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Saturday, October 24, 2015

Chimpanzee–human last common ancestor


From Wikipedia, the free encyclopedia

Chimpanzee–human last common ancestor
Temporal range: 5.4–0 Ma
O
S
D
C
P
T
J
K
N
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Primates
Infraorder: Simiiformes
Superfamily: Hominoidea
Family: Hominidae
Subfamily: Homininae
Type species
Homo sapiens
Linnaeus, 1758
Genera
Tribe Hominini
The chimpanzee–human last common ancestor, or CHLCA, is the last species shared as a common ancestor by humans and chimpanzees; it represents the node point at which the line to genus Homo split from genus Pan. The last common ancestor of humans and chimps is estimated to have lived during the late Miocene, but possibly as late as Pliocene times—that is, more recent than 5.3 million years ago.

Speciation from Pan to Homo appears to have been a long, drawn-out process. After the "original" divergence(s), there were, according to Patterson (2006), periods of hybridization between population groups and a process of alternating divergence and hybridization that lasted over several millions of years.[1] Sometime during the late Miocene or early Pliocene the earliest members of the human clade completed a final separation from the lineage of Pan—with dates estimated by several specialists ranging from 13 million [2] to as recent as 4 million years ago.[3] The latter date and the argument for hybridization events are rejected by Wakeley[4] (see current estimates regarding complex speciation).

Richard Wrangham (2001) argued that the CHLCA species was very similar to the common chimpanzee (Pan troglodytes)—so much so that it should be classified as a member of the Pan genus and be given the taxonomic name Pan prior.[5] However, to date no fossil has been identified as a probable candidate for the CHLCA or the taxon Pan prior.

In human genetic studies, the CHLCA is useful as an anchor point for calculating single-nucleotide polymorphism (SNP) rates in human populations where chimpanzees are used as an outgroup, that is, as the extant species most genetically similar to Homo sapiens.

Time estimates

Historical studies

The earliest studies[year needed] of apes suggested the CHLCA may have been as old as 25 million years; however, protein studies in the 1970s suggested the CHLCA was less than 8 million years in age. Genetic methods based on orangutan–human and gibbon–human LCA times were then used to estimate a chimpanzee–human LCA of 5 to 7 million years.

Some researchers tried to estimate the age of the CHLCA (TCHLCA) using biopolymer structures that differ slightly between closely related animals. Among these researchers, Allan C. Wilson and Vincent Sarich were pioneers in the development of the molecular clock for humans. Working on protein sequences they eventually (1971) determined that apes were closer to humans than some paleontologists perceived based on the fossil record. [7] Later,[year needed] Vincent Sarich concluded that the TCHLCA was no greater than 8 million years in age, with a favored range between 4 and 6 million years before present.

This paradigmatic age has stuck with molecular anthropology until the late 1990s. Since the 1990s, the estimate has again been pushed towards more-remote times, because studies have found evidence for a slowing of the molecular clock as apes evolved from a common monkey-like ancestor with monkeys and humans evolved from a common ape-like ancestor with non-human apes.[8]

Current estimates

Since the 1990s, the estimation of the TCHLCA has become less certain, and there is genetic as well as paleontological support for increasing TCHLCA beyond the 5 to 7 million years range accepted during the 1970s and 1980s. An estimate of TCHLCA at 10 to 13 million years was proposed in 1998,[9] and a range of 7 to 10 million years ago is assumed by White et a. (2009):
In effect, there is now no a priori reason to presume that human-chimpanzee split times are especially recent, and the fossil evidence is now fully compatible with older chimpanzee–human divergence dates [7 to 10 Ma...
— White et al. (2009), [10]
A source of confusion in determining the exact age of the PanHomo split is evidence of a more complex speciation process rather than a clean split between the two lineages. Different chromosomes appear to have split at different times, possibly over as much as a 4-million-year period, indicating a long and drawn out speciation process with large-scale hybridization events between the two emerging lineages as late as 6.3 to 5.4 million years ago according to Patterson et al. (2006).[11] The assumption of late hybridization was in particular based on the similarity of the X chromosome in humans and chimpanzees, suggesting a divergence as late as some 4 million years ago. This conclusion was rejected as unwarranted by Wakeley (2008), who suggested alternative explanations, including selection pressure on the X chromosome in the populations ancestral to the CHLCA.[12]

Complex speciation and incomplete lineage sorting of genetic sequences seem to also have happened in the split between the human lineage and that of the gorilla, indicating "messy" speciation is the rule rather than the exception in large primates.[13][14] Such a scenario would explain why the divergence age between the Homo and Pan has varied with the chosen method and why a single point has so far been hard to track down.

Taxonomy

The taxon 'tribe Hominini' was proposed on basis of the idea that, regarding a trichotomy, the least similar species should be separated from the other two. Originally, this produced a separated Homo genus, which, predictably, was deemed the 'most different' among the three genera that includes Pan and Gorilla. However, later discoveries and analyses revealed that Pan and Homo are closer genetically than are Pan and Gorilla; thus, Pan was referred to the tribe Hominini with HomoGorilla now became the separated genus and was referred to the new taxon 'tribe Gorillini' (see evolutionary tree here).

Mann and Weiss (1996), proposed that the tribe Hominini should encompass Pan as well as Homo, but grouped within separate subtribes.[15] They would classify Homo and all bipedal apes to the subtribe Hominina and Pan to the subtribe Panina. (Wood (2010) discusses the different views of this taxonomy.)[16] Richard Wrangham (2001) argued that the CHLCA species was very similar to chimpanzees (Pan troglodytes)—so much so that it should be classified as a member of the Pan genus and be given the taxonomic name Pan prior.[5] To date, no fossil has been identified as a potential candidate for the CHLCA or the taxon Pan prior.

The 'human-side' descendants of the CHLCA species are specified as members of the tribe Hominini, that is, to the inclusion of the genus Homo and its closely related genus Australopithecus, but to the exclusion of the genus Pan—meaning all those human-related genera of tribe Hominini that arose after speciation from the line with Pan. Such grouping represents "the human clade" and its members are called "hominins".[17] A "chimpanzee clade" was posited by Wood and Richard, who referred it to a 'Tribe Panini', which was envisioned from the family Hominidae being composed of a trifurcation of subfamilies.[18]

Sahelanthropus tchadensis is an extinct hominid species with a morphology apparently as expected of the CHLCA; and it lived some 7 million years ago—which is very close to the time of the chimpanzee–human divergence. But it is unclear whether it should be classified as a member of the Hominini tribe, that is, a hominin, or as a direct ancestor of Homo and Pan and a potential candidate for the CHLCA species itself.

Few fossil specimens on the 'chimpanzee-side' of the split have been found; the first fossil chimpanzee, dating between 545 and 284 kyr (thousand years, radiometric), was discovered in Kenya's East African Rift Valley (McBrearty, 2005).[19] All extinct genera listed in the taxobox are ancestral to Homo, or are offshoots of such. However, both Orrorin and Sahelanthropus existed around the time of the divergence, and so either one or both may be ancestral to both genera Homo and Pan.

Thursday, October 22, 2015

Why Is Venus So Hot?























Surface temperatures of Venus are around 500 degrees C, some 485 degrees hotter than Earth (15 degrees C).  Although Venus has for decades been cited as a severe example of the "runaway greenhouse effect"  [http://www.esa.int/Our_Activities/Space_Science/Venus_Express/Greenhouse_effect_clouds_and_winds] for one example, there are problems with this theory [http://theendofthemystery.blogspot.gr/2010/11/venus-no-greenhouse-effect.html],
[http://www.perthnow.com.au/news/opinion/miranda-devine-perth-electrical-engineers-discovery-will-change-climate-change-debate/news-story/d1fe0f22a737e8d67e75a5014d0519c6], and [http://science.nasa.gov/science-news/science-at-nasa/2012/22mar_saber/]. In fact, it appears that it is Venus' dense atmosphere which accounts for most of the temperature diffence [https://en.wikipedia.org/wiki/Atmosphere_of_Venus-venus-atmosphere.html].

I was interested in using the Arrhenius equation to calculate how much greenhouse warming did contribute to Venus' lead-melting surface temperature. This equation can be summarized as

delta T = k * ln(C/C0).

In this case, I will use C = concentration of CO2 in Venus' atmosphere, and C0 as the concentration in Eath's atmosphere, and k as approximately 2.  Thus

2 * ln(90/0.0025) = 21 degrees C, or only 4% of the temperature difference between Earth and Venus!  This, it is important to add, does not include greenhouse cooling at the top of the atmosphere by CO2 deflecting incoming infrared, an effect that might reduce greenhouse heating by 80% or more.

Incidentally, if you want to know how hot Earth would get if the entire atmosphere were pure CO2 (2500 times greater), the same equation yields about a 10 degree rise (overlooking cooling and other effects of such high CO2 levels).

I said that the density of the Venusian atmosphere is responsible for (most of) its temperature.  There is a simple method of demonstrating this, although under the conditions we will consider there are some serious caveats to this method.  If you have ever studied gases in high school or college, you may have seen the equation

PV = nRT,

otherwise known as the ideal gas equation, where PV is the pressure times the volume of the gas, and nRT is the number of mols of gas (6.203E23 = molecules/mol), R is the ideal gas constant, and T is the temperature (in kelvins).  This is where one caveat comes in:  in principle there is no such thing as an "ideal gas", but most gases under Earthlike conditions come close enough for the equation to provide a reasonable approximation.  On Venus, however, things are trickier.

You probably know that when you compress a gas, its temperature rises, and vice versa.  (Bear in mind, you can compress with a constant pressure, which means P does not change.)  Starting with Earth's atmosphere (for it is closest to an ideal gas), we rearrange the equation above equation to:

T  = PV/nR, where we treat P/nR as a single constant, called K.  Then T(e) = K*V(e), and T(v) = K*V(v).  Then, again rearranging, we have T(v)/T(e) = V(v)/V(e) (K's cancel).  If you reduce the volume V(e) to the same density of V(v) -- a factor of about 90 -- then T(v) = 90*T(e).

Here, we run into our caveat, because the density of the Venusian gas is 90 times that of Earth's, and the temperature is about 2.67 times that of our planet (not 90).  Gas at such high density and temperature behaves in a highly non-ideal fasion; in fact, at the surface of Venus the carbon dioxide isn't even a gas anymore, but what's known as a "super critical" fluid, which absorbs much more heat than regular gases under Earth-like conditions.

Another factor comes into play here.  All gases, especially gases at high densities, are excellect heat insultors, due to conduction and convection.  I can think of no better example than our sun (or any star), in which a photon generated at the core takes thousands (or is it millions?) of years to reach the top of the atmosphere (the photosphere, the part we see) before zipping off into space at the speed of light.  Given that stars are almost entirely hydrogen and helium, non-greenhouse gases, that is clearly not the reason for this phenomenon.

Wednesday, October 21, 2015

Carboniferous


From Wikipedia, the free encyclopedia

Carboniferous Period
358.9–298.9 million years ago

Mean atmospheric O
2
content over period duration
ca. 32.5 vol %[1]
(163 % of modern level)
Mean atmospheric CO
2
content over period duration
ca. 800 ppm[2]
(3 times pre-industrial level)
Mean surface temperature over period duration ca. 14 °C[3]
(0 °C above modern level)
Sea level (above present day) Falling from 120 m to present day level throughout Mississippian, then rising steadily to about 80 m at end of period[4]

The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, at 358.9 ± 0.4 million years ago, to the beginning of the Permian Period, at 298.9 ± 0.15 Ma. The name Carboniferous means "coal-bearing" and derives from the Latin words carbō (“coal”) and ferō (“I bear, I carry”), and was coined by geologists William Conybeare and William Phillips in 1822.[5] Based on a study of the British rock succession, it was the first of the modern 'system' names to be employed, and reflects the fact that many coal beds were formed globally during this time.[6] The Carboniferous is often treated in North America as two geological periods, the earlier Mississippian and the later Pennsylvanian.[7]

Terrestrial life was well established by the Carboniferous period.[8] Amphibians were the dominant land vertebrates, of which one branch would eventually evolve into reptiles, the first fully terrestrial vertebrates. Arthropods were also very common, and many (such as Meganeura), were much larger than those of today. Vast swaths of forest covered the land, which would eventually be laid down and become the coal beds characteristic of the Carboniferous system. The atmospheric content of oxygen also reached their highest levels in history during the period, 35%[9] compared with 21% today. This increased the atmospheric density by a third over today’s value.[9] A minor marine and terrestrial extinction event occurred in the middle of the period, caused by a change in climate.[10] The later half of the period experienced glaciations, low sea level, and mountain building as the continents collided to form Pangaea.

Subdivisions

In the United States the Carboniferous is usually broken into Mississippian (earlier) and Pennsylvanian (later) subperiods. The Mississippian is about twice as long as the Pennsylvanian, but due to the large thickness of coal bearing deposits with Pennsylvanian ages in Europe and North America, the two subperiods were long thought to have been more or less equal in duration.[11] In Europe the Lower Carboniferous sub-system is known as the Dinantian, comprising the Tournaisian and Visean Series, dated at 362.5-332.9 Ma, and the Upper Carboniferous sub-system is known as the Silesian, comprising the Namurian, Westphalian, and Stephanian Series, dated at 332.9-290 Ma. The Silesian is roughly contemporaneous with the late Mississippian Serpukhovian plus the Pennsylvanian. In Britain the Dinantian is traditionally known as the Carboniferous Limestone, the Namurian as the Millstone Grit, and the Westphalian as the Coal Measures and Pennant Sandstone.
The faunal stages from youngest to oldest, together with some of their subdivisions, are:

Late Pennsylvanian: Gzhelian (most recent)
  • Noginskian / Virgilian (part)
Late Pennsylvanian: Kasimovian
  • Klazminskian
  • Dorogomilovksian / Virgilian (part)
  • Chamovnicheskian / Cantabrian / Missourian
  • Krevyakinskian / Cantabrian / Missourian
Middle Pennsylvanian: Moscovian
  • Myachkovskian / Bolsovian / Desmoinesian
  • Podolskian / Desmoinesian
  • Kashirskian / Atokan
  • Vereiskian / Bolsovian / Atokan
Early Pennsylvanian: Bashkirian / Morrowan
  • Melekesskian / Duckmantian
  • Cheremshanskian / Langsettian
  • Yeadonian
  • Marsdenian
  • Kinderscoutian
Late Mississippian: Serpukhovian
  • Alportian
  • Chokierian / Chesterian / Elvirian
  • Arnsbergian / Elvirian
  • Pendleian
Middle Mississippian: Visean
  • Brigantian / St Genevieve / Gasperian / Chesterian
  • Asbian / Meramecian
  • Holkerian / Salem
  • Arundian / Warsaw / Meramecian
  • Chadian / Keokuk / Osagean (part) / Osage (part)
Early Mississippian: Tournaisian (oldest)
  • Ivorian / (part) / Osage (part)
  • Hastarian / Kinderhookian / Chouteau

Paleogeography

A global drop in sea level at the end of the Devonian reversed early in the Carboniferous; this created the widespread epicontinental seas and carbonate deposition of the Mississippian.[12] There was also a drop in south polar temperatures; southern Gondwanaland was glaciated throughout the period, though it is uncertain if the ice sheets were a holdover from the Devonian or not.[12] These conditions apparently had little effect in the deep tropics, where lush coal swamps flourished within 30 degrees of the northernmost glaciers.[12]

Generalized geographic map of the United States in Middle Pennsylvanian time.

A mid-Carboniferous drop in sea level precipitated a major marine extinction, one that hit crinoids and ammonites especially hard.[12] This sea level drop and the associated unconformity in North America separate the Mississippian subperiod from the Pennsylvanian subperiod.[12] This happened about 318 million years ago, at the onset of the Permo-Carboniferous Glaciation.[citation needed]

The Carboniferous was a time of active mountain-building, as the supercontinent Pangaea came together. The southern continents remained tied together in the supercontinent Gondwana, which collided with North America–Europe (Laurussia) along the present line of eastern North America. This continental collision resulted in the Hercynian orogeny in Europe, and the Alleghenian orogeny in North America; it also extended the newly uplifted Appalachians southwestward as the Ouachita Mountains.[12] In the same time frame, much of present eastern Eurasian plate welded itself to Europe along the line of the Ural mountains. Most of the Mesozoic supercontinent of Pangea was now assembled, although North China (which would collide in the Latest Carboniferous), and South China continents were still separated from Laurasia. The Late Carboniferous Pangaea was shaped like an "O."

There were two major oceans in the Carboniferous—Panthalassa and Paleo-Tethys, which was inside the "O" in the Carboniferous Pangaea. Other minor oceans were shrinking and eventually closed - Rheic Ocean (closed by the assembly of South and North America), the small, shallow Ural Ocean (which was closed by the collision of Baltica and Siberia continents, creating the Ural Mountains) and Proto-Tethys Ocean (closed by North China collision with Siberia/Kazakhstania).

Climate and weather

Average global temperatures in the Early Carboniferous Period were high: approximately 20 °C (68 °F). However, cooling during the Middle Carboniferous reduced average global temperatures to about 12 °C (54 °F). Glaciations in Gondwana, triggered by Gondwana's southward movement, continued into the Permian and because of the lack of clear markers and breaks, the deposits of this glacial period are often referred to as Permo-Carboniferous in age.

The thicker atmosphere and stronger coriolis effect due to Earth's faster rotation (a day lasted for 22.4 hours in early Carboniferous) created significantly stronger winds than today.[13]

The cooling and drying of the climate led to the Carboniferous Rainforest Collapse (CRC). Tropical rainforests fragmented and then were eventually devastated by climate change.[10][14]

Rocks and coal


Lower Carboniferous marble in Big Cottonwood Canyon, Wasatch Mountains, Utah.

Carboniferous rocks in Europe and eastern North America largely consist of a repeated sequence of limestone, sandstone, shale and coal beds.[15] In North America, the early Carboniferous is largely marine limestone, which accounts for the division of the Carboniferous into two periods in North American schemes. The Carboniferous coal beds provided much of the fuel for power generation during the Industrial Revolution and are still of great economic importance.

The large coal deposits of the Carboniferous may owe their existence primarily to two factors. The first of these is the appearance of wood tissue and bark-bearing trees. The evolution of the wood fiber lignin and the bark-sealing, waxy substance suberin variously opposed decay organisms so effectively that dead materials accumulated long enough to fossilise on a large scale. The second factor was the lower sea levels that occurred during the Carboniferous as compared to the preceding Devonian period. This promoted the development of extensive lowland swamps and forests in North America and Europe. Based on a genetic analysis of mushroom fungi, David Hibbett and colleagues proposed that large quantities of wood were buried during this period because animals and decomposing bacteria had not yet evolved enzymes that could effectively digest the resistant phenolic lignin polymers and waxy suberin polymers. They suggest that fungi that could break those substances down effectively only became dominant towards the end of the period, making subsequent coal formation much rarer.[16][17][18]

The Carboniferous trees made extensive use of lignin. They had bark to wood ratios of 8 to 1, and even as high as 20 to 1. This compares to modern values less than 1 to 4. This bark, which must have been used as support as well as protection, probably had 38% to 58% lignin. Lignin is insoluble, too large to pass through cell walls, too heterogeneous for specific enzymes, and toxic, so that few organisms other than Basidiomycetes fungi can degrade it. To oxidize it requires an atmosphere of greater than 5% oxygen, or compounds such as peroxides. It can linger in soil for thousands of years and its toxic breakdown products inhibit decay of other substances.[19] Probably the reason for its high percentages is protection from insect herbivory in a world containing very effective insect herbivores, but nothing remotely as effective as modern insectivores and probably many fewer poisons than currently. In any case coal measures could easily have made thick deposits on well drained soils as well as swamps. The extensive burial of biologically produced carbon led to an increase in oxygen levels in the atmosphere; estimates place the peak oxygen content as high as 35%, compared to 21% today.[20] This oxygen level may have increased wildfire activity. It also may have promoted gigantism of insects and amphibians — creatures that have been constrained in size by respiratory systems that are limited in their physiological ability to transport and distribute oxygen at the lower atmospheric concentrations that have since been available.[21]

In eastern North America, marine beds are more common in the older part of the period than the later part and are almost entirely absent by the late Carboniferous. More diverse geology existed elsewhere, of course. Marine life is especially rich in crinoids and other echinoderms. Brachiopods were abundant. Trilobites became quite uncommon. On land, large and diverse plant populations existed. Land vertebrates included large amphibians.

Life

Plants


Etching depicting some of the most significant plants of the Carboniferous.
Early Carboniferous land plants, some of which were preserved in coal balls, were very similar to those of the preceding Late Devonian, but new groups also appeared at this time.

Ancient in situ lycopsid, probably Sigillaria, with attached stigmarian roots.

Base of a lycopsid showing connection with bifurcating stigmarian roots.

The main Early Carboniferous plants were the Equisetales (horse-tails), Sphenophyllales (scrambling plants), Lycopodiales (club mosses), Lepidodendrales (scale trees), Filicales (ferns), Medullosales (informally included in the "seed ferns", an artificial assemblage of a number of early gymnosperm groups) and the Cordaitales. These continued to dominate throughout the period, but during late Carboniferous, several other groups, Cycadophyta (cycads), the Callistophytales (another group of "seed ferns"), and the Voltziales (related to and sometimes included under the conifers), appeared.
The Carboniferous lycophytes of the order Lepidodendrales, which are cousins (but not ancestors) of the tiny club-moss of today, were huge trees with trunks 30 meters high and up to 1.5 meters in diameter. These included Lepidodendron (with its cone called Lepidostrobus), Anabathra, Lepidophloios and Sigillaria. The roots of several of these forms are known as Stigmaria. Unlike present day trees, their secondary growth took place in the cortex, which also provided stability, instead of the xylem.[22] The Cladoxylopsids were large trees, that were ancestors of ferns, first arising in the Carboniferous.[23]

The fronds of some Carboniferous ferns are almost identical with those of living species. Probably many species were epiphytic. Fossil ferns and "seed ferns" include Pecopteris, Cyclopteris, Neuropteris, Alethopteris, and Sphenopteris; Megaphyton and Caulopteris were tree ferns.

The Equisetales included the common giant form Calamites, with a trunk diameter of 30 to 60 cm (24 in) and a height of up to 20 m (66 ft). Sphenophyllum was a slender climbing plant with whorls of leaves, which was probably related both to the calamites and the lycopods.

Cordaites, a tall plant (6 to over 30 meters) with strap-like leaves, was related to the cycads and conifers; the catkin-like reproductive organs, which bore ovules/seeds, is called Cardiocarpus. These plants were thought to live in swamps and mangroves. True coniferous trees (Walchia, of the order Voltziales) appear later in the Carboniferous, and preferred higher drier ground.

Marine invertebrates

In the oceans the most important marine invertebrate groups are the Foraminifera, corals, Bryozoa, Ostracoda, brachiopods, ammonoids, hederelloids, microconchids and echinoderms (especially crinoids). For the first time foraminifera take a prominent part in the marine faunas. The large spindle-shaped genus Fusulina and its relatives were abundant in what is now Russia, China, Japan, North America; other important genera include Valvulina, Endothyra, Archaediscus, and Saccammina (the latter common in Britain and Belgium). Some Carboniferous genera are still extant.
The microscopic shells of radiolarians are found in cherts of this age in the Culm of Devon and Cornwall, and in Russia, Germany and elsewhere. Sponges are known from spicules and anchor ropes, and include various forms such as the Calcispongea Cotyliscus and Girtycoelia, the demosponge Chaetetes, and the genus of unusual colonial glass sponges Titusvillia.

Both reef-building and solitary corals diversify and flourish; these include both rugose (for example, Caninia, Corwenia, Neozaphrentis), heterocorals, and tabulate (for example, Chladochonus, Michelinia) forms. Conularids were well represented by Conularia

Bryozoa are abundant in some regions; the fenestellids including Fenestella, Polypora, and Archimedes, so named because it is in the shape of an Archimedean screw. Brachiopods are also abundant; they include productids, some of which (for example, Gigantoproductus) reached very large (for brachiopods) size and had very thick shells, while others like Chonetes were more conservative in form. Athyridids, spiriferids, rhynchonellids, and terebratulids are also very common. Inarticulate forms include Discina and Crania. Some species and genera had a very wide distribution with only minor variations.

Annelids such as Serpulites are common fossils in some horizons. Among the mollusca, the bivalves continue to increase in numbers and importance. Typical genera include Aviculopecten, Posidonomya, Nucula, Carbonicola, Edmondia, and Modiola Gastropods are also numerous, including the genera Murchisonia, Euomphalus, Naticopsis. Nautiloid cephalopods are represented by tightly coiled nautilids, with straight-shelled and curved-shelled forms becoming increasingly rare. Goniatite ammonoids are common.

Trilobites are rarer than in previous periods, on a steady trend towards extinction, represented only by the proetid group. Ostracoda, a class of crustaceans, were abundant as representatives of the meiobenthos; genera included Amphissites, Bairdia, Beyrichiopsis, Cavellina, Coryellina, Cribroconcha, Hollinella, Kirkbya, Knoxiella, and Libumella.

Amongst the echinoderms, the crinoids were the most numerous. Dense submarine thickets of long-stemmed crinoids appear to have flourished in shallow seas, and their remains were consolidated into thick beds of rock. Prominent genera include Cyathocrinus, Woodocrinus, and Actinocrinus. Echinoids such as Archaeocidaris and Palaeechinus were also present. The blastoids, which included the Pentreinitidae and Codasteridae and superficially resembled crinoids in the possession of long stalks attached to the seabed, attain their maximum development at this time.

Freshwater and lagoonal invertebrates

Freshwater Carboniferous invertebrates include various bivalve molluscs that lived in brackish or fresh water, such as Anthraconaia, Naiadites, and Carbonicola; diverse crustaceans such as Candona, Carbonita, Darwinula, Estheria, Acanthocaris, Dithyrocaris, and Anthrapalaemon.

The upper Carboniferous giant spider-like eurypterid Megarachne grew to legspans of 50 cm (20 in).

The Eurypterids were also diverse, and are represented by such genera as Anthraconectes, Megarachne (originally misinterpreted as a giant spider) and the specialised very large Hibbertopterus. Many of these were amphibious.

Frequently a temporary return of marine conditions resulted in marine or brackish water genera such as Lingula, Orbiculoidea, and Productus being found in the thin beds known as marine bands.

Terrestrial invertebrates

Fossil remains of air-breathing insects,[24] myriapods and arachnids[25] are known from the late Carboniferous, but so far not from the early Carboniferous.[8] The first true priapulids appeared during this period. Their diversity when they do appear, however, shows that these arthropods were both well developed and numerous. Their large size can be attributed to the moistness of the environment (mostly swampy fern forests) and the fact that the oxygen concentration in the Earth's atmosphere in the Carboniferous was much higher than today.[26] This required less effort for respiration and allowed arthropods to grow larger with the up to 2.6 metres long millipede-like Arthropleura being the largest known land invertebrate of all time. Among the insect groups are the huge predatory Protodonata (griffinflies), among which was Meganeura, a giant dragonfly-like insect and with a wingspan of ca. 75 cm (30 in) — the largest flying insect ever to roam the planet. Further groups are the Syntonopterodea (relatives of present-day mayflies), the abundant and often large sap-sucking Palaeodictyopteroidea, the diverse herbivorous Protorthoptera, and numerous basal Dictyoptera (ancestors of cockroaches).[24] Many insects have been obtained from the coalfields of Saarbrücken and Commentry, and from the hollow trunks of fossil trees in Nova Scotia. Some British coalfields have yielded good specimens: Archaeoptitus, from the Derbyshire coalfield, had a spread of wing extending to more than 35 cm; some specimens (Brodia) still exhibit traces of brilliant wing colors. In the Nova Scotian tree trunks land snails (Archaeozonites, Dendropupa) have been found.

Fish

Many fish inhabited the Carboniferous seas; predominantly Elasmobranchs (sharks and their relatives). These included some, like Psammodus, with crushing pavement-like teeth adapted for grinding the shells of brachiopods, crustaceans, and other marine organisms. Other sharks had piercing teeth, such as the Symmoriida; some, the petalodonts, had peculiar cycloid cutting teeth. Most of the sharks were marine, but the Xenacanthida invaded fresh waters of the coal swamps. Among the bony fish, the Palaeonisciformes found in coastal waters also appear to have migrated to rivers. Sarcopterygian fish were also prominent, and one group, the Rhizodonts, reached very large size.

Most species of Carboniferous marine fish have been described largely from teeth, fin spines and dermal ossicles, with smaller freshwater fish preserved whole.
Freshwater fish were abundant, and include the genera Ctenodus, Uronemus, Acanthodes, Cheirodus, and Gyracanthus.

Sharks (especially the Stethacanthids) underwent a major evolutionary radiation during the Carboniferous.[27] It is believed that this evolutionary radiation occurred because the decline of the placoderms at the end of the Devonian period caused many environmental niches to become unoccupied and allowed new organisms to evolve and fill these niches.[27] As a result of the evolutionary radiation carboniferous sharks assumed a wide variety of bizarre shapes including Stethacanthus which possessed a flat brush-like dorsal fin with a patch of denticles on its top.[27] Stethacanthus' unusual fin may have been used in mating rituals.[27]

Tetrapods

Carboniferous amphibians were diverse and common by the middle of the period, more so than they are today; some were as long as 6 meters, and those fully terrestrial as adults had scaly skin.[28] They included a number of basal tetrapod groups classified in early books under the Labyrinthodontia. These had long bodies, a head covered with bony plates and generally weak or undeveloped limbs. The largest were over 2 meters long. They were accompanied by an assemblage of smaller amphibians included under the Lepospondyli, often only about 15 cm (6 in) long. Some Carboniferous amphibians were aquatic and lived in rivers (Loxomma, Eogyrinus, Proterogyrinus); others may have been semi-aquatic (Ophiderpeton, Amphibamus, Hyloplesion) or terrestrial (Dendrerpeton, Tuditanus, Anthracosaurus).

The Carboniferous Rainforest Collapse slowed the evolution of amphibians who could not survive as well in the cooler, drier conditions. Reptiles, however prospered due to specific key adaptations.[10] One of the greatest evolutionary innovations of the Carboniferous was the amniote egg, which allowed for the further exploitation of the land by certain tetrapods. These included the earliest sauropsid reptiles (Hylonomus), and the earliest known synapsid (Archaeothyris). These small lizard-like animals quickly gave rise to many descendants. The amniote egg allowed these ancestors of all later birds, mammals, and reptiles to reproduce on land by preventing the desiccation, or drying-out, of the embryo inside.

Reptiles underwent a major evolutionary radiation in response to the drier climate that preceded the rainforest collapse.[10][29] By the end of the Carboniferous period, amniotes had already diversified into a number of groups, including protorothyridids, captorhinids, araeoscelids, and several families of pelycosaurs.

Fungi

Because plants and animals were growing in size and abundance in this time (for example, Lepidodendron), land fungi diversified further. Marine fungi still occupied the oceans. All modern classes of fungi were present in the Late Carboniferous (Pennsylvanian Epoch).[30]

Extinction events

Romer's gap

The first 15 million years of the Carboniferous had very limited terrestrial fossils. This gap in the fossil record is called Romer's gap after the American palaentologist Alfred Romer. While it has long been debated whether the gap is a result of fossilisation or relates to an actual event, recent work indicates the gap period saw a drop in atmospheric oxygen levels, indicating some sort of ecological collapse.[31] The gap saw the demise of the Devonian fish-like ichthyostegalian labyrinthodonts, and the rise of the more advanced temnospondyl and reptiliomorphan amphibians that so typify the Carboniferous terrestrial vertebrate fauna.

Carboniferous rainforest collapse

Before the end of the Carboniferous Period, an extinction event occurred. On land this event is referred to as the Carboniferous Rainforest Collapse (CRC).[10] Vast tropical rainforests collapsed suddenly as the climate changed from hot and humid to cool and arid. This was likely caused by intense glaciation and a drop in sea levels.[32]
The new climatic conditions were not favorable to the growth of rainforest and the animals within them. Rainforests shrank into isolated islands, surrounded by seasonally dry habitats. Towering lycopsid forests with a heterogeneous mixture of vegetation were replaced by much less diverse tree-fern dominated flora.

Amphibians, the dominant vertebrates at the time, fared poorly through this event with large losses in biodiversity; reptiles continued to diversify due to key adaptations that let them survive in the drier habitat, specifically the hard-shelled egg and scales, both of which retain water better than their amphibian counterparts.[10]