during the late
Changhsingian before the more abrupt main phase of the extinction at the
Permian-Triassic boundary that disproportionately affected tropical and
subtropical species.
The persistence of hyperaridity varied regionally as well. In the
North China Basin, highly arid climatic conditions are recorded during
the latest Permian, near the Permian-Triassic boundary, with a swing
towards increased precipitation during the Early Triassic, the latter
likely assisting biotic recovery following the mass extinction.
Elsewhere, such as in the Karoo Basin, episodes of dry climate recurred
regularly in the Early Triassic, with profound effects on terrestrial
tetrapods.
The types and diversity of ichnofossils in a locality has been
used as an indicator measuring aridity. Nurra, an ichnofossil site on
the island of Sardinia,
shows evidence of major drought-related stress among crustaceans.
Whereas the Permian subnetwork at Nurra displays extensive horizontal
backfilled traces and high ichnodiversity, the Early Triassic subnetwork
is characterised by an absence of backfilled traces, an ichnological
sign of aridification.
Ozone depletion
A collapse of the atmospheric ozone shield has been invoked as an explanation for the mass extinction, particularly that of terrestrial plants.
Ozone production may have been reduced by 60-70%, increasing the flux
of ultraviolet radiation by 400% at equatorial latitudes and 5,000% at
polar latitudes.
The hypothesis has the advantage of explaining the mass extinction of
plants, which would have added to the methane levels and should
otherwise have thrived in an atmosphere with a high level of carbon
dioxide. Fossil spores from the end-Permian further support the theory;
many spores show deformities that could have been caused by ultraviolet radiation, which would have been more intense after hydrogen sulfide emissions weakened the ozone layer.
Malformed plant spores from the time of the extinction event show an
increase in ultraviolet B absorbing compounds, confirming that increased
ultraviolet radiation played a role in the environmental catastrophe
and excluding other possible causes of mutagenesis, such as heavy metal
toxicity, in these mutated spores. Extremely positive Δ33S anomalies provide evidence of photolysis of volcanic SO2, indicating increased ultraviolet radiation flux.
Sulphur isotope data from North China are inconsistent with a total
collapse of the ozone layer, however, suggesting it may have not been as
major a kill mechanism as others.
Multiple mechanisms could have reduced the ozone shield and
rendered it ineffective. Computer modelling shows high atmospheric
methane levels are associated with ozone shield decline and may have
contributed to its reduction during the PTME. Volcanic emissions of sulphate aerosols into the stratosphere would have dealt significant destruction to the ozone layer. As mentioned previously, the rocks in the region where the Siberian Traps were emplaced are extremely rich in halogens. The intrusion of Siberian Traps volcanism into deposits rich in organohalogens, such as methyl bromide and methyl chloride, would have been another source of ozone destruction.
An uptick in wildfires, a natural source of methyl chloride, would have
had further deleterious effects still on the atmospheric ozone shield. Upwelling of euxinic water may have released massive hydrogen sulphide emissions into the atmosphere and would poison terrestrial plants and animals and severely weaken the ozone layer, exposing much of the life that remained to fatal levels of UV radiation, although other modelling work has found that the release of this gas would not have significantly damaged the ozone layer. Indeed, biomarker evidence for anaerobic photosynthesis by Chlorobiaceae
(green sulfur bacteria) from the Late-Permian into the Early Triassic
indicates that hydrogen sulphide did upwell into shallow waters because
these bacteria are restricted to the photic zone and use sulfide as an electron donor.
Asteroid impact
Artist's impression of a major impact event: A collision between Earth and an asteroid a few kilometers in diameter would release as much energy as the detonation of several million nuclear weapons.
Evidence that an impact event may have caused the Cretaceous–Paleogene extinction
has led to speculation that similar impacts may have been the cause of
other extinction events, including the P–Tr extinction, and thus to a
search for evidence of impacts at the times of other extinctions, such
as large impact craters of the appropriate age. However, suggestions that an asteroid impact was the trigger of the Permian-Triassic extinction are now largely rejected.
Reported evidence for an impact event from the P–Tr boundary level includes rare grains of shocked quartz in Australia and Antarctica; fullerenes trapping extraterrestrial noble gases; meteorite fragments in Antarctica; and grains rich in iron, nickel, and silicon, which may have been created by an impact. However, the accuracy of most of these claims has been challenged. For example, quartz from Graphite Peak
in Antarctica, once considered "shocked", has been re-examined by
optical and transmission electron microscopy. The observed features were
concluded to be due not to shock, but rather to plastic deformation, consistent with formation in a tectonic environment such as volcanism.
Iridium levels in many sites straddling the Permian-Triassic boundaries
are not anomalous, providing evidence against an extraterrestrial
impact as the PTME's cause.
An impact crater on the seafloor would be evidence of a possible
cause of the P–Tr extinction, but such a crater would by now have
disappeared. As 70% of the Earth's surface is currently sea, an asteroid or comet
fragment is now perhaps more than twice as likely to hit the ocean as
it is to hit land. However, Earth's oldest ocean-floor crust is only
200 million years old as it is continually being destroyed and renewed
by spreading and subduction. Furthermore, craters produced by very large impacts may be masked by extensive flood basalting from below after the crust is punctured or weakened.
Yet, subduction should not be entirely accepted as an explanation for
the lack of evidence: as with the K-T event, an ejecta blanket stratum
rich in siderophilic elements (such as iridium) would be expected in formations from the time.
A large impact might have triggered other mechanisms of extinction described above, such as the Siberian Traps eruptions at either an impact site or the antipode of an impact site. The abruptness of an impact also explains why more species did not rapidly evolve to survive, as would be expected if the Permian–Triassic event had been slower and less global than a meteorite impact.
Bolide impact claims have been criticised on the grounds that
they are unnecessary as explanations for the extinctions, and they do
not fit the known data compatible with a protracted extinction spanning
thousands of years. Additionally, many sites spanning the Permian-Triassic boundary display a complete lack of evidence of an impact event.
Possible impact sites
Possible impact craters proposed as the site of an impact causing the P–Tr extinction include the 250 km (160 mi) Bedout structure off the northwest coast of Australia and the hypothesized 480 km (300 mi) Wilkes Land crater of East Antarctica.
An impact has not been proved in either case, and the idea has been
widely criticized. The Wilkes Land geophysical feature is of very
uncertain age, possibly later than the Permian–Triassic extinction.
Another impact hypothesis postulates that the impact event which formed the Araguainha crater, whose formation has been dated to 254.7 ± 2.5 million, a possible temporal range overlapping with the end-Permian extinction, precipitated the mass extinction.
The impact occurred around extensive deposits of oil shale in the
shallow marine Paraná–Karoo Basin, whose perturbation by the seismicity
resulting from impact likely discharged about 1.6 teratonnes of methane
into Earth's atmosphere, buttressing the already rapid warming caused by
hydrocarbon release due to the Siberian Traps. The large earthquakes generated by the impact would have additionally generated massive tsunamis across much of the globe.
Despite this, most palaeontologists reject the impact as being a
significant driver of the extinction, citing the relatively low energy
(equivalent to 105 to 106 (tons?) of TNT, around two
orders of magnitude lower than the impact energy believed to be required
to induce mass extinctions) released by the impact.
A 2017 paper noted the discovery of a circular gravity anomaly near the Falkland Islands which might correspond to an impact crater with a diameter of 250 km (160 mi),
as supported by seismic and magnetic evidence. Estimates for the age of
the structure range up to 250 million years old. This would be
substantially larger than the well-known 180 km (110 mi) Chicxulub impact crater
associated with a later extinction. However, Dave McCarthy and
colleagues from the British Geological Survey illustrated that the
gravity anomaly is not circular and also that the seismic data presented
by Rocca, Rampino and Baez Presser did not cross the proposed crater or
provide any evidence for an impact crater.
Methanogens
A hypothesis published in 2014 posits that a genus of anaerobic methanogenic archaea known as Methanosarcina was responsible for the event. Three lines of evidence suggest that these microbes acquired a new metabolic pathway via gene transfer
at about that time, enabling them to efficiently metabolize acetate
into methane. That would have led to their exponential reproduction,
allowing them to rapidly consume vast deposits of organic carbon that
had accumulated in marine sediment. The result would have been a sharp
buildup of methane and carbon dioxide in the oceans and atmosphere, in a
manner that may be consistent with the 13C/12C isotopic record. Massive volcanism facilitated this process by releasing large amounts of nickel, a scarce metal which is a cofactor for enzymes involved in producing methane.
Chemostratigraphic analysis of Permian-Triassic boundary sediments in
Chaotian demonstrates a methanogenic burst could be responsible for some
percentage of the carbon isotopic fluctuations. On the other hand, in the canonical Meishan sections, the nickel concentration increases somewhat after the δ13C concentrations have begun to fall.
Interstellar dust
John Gribbin argues that the Solar System last passed through a spiral arm of the Milky Way around 250 million years ago and that the resultant dusty gas clouds may have caused a dimming of the Sun, which combined with the effect of Pangaea to produce an ice age.
Comparison to present global warming
The PTME has been compared to the current anthropogenic global warming situation and Holocene extinction
due to sharing the common characteristic of rapid rates of carbon
dioxide release. Though the current rate of greenhouse gas emissions is
more than an order of magnitude greater than the rate measured over the
course of the PTME, the discharge of greenhouse gases during the PTME is
poorly constrained geo-chronologically and was most likely pulsed and
constrained to a few key, short intervals, rather than continuously
occurring at a constant rate for the whole extinction interval; the rate
of carbon release within these intervals was likely to have been
similar in timing to modern anthropogenic emissions.
As they did during the PTME, oceans in the present day are experiencing
drops in pH and in oxygen levels, prompting further comparisons between
modern anthropogenic ecological conditions and the PTME.
Another biocalcification event similar in its effects on modern marine
ecosystems is predicted to occur if carbon dioxide levels continue to
rise.
The changes in plant-insect interactions resulting from the PTME have
also been invoked as possible indicators of the world's future ecology.
The similarities between the two extinction events have led to warnings
from geologists about the urgent need for reducing carbon dioxide
emissions if an event similar to the PTME is to be prevented from
occurring.
Just as during the PTME, contemporary oceans experience their extreme
change-change in the form of a decline in pH and oxygen levels, which
further strengthens the pull between the two events. This is emphasised
by geologistLee Kump:
“The
Permian-Triassic mass extinction provides a stark reminder of the
consequences of rapid carbon dioxide emissions. During the PTME,
volcanic activity unleashed massive amounts of CO₂, leading to ocean
acidification, deoxygenation, and widespread ecological collapse. Today,
we see human activities driving similar processes at an even faster
rate. The geological record shows that once these tipping points are
reached, the cascading effects on ecosystems can last for millions of
years.”
If
it continues to rise, the consequence could be another
bio-calcification crisis, as seems to have occurred in the fossil
record, which would have disastrous consequences for modern marine
ecosystems.
