The coelacanths were thought to have gone extinct 66 million years ago, until a living specimen belonging to the order was discovered in 1938.
A living fossil is an extant taxon
that phenotypically resembles related species known only from the
fossil record. To be considered a living fossil, the fossil species must
be old relative to the time of origin of the extant clade.
Living fossils commonly are of species-poor lineages, but they need not
be. While the body plan of a living fossil remains superficially
similar, it is never the same species as the remote relatives it
resembles, because genetic drift would inevitably change its chromosomal structure.
Living fossils exhibit stasis
(also called "bradytely") over geologically long time scales. Popular
literature may wrongly claim that a "living fossil" has undergone no
significant evolution since fossil times, with practically no molecular evolution or morphological changes. Scientific investigations have repeatedly discredited such claims.
Living fossils have two main characteristics, although some have a third:
Living organisms that are members of a taxon that has remained recognisable in the fossil record over an unusually long time span.
They show little morphological divergence, whether from early members of the lineage, or among extant species.
They tend to have little taxonomic diversity.
The first two are required for recognition as a living fossil; some
authors also require the third, others merely note it as a frequent
trait.
Such criteria are neither well-defined nor clearly quantifiable,
but modern methods for analyzing evolutionary dynamics can document the
distinctive tempo of stasis.
Lineages that exhibit stasis over very short time scales are not
considered living fossils; what is poorly-defined is the time scale over
which the morphology must persist for that lineage to be recognized as a
living fossil.
The term living fossil is much misunderstood in popular
media in particular, in which it often is used meaninglessly. In
professional literature the expression seldom appears and must be used
with far more caution, although it has been used inconsistently.
One example of a concept that could be confused with "living fossil" is that of a "Lazarus taxon", but the two are not equivalent; a Lazarus taxon (whether a single species or a group of related species) is one that suddenly reappears, either in the fossil record or in nature, as if the fossil had "come to life again".
In contrast to "Lazarus taxa", a living fossil in most senses is a
species or lineage that has undergone exceptionally little change
throughout a long fossil record, giving the impression that the extant
taxon had remained identical through the entire fossil and modern
period. Because of the mathematical inevitability of genetic drift,
though, the DNA of the modern species is necessarily different from
that of its distant, similar-looking ancestor. They almost certainly
would not be able to cross-reproduce, and are not the same species.
The average species turnover time, meaning the time between when a species first is established and when it finally disappears, varies widely among phyla, but averages about 2–3million years.
A living taxon that had long been thought to be extinct could be called
a Lazarus taxon once it was discovered to be still extant. A dramatic
example was the order Coelacanthiformes, of which the genus Latimeria was found to be extant in 1938. About that there is little debate – however, whether Latimeria
resembles early members of its lineage sufficiently closely to be
considered a living fossil as well as a Lazarus taxon has been denied by
some authors in recent years.
Coelacanths disappeared from the fossil record some 80million years ago (in the upper Cretaceous
period) and, to the extent that they exhibit low rates of morphological
evolution, extant species qualify as living fossils. It must be
emphasised that this criterion reflects fossil evidence, and is totally
independent of whether the taxa had been subject to selection at all,
which all living populations continuously are, whether they remain
genetically unchanged or not.
This apparent stasis, in turn, gives rise to a great deal of
confusion – for one thing, the fossil record seldom preserves much more
than the general morphology of a specimen. To determine much about its
physiology is seldom possible; not even the most dramatic examples of
living fossils can be expected to be without changes, no matter how
persistently constant their fossils and the extant specimens might seem.
To determine much about noncoding DNA
is hardly ever possible, but even if a species were hypothetically
unchanged in its physiology, it is to be expected from the very nature
of the reproductive processes, that its non-functional genomic
changes would continue at more-or-less standard rates. Hence, a fossil
lineage with apparently constant morphology need not imply equally
constant physiology, and certainly neither implies any cessation of the
basic evolutionary processes such as natural selection, nor reduction in
the usual rate of change of the noncoding DNA.
Some living fossils are taxa that were known from
palaeontological fossils before living representatives were discovered.
The most famous examples of this are:
All the above include taxa that originally were described as fossils but now are known to include still-extant species.
Other examples of living fossils are single living species that
have no close living relatives, but are survivors of large and
widespread groups in the fossil record. For example:
All of these were described from fossils before later being found alive.
The fact that a living fossil is a surviving representative of an
archaic lineage does not imply that it must retain all the "primitive"
features (plesiomorphies)
of its ancestral lineage. Although it is common to say that living
fossils exhibit "morphological stasis", stasis, in the scientific
literature, does not mean that any species is strictly identical to its
ancestor, much less remote ancestors.
Some living fossils are relicts of formerly diverse and
morphologically varied lineages, but not all survivors of ancient
lineages necessarily are regarded as living fossils. See for example the
uniquely and highly autapomorphic oxpeckers, which appear to be the only survivors of an ancient lineage related to starlings and mockingbirds.
Evolution and living fossils
The term living fossil
is usually reserved for species or larger clades that are exceptional
for their lack of morphological diversity and their exceptional
conservatism, and several hypotheses could explain morphological stasis
on a geologically long time-scale. Early analyses of evolutionary rates
emphasized the persistence of a taxon rather than rates of evolutionary
change.
Contemporary studies instead analyze rates and modes of phenotypic
evolution, but most have focused on clades that are thought to be
adaptive radiations rather than on those thought to be living fossils.
Thus, very little is presently known about the evolutionary mechanisms
that produce living fossils or how common they might be. Some recent
studies have documented exceptionally low rates of ecological and
phenotypic evolution despite rapid speciation.
This has been termed a "non-adaptive radiation" referring to
diversification not accompanied by adaptation into various significantly
different niches.
Such radiations are explanation for groups that are morphologically
conservative. Persistent adaptation within an adaptive zone is a common
explanation for morphological stasis.
The subject of very low evolutionary rates, however, has received much
less attention in the recent literature than that of high rates.
Living fossils are not expected to exhibit exceptionally low
rates of molecular evolution, and some studies have shown that they do
not. For example, on tadpole shrimp (Triops),
one article notes, "Our work shows that organisms with conservative
body plans are constantly radiating, and presumably, adapting to novel
conditions... I would favor retiring the term 'living fossil'
altogether, as it is generally misleading."
Some scientists instead prefer a new term stabilomorph, being defined
as "an effect of a specific formula of adaptative strategy among
organisms whose taxonomic status does not exceed genus-level. A high
effectiveness of adaptation significantly reduces the need for
differentiated phenotypic variants in response to environmental changes
and provides for long-term evolutionary success."
The question posed by several recent studies pointed out that the
morphological conservatism of coelacanths is not supported by
paleontological data.
In addition, it was shown recently that studies concluding that a slow
rate of molecular evolution is linked to morphological conservatism in
coelacanths are biased by the a priori hypothesis that these species are 'living fossils'.
Accordingly, the genome stasis hypothesis is challenged by the recent
finding that the genome of the two extant coelacanth species L. chalumnae and L. menadoensis
contain multiple species-specific insertions, indicating transposable
element recent activity and contribution to post-speciation genome
divergence.
Such studies, however, challenge only a genome stasis hypothesis, not
the hypothesis of exceptionally low rates of phenotypic evolution.
All fresh-water basins, taken
together, make a small area compared with that of the sea or of the
land; and, consequently, the competition between fresh-water productions
will have been less severe than elsewhere; new forms will have been
more slowly formed, and old forms more slowly exterminated. And it is in
fresh water that we find seven genera of Ganoid fishes, remnants of a
once preponderant order: and in fresh water we find some of the most
anomalous forms now known in the world, as the Ornithorhynchus and Lepidosiren,
which, like fossils, connect to a certain extent orders now widely
separated in the natural scale. These anomalous forms may almost be
called living fossils; they have endured to the present day, from having
inhabited a confined area, and from having thus been exposed to less
severe competition.
A living taxon that lived through a large portion of geologic time.
The Australian lungfish (Neoceratodus fosteri),
also known as the Queensland lungfish, is an example of an organism
that meets this criterion. Fossils identical to modern specimens have
been dated at over 100million years old. Modern Queensland lungfish have existed as a species for almost 30million years. The contemporary nurse shark has existed for more than 112million years, making this species one of the oldest, if not actually the oldest extant vertebrate species.
Resembles ancient species
A living taxon morphologically and/or physiologically resembling a fossil taxon through a large portion of geologic time (morphological stasis).
Retains many ancient traits
More primitive trapdoor spiders, such as this female Liphistius sp.,
have segmented plates on the dorsal surface of the abdomen and
cephalothorax, a character shared with scorpions, making it probable
that after the spiders diverged from the scorpions, the earliest unique
ancestor of trapdoor species was the first to split off from the lineage
that contains all other extant spiders.
A living taxon with many characteristics believed to be primitive.
This is a more neutral definition. However, it does not make it clear
whether the taxon is truly old, or it simply has many plesiomorphies.
Note that, as mentioned above, the converse may hold for true living
fossil taxa; that is, they may possess a great many derived features (autapomorphies), and not be particularly "primitive" in appearance.
Some paleontologists believe that living fossils with large distributions (such as Triops cancriformis) are not real living fossils. In the case of Triops cancriformis (living from the Triassic until now), the Triassic specimens lost most of their appendages (mostly only carapaces remain), and they have not been thoroughly examined since 1938.
Low diversity
Any of the first three definitions, but the clade also has a low taxonomic diversity (low diversity lineages).
Oxpeckers are morphologically somewhat similar to starlings
due to shared plesiomorphies, but are uniquely adapted to feed on
parasites and blood of large land mammals, which has always obscured
their relationships. This lineage forms part of a radiation that
includes Sturnidae and Mimidae, but appears to be the most ancient of these groups. Biogeography strongly suggests that oxpeckers originated in eastern Asia and only later arrived in Africa, where they now have a relict distribution.
The two living species thus seem to represent an entirely extinct and (as Passerida
go) rather ancient lineage, as certainly as this can be said in the
absence of actual fossils. The latter is probably due to the fact that
the oxpecker lineage never occurred in areas where conditions were good
for fossilization of small bird bones, but of course, fossils of
ancestral oxpeckers may one day turn up enabling this theory to be
tested.
Operational definition
An
operational definition was proposed in 2017, where a 'living fossil'
lineage has a slow rate of evolution and occurs close to the middle of
morphological variation (the centroid of morphospace) among related taxa
(i.e. a species is morphologically conservative among relatives). The scientific accuracy of the morphometric analyses used to classify tuatara as a living fossil under this definition have been criticised however, which prompted a rebuttal from the original authors.
Examples
Some of these are informally known as "living fossils".
Paleontology is the study of fossils: their age, method of formation, and evolutionary significance. Specimens are usually considered to be fossils if they are over 10,000 years old. The oldest fossils are around 3.48 billion years old to 4.1 billion years old. The observation in the 19th century that certain fossils were associated with certain rock strata led to the recognition of a geological timescale and the relative ages of different fossils. The development of radiometric dating techniques in the early 20th century allowed scientists to quantitatively measure the absolute ages of rocks and the fossils they host.
Fossils vary in size from one-micrometre (1 µm) bacteria to dinosaurs
and trees, many meters long and weighing many tons. A fossil normally
preserves only a portion of the deceased organism, usually that portion
that was partially mineralized during life, such as the bones and teeth of vertebrates, or the chitinous or calcareous exoskeletons of invertebrates. Fossils may also consist of the marks left behind by the organism while it was alive, such as animal tracks or feces (coprolites). These types of fossil are called trace fossils or ichnofossils, as opposed to body fossils. Some fossils are biochemical and are called chemofossils or biosignatures.
Though the fossil record is incomplete, numerous studies have
demonstrated that there is enough information available to give us a
good understanding of the pattern of diversification of life on Earth.In addition, the record can predict and fill gaps such as the discovery of Tiktaalik in the arctic of Canada.
Fossilization processes
The process of fossilization varies according to tissue type and external conditions:
Permineralization
is a process of fossilization that occurs when an organism is buried.
The empty spaces within an organism (spaces filled with liquid or gas
during life) become filled with mineral-rich groundwater.
Minerals precipitate from the groundwater, occupying the empty spaces.
This process can occur in very small spaces, such as within the cell wall of a plant cell. Small scale permineralization can produce very detailed fossils.
For permineralization to occur, the organism must become covered by
sediment soon after death, otherwise the remains are destroyed by
scavengers or decomposition.
The degree to which the remains are decayed when covered determines the
later details of the fossil. Some fossils consist only of skeletal
remains or teeth; other fossils contain traces of skin, feathers or even soft tissues. This is a form of diagenesis.
In some cases, the original remains of the organism completely
dissolve or are otherwise destroyed. The remaining organism-shaped hole
in the rock is called an external mold. If this void is later filled with sediment, the resulting cast resembles what the organism looked like. An endocast, or internal mold, is the result of sediments filling an organism's interior, such as the inside of a bivalve or snail or the hollow of a skull. Endocasts are sometimes termed Steinkerns, especially when bivalves are preserved this way.
Authigenic mineralization
This is a special form of cast and mold formation. If the chemistry
is right, the organism (or fragment of organism) can act as a nucleus
for the precipitation of minerals such as siderite,
resulting in a nodule forming around it. If this happens rapidly before
significant decay to the organic tissue, very fine three-dimensional
morphological detail can be preserved. Nodules from the Carboniferous Mazon Creek fossil beds of Illinois, US, are among the best documented examples of such mineralization.
Replacement and recrystallization
Silicified (replaced with silica) fossils from the Road Canyon Formation (Middle Permian of Texas)Recrystallized scleractinian coral (aragonite to calcite) from the Jurassic of southern Israel
Replacement occurs when the shell, bone, or other tissue is replaced
with another mineral. In some cases mineral replacement of the original
shell occurs so gradually and at such fine scales that microstructural
features are preserved despite the total loss of original material. A
shell is said to be recrystallized when the original skeletal compounds are still present but in a different crystal form, as from aragonite to calcite.
Adpression (compression-impression)
Compression fossils,
such as those of fossil ferns, are the result of chemical reduction of
the complex organic molecules composing the organism's tissues. In this
case the fossil consists of original material, albeit in a geochemically
altered state. This chemical change is an expression of diagenesis. Often what remains is a carbonaceous film
known as a phytoleim, in which case the fossil is known as a
compression. Often, however, the phytoleim is lost and all that remains
is an impression of the organism in the rock—an impression fossil. In
many cases, however, compressions and impressions occur together. For
instance, when the rock is broken open, the phytoleim will often be
attached to one part (compression), whereas the counterpart will just be
an impression. For this reason, one term covers the two modes of
preservation: adpression.
Soft tissue, cell and molecular preservation
Because of their antiquity, an unexpected exception to the alteration
of an organism's tissues by chemical reduction of the complex organic
molecules during fossilization has been the discovery of soft tissue in
dinosaur fossils, including blood vessels, and the isolation of proteins
and evidence for DNA fragments. In 2014, Mary Schweitzer and her colleagues reported the presence of iron particles (goethite-aFeO(OH))
associated with soft tissues recovered from dinosaur fossils. Based on
various experiments that studied the interaction of iron in haemoglobin with blood vessel tissue they proposed that solution hypoxia coupled with iron chelation
enhances the stability and preservation of soft tissue and provides the
basis for an explanation for the unforeseen preservation of fossil soft
tissues. However, a slightly older study based on eight taxa ranging in time from the Devonian to the Jurassic found that reasonably well-preserved fibrils that probably represent collagen
were preserved in all these fossils and that the quality of
preservation depended mostly on the arrangement of the collagen fibers,
with tight packing favoring good preservation. There seemed to be no correlation between geological age and quality of preservation, within that timeframe.
Carbonization and coalification
Fossils that are carbonized or coalified consist of the organic
remains which have been reduced primarily to the chemical element
carbon. Carbonized fossils consist of a thin film which forms a
silhouette of the original organism, and the original organic remains
were typically soft tissues. Coalified fossils consist primarily of
coal, and the original organic remains were typically woody in
composition.
Partially coalified axis (branch) of a lycopod from the Devonian of Wisconsin.
Bioimmuration
The star-shaped holes (Catellocaula vallata) in this Upper Ordovician bryozoan represent a soft-bodied organism preserved by bioimmuration in the bryozoan skeleton.
Bioimmuration occurs when a skeletal organism overgrows or otherwise
subsumes another organism, preserving the latter, or an impression of
it, within the skeleton. Usually it is a sessile skeletal organism, such as a bryozoan or an oyster, which grows along a substrate, covering other sessile sclerobionts.
Sometimes the bioimmured organism is soft-bodied and is then preserved
in negative relief as a kind of external mold. There are also cases
where an organism settles on top of a living skeletal organism that
grows upwards, preserving the settler in its skeleton. Bioimmuration is
known in the fossil record from the Ordovician to the Recent.
Index fossils (also known as guide fossils, indicator fossils or zone fossils) are fossils used to define and identify geologic periods (or faunal stages). They work on the premise that, although different sediments may look different depending on the conditions under which they were deposited, they may include the remains of the same species
of fossil. The shorter the species' time range, the more precisely
different sediments can be correlated, and so rapidly evolving species'
fossils are particularly valuable. The best index fossils are common,
easy to identify at species level and have a broad
distribution—otherwise the likelihood of finding and recognizing one in
the two sediments is poor.
Trace fossils consist mainly of tracks and burrows, but also include coprolites (fossil feces) and marks left by feeding.
Trace fossils are particularly significant because they represent a
data source that is not limited to animals with easily fossilized hard
parts, and they reflect animal behaviours. Many traces date from
significantly earlier than the body fossils of animals that are thought
to have been capable of making them.
Whilst exact assignment of trace fossils to their makers is generally
impossible, traces may for example provide the earliest physical
evidence of the appearance of moderately complex animals (comparable to earthworms).
Coprolites are classified as trace fossils as opposed to body
fossils, as they give evidence for the animal's behaviour (in this case,
diet) rather than morphology. They were first described by William Buckland in 1829. Prior to this they were known as "fossil fir cones" and "bezoar
stones." They serve a valuable purpose in paleontology because they
provide direct evidence of the predation and diet of extinct organisms. Coprolites may range in size from a few millimetres to over 60 centimetres.
A transitional fossil is any fossilized remains of a life form
that exhibits traits common to both an ancestral group and its derived
descendant group.
This is especially important where the descendant group is sharply
differentiated by gross anatomy and mode of living from the ancestral
group. Because of the incompleteness of the fossil record, there is
usually no way to know exactly how close a transitional fossil is to the
point of divergence. These fossils serve as a reminder that taxonomic
divisions are human constructs that have been imposed in hindsight on a
continuum of variation.
Microfossil
is a descriptive term applied to fossilized plants and animals whose
size is just at or below the level at which the fossil can be analyzed
by the naked eye. A commonly applied cutoff point between "micro" and "macro" fossils is 1 mm. Microfossils may either be complete (or near-complete) organisms in themselves (such as the marine plankters foraminifera and coccolithophores) or component parts (such as small teeth or spores) of larger animals or plants. Microfossils are of critical importance as a reservoir of paleoclimate information, and are also commonly used by biostratigraphers to assist in the correlation of rock units.
Fossil resin (colloquially called amber) is a natural polymer found in many types of strata throughout the world, even the Arctic. The oldest fossil resin dates to the Triassic, though most dates to the Cenozoic. The excretion of the resin by certain plants is thought to be an evolutionary adaptation
for protection from insects and to seal wounds. Fossil resin often
contains other fossils called inclusions that were captured by the
sticky resin. These include bacteria, fungi, other plants, and animals.
Animal inclusions are usually small invertebrates, predominantly arthropods such as insects and spiders, and only extremely rarely a vertebrate such as a small lizard. Preservation of inclusions can be exquisite, including small fragments of DNA.
Eroded Jurassicplesiosaur vertebral centrum found in the Lower Cretaceous Faringdon Sponge Gravels in Faringdon, England. An example of a remanié fossil.
A derived, reworked or remanié fossil is a fossil found in rock that accumulated significantly later than when the fossilized animal or plant died.
Reworked fossils are created by erosion exhuming (freeing) fossils from
the rock formation in which they were originally deposited and their
redeposition in a younger sedimentary deposit.
Polished section of petrified wood showing annual rings
Fossil wood is wood that is preserved in the fossil record. Wood is
usually the part of a plant that is best preserved (and most easily
found). Fossil wood may or may not be petrified. The fossil wood may be the only part of the plant that has been preserved; therefore such wood may get a special kind of botanical name. This will usually include "xylon" and a term indicating its presumed affinity, such as Araucarioxylon (wood of Araucaria or some related genus), Palmoxylon (wood of an indeterminate palm), or Castanoxylon (wood of an indeterminate chinkapin).
The term subfossil can be used to refer to remains, such as bones, nests, or fecal deposits,
whose fossilization process is not complete, either because the length
of time since the animal involved was living is too short or because the
conditions in which the remains were buried were not optimal for
fossilization. Subfossils are often found in caves or other shelters where they can be preserved for thousands of years. The main importance of subfossil vs. fossil remains is that the former contain organic material, which can be used for radiocarbon dating or extraction and sequencing of DNA, protein, or other biomolecules. Additionally, isotope
ratios can provide much information about the ecological conditions
under which extinct animals lived. Subfossils are useful for studying
the evolutionary history of an environment and can be important to
studies in paleoclimatology.
Subfossils are often found in depositionary environments, such as
lake sediments, oceanic sediments, and soils. Once deposited, physical
and chemical weathering can alter the state of preservation, and small subfossils can also be ingested by living organisms. Subfossil remains that date from the Mesozoic are exceptionally rare, are usually in an advanced state of decay, and are consequently much disputed. The vast bulk of subfossil material comes from Quaternary sediments, including many subfossilized chironomid head capsules, ostracodcarapaces, diatoms, and foraminifera.
For remains such as molluscan seashells,
which frequently do not change their chemical composition over
geological time, and may occasionally even retain such features as the
original color markings for millions of years, the label 'subfossil' is
applied to shells that are understood to be thousands of years old, but
are of Holocene age, and therefore are not old enough to be from the Pleistocene epoch.
Chemical fossils, or chemofossils, are chemicals found in rocks and fossil fuels (petroleum, coal, and natural gas) that provide an organic signature for ancient life. Molecular fossils and isotope ratios represent two types of chemical fossils. The oldest traces of life on Earth are fossils of this type, including carbon isotope anomalies found in zircons that imply the existence of life as early as 4.1 billion years ago.
Paleontology seeks to map out how life evolved across geologic time. A
substantial hurdle is the difficulty of working out fossil ages. Beds
that preserve fossils typically lack the radioactive elements needed for
radiometric dating.
This technique is our only means of giving rocks greater than about
50 million years old an absolute age, and can be accurate to within 0.5%
or better.
Although radiometric dating requires careful laboratory work, its basic
principle is simple: the rates at which various radioactive elements decay
are known, and so the ratio of the radioactive element to its decay
products shows how long ago the radioactive element was incorporated
into the rock. Radioactive elements are common only in rocks with a
volcanic origin, and so the only fossil-bearing rocks that can be dated
radiometrically are volcanic ash layers, which may provide termini for
the intervening sediments.
Stratigraphy
Consequently, palaeontologists rely on stratigraphy to date fossils. Stratigraphy is the science of deciphering the "layer-cake" that is the sedimentary record.
Rocks normally form relatively horizontal layers, with each layer
younger than the one underneath it. If a fossil is found between two
layers whose ages are known, the fossil's age is claimed to lie between
the two known ages. Because rock sequences are not continuous, but may be broken up by faults or periods of erosion,
it is very difficult to match up rock beds that are not directly
adjacent. However, fossils of species that survived for a relatively
short time can be used to match isolated rocks: this technique is called
biostratigraphy. For instance, the conodont Eoplacognathus pseudoplanus has a short range in the Middle Ordovician period. If rocks of unknown age have traces of E. pseudoplanus, they have a mid-Ordovician age. Such index fossils
must be distinctive, be globally distributed and occupy a short time
range to be useful. Misleading results are produced if the index fossils
are incorrectly dated. Stratigraphy and biostratigraphy can in general provide only relative dating (A was before B),
which is often sufficient for studying evolution. However, this is
difficult for some time periods, because of the problems involved in
matching rocks of the same age across continents.
Family-tree relationships also help to narrow down the date when
lineages first appeared. For instance, if fossils of B or C date to
X million years ago and the calculated "family tree" says A was an
ancestor of B and C, then A must have evolved earlier.
It is also possible to estimate how long ago two living clades
diverged, in other words approximately how long ago their last common
ancestor must have lived, by assuming that DNA mutations accumulate at a constant rate. These "molecular clocks",
however, are fallible, and provide only approximate timing: for
example, they are not sufficiently precise and reliable for estimating
when the groups that feature in the Cambrian explosion first evolved, and estimates produced by different techniques may vary by a factor of two.
Some of the most remarkable gaps in the fossil record (as of October 2013) show slanting toward organisms with hard parts.
Organisms are only rarely preserved as fossils in the best of
circumstances, and only a fraction of such fossils have been discovered.
This is illustrated by the fact that the number of species known
through the fossil record is less than 5% of the number of known living
species, suggesting that the number of species known through fossils
must be far less than 1% of all the species that have ever lived.
Because of the specialized and rare circumstances required for a
biological structure to fossilize, only a small percentage of life-forms
can be expected to be represented in discoveries, and each discovery
represents only a snapshot of the process of evolution. The transition
itself can only be illustrated and corroborated by transitional fossils,
which will never demonstrate an exact half-way point.
Stromatolites are layered accretionarystructures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria. Stromatolites provide some of the most ancient fossil records of life on Earth, dating back more than 3.5 billion years ago.
A 2009 discovery provides strong evidence of microbial stromatolites extending as far back as 3.45 billion years ago.
Stromatolites are a major constituent of the fossil record for
life's first 3.5 billion years, peaking about 1.25 billion years ago. They subsequently declined in abundance and diversity,
which by the start of the Cambrian had fallen to 20% of their peak. The
most widely supported explanation is that stromatolite builders fell
victims to grazing creatures (the Cambrian substrate revolution), implying that sufficiently complex organisms were common over 1 billion years ago.
The connection between grazer and stromatolite abundance is well documented in the younger Ordovicianevolutionary radiation; stromatolite abundance also increased after the end-Ordovician and end-Permian extinctions decimated marine animals, falling back to earlier levels as marine animals recovered. Fluctuations in metazoan
population and diversity may not have been the only factor in the
reduction in stromatolite abundance. Factors such as the chemistry of
the environment may have been responsible for changes.
While prokaryotic cyanobacteria themselves reproduce asexually through cell division, they were instrumental in priming the environment for the evolutionary development of more complex eukaryotic organisms. Cyanobacteria (as well as extremophileGammaproteobacteria) are thought to be largely responsible for increasing the amount of oxygen in the primeval Earth's atmosphere through their continuing photosynthesis. Cyanobacteria use water, carbon dioxide and sunlight to create their food. A layer of mucus
often forms over mats of cyanobacterial cells. In modern microbial
mats, debris from the surrounding habitat can become trapped within the
mucus, which can be cemented by the calcium carbonate to grow thin
laminations of limestone.
These laminations can accrete over time, resulting in the banded
pattern common to stromatolites. The domal morphology of biological
stromatolites is the result of the vertical growth necessary for the
continued infiltration of sunlight to the organisms for photosynthesis.
Layered spherical growth structures termed oncolites are similar to stromatolites and are also known from the fossil record. Thrombolites
are poorly laminated or non-laminated clotted structures formed by
cyanobacteria common in the fossil record and in modern sediments.
The Zebra River Canyon area of the Kubis platform in the deeply dissected Zaris Mountains of southwestern Namibia
provides an extremely well exposed example of the
thrombolite-stromatolite-metazoan reefs that developed during the
Proterozoic period, the stromatolites here being better developed in
updip locations under conditions of higher current velocities and
greater sediment influx.
Astrobiology
It has been suggested that biominerals could be important indicators of extraterrestrial life and thus could play an important role in the search for past or present life on the planet Mars. Furthermore, organic components (biosignatures) that are often associated with biominerals are believed to play crucial roles in both pre-biotic and biotic reactions.
Pseudofossils are visual patterns in rocks that are produced
by geologic processes rather than biologic processes. They can easily be
mistaken for real fossils. Some pseudofossils, such as geological dendrite
crystals, are formed by naturally occurring fissures in the rock that
get filled up by percolating minerals. Other types of pseudofossils are
kidney ore (round shapes in iron ore) and moss agates, which look like moss or plant leaves. Concretions, spherical or ovoid-shaped nodules found in some sedimentary strata, were once thought to be dinosaur eggs, and are often mistaken for fossils as well.
Gathering fossils dates at least to the beginning of recorded
history. The fossils themselves are referred to as the fossil record.
The fossil record was one of the early sources of data underlying the
study of evolution and continues to be relevant to the history of life on Earth. Paleontologists examine the fossil record to understand the process of evolution and the way particular species have evolved.
Ancient civilizations
Fossils have been visible and common throughout most of natural
history, and so documented human interaction with them goes back as far
as recorded history, or earlier.
There are many examples of paleolithic stone knives in Europe, with fossil echinoderms set precisely at the hand grip, going all the way back to Homo heidelbergensis and Neanderthals.
These ancient peoples also drilled holes through the center of those
round fossil shells, apparently using them as beads for necklaces.
The ancient Egyptians gathered fossils of species that resembled the bones of modern species they worshipped. The god Set was associated with the hippopotamus, therefore fossilized bones of hippo-like species were kept in that deity's temples. Five-rayed fossil sea urchin shells were associated with the deity Sopdu, the Morning Star, equivalent of Venus in Roman mythology.
Ceratopsian skulls are common in the Dzungarian Gate
mountain pass in Asia, an area once famous for gold mines, as well as
its endlessly cold winds. This has been attributed to legends of both
gryphons and the land of Hyperborea.
Fossils appear to have directly contributed to the mythology of many
civilizations, including the ancient Greeks. Classical Greek historian Herodotos wrote of an area near Hyperborea where gryphons protected golden treasure. There was indeed gold mining in that approximate region, where beaked Protoceratops skulls were common as fossils.
A later Greek scholar, Aristotle,
eventually realized that fossil seashells from rocks were similar to
those found on the beach, indicating the fossils were once living
animals. He had previously explained them in terms of vaporousexhalations, which Persian polymath Avicenna modified into the theory of petrifyingfluids (succus lapidificatus). Recognition of fossil seashells as originating in the sea was built upon in the 14th century by Albert of Saxony, and accepted in some form by most naturalists by the 16th century.
Roman naturalist Pliny the Elder wrote of "tongue stones", which he called glossopetra. These were fossil shark teeth, thought by some classical cultures to look like the tongues of people or snakes. He also wrote about the horns of Ammon, which are fossil ammonites,
whence the group of shelled octopus-cousins ultimately draws its modern
name. Pliny also makes one of the earlier known references to toadstones,
thought until the 18th century to be a magical cure for poison
originating in the heads of toads, but which are fossil teeth from Lepidotes, a Cretaceous ray-finned fish.
The Plains tribes
of North America are thought to have similarly associated fossils, such
as the many intact pterosaur fossils naturally exposed in the region,
with their own mythology of the thunderbird.
There is no such direct mythological connection known from
prehistoric Africa, but there is considerable evidence of tribes there
excavating and moving fossils to ceremonial sites, apparently treating
them with some reverence.
In Japan, fossil shark teeth were associated with the mythical tengu, thought to be the razor-sharp claws of the creature, documented some time after the 8th century AD.
In medieval China, the fossil bones of ancient mammals including Homo erectus were often mistaken for "dragon bones" and used as medicine and aphrodisiacs.
In addition, some of these fossil bones are collected as "art" by
scholars, who left scripts on various artifacts, indicating the time
they were added to a collection. One good example is the famous scholar Huang Tingjian of the Song Dynasty during the 11th century, who kept a specific seashell fossil with his own poem engraved on it. In his Dream Pool Essays published in 1088, Song dynasty Chinese scholar-officialShen Kuo hypothesized that marine fossils found in a geological stratum of mountains located hundreds of miles from the Pacific Ocean was evidence that a prehistoric seashore had once existed there and shifted over centuries of time. His observation of petrifiedbamboos in the dry northern climate zone of what is now Yan'an, Shaanxi province, China, led him to advance early ideas of gradual climate change due to bamboo naturally growing in wetter climate areas.
In medieval Christendom, fossilized sea creatures on mountainsides were seen as proof of the biblical deluge of Noah's Ark. After observing the existence of seashells in mountains, the ancient Greek philosopherXenophanes (c. 570 – 478 BC) speculated that the world was once inundated in a great flood that buried living creatures in drying mud.
If what is said concerning the
petrifaction of animals and plants is true, the cause of this
(phenomenon) is a powerful mineralizing and petrifying virtue which
arises in certain stony spots, or emanates suddenly from the earth
during earthquake and subsidences, and petrifies whatever comes into
contact with it. As a matter of fact, the petrifaction of the bodies of
plants and animals is not more extraordinary than the transformation of
waters.
From the 13th century to the present day, scholars pointed out that the fossil skulls of Deinotherium giganteum, found in Crete and Greece, might have been interpreted as being the skulls of the Cyclopes of Greek mythology, and are possibly the origin of that Greek myth.Their skulls appear to have a single eye-hole in the front, just like their modern elephant cousins, though in fact it's actually the opening for their trunk.
Fossil shells from the cretaceous era sea urchin, Micraster,
were used in medieval times as both shepherd's crowns to protect
houses, and as painted fairy loaves by bakers to bring luck to their
bread-making.
In Norse mythology, echinoderm shells (the round five-part button left over from a sea urchin) were associated with the god Thor, not only being incorporated in thunderstones,
representations of Thor's hammer and subsequent hammer-shaped crosses
as Christianity was adopted, but also kept in houses to garner Thor's
protection.
These grew into the shepherd's crowns of English folklore, used for decoration and as good luck charms, placed by the doorway of homes and churches. In Suffolk, a different species was used as a good-luck charm by bakers, who referred to them as fairy loaves, associating them with the similarly shaped loaves of bread they baked.
Early modern explanations
More scientific views of fossils emerged during the Renaissance. Leonardo da Vinci concurred with Aristotle's view that fossils were the remains of ancient life. For example, Leonardo noticed discrepancies with the biblical flood narrative as an explanation for fossil origins:
If the Deluge had carried the
shells for distances of three and four hundred miles from the sea it
would have carried them mixed with various other natural objects all
heaped up together; but even at such distances from the sea we see the
oysters all together and also the shellfish and the cuttlefish and all
the other shells which congregate together, found all together dead; and
the solitary shells are found apart from one another as we see them
every day on the sea-shores.
And we find oysters together in
very large families, among which some may be seen with their shells
still joined together, indicating that they were left there by the sea
and that they were still living when the strait of Gibraltar was cut
through. In the mountains of Parma and Piacenza multitudes of shells and
corals with holes may be seen still sticking to the rocks....
Georges Cuvier's 1812 skeletal reconstruction of Anoplotherium commune based on fossil remains of the extinct artiodactyl from Montmartre in Paris, France
In 1666, Nicholas Steno
examined a shark, and made the association of its teeth with the
"tongue stones" of ancient Greco-Roman mythology, concluding that those
were not in fact the tongues of venomous snakes, but the teeth of some
long-extinct species of shark.
Robert Hooke (1635–1703) included micrographs of fossils in his Micrographia and was among the first to observe fossil forams.
His observations on fossils, which he stated to be the petrified
remains of creatures some of which no longer existed, were published
posthumously in 1705.
William Smith (1769–1839), an English canal engineer, observed that rocks of different ages (based on the law of superposition)
preserved different assemblages of fossils, and that these assemblages
succeeded one another in a regular and determinable order. He observed
that rocks from distant locations could be correlated based on the
fossils they contained. He termed this the principle of faunal succession. This principle became one of Darwin's chief pieces of evidence that biological evolution was real.
Georges Cuvier
came to believe that most if not all the animal fossils he examined
were remains of extinct species. This led Cuvier to become an active
proponent of the geological school of thought called catastrophism. Near the end of his 1796 paper on living and fossil elephants he said:
All of these facts, consistent
among themselves, and not opposed by any report, seem to me to prove the
existence of a world previous to ours, destroyed by some kind of
catastrophe.
Interest in fossils, and geology more generally, expanded during the early nineteenth century. In Britain, Mary Anning's discoveries of fossils, including the first complete ichthyosaur and a complete plesiosaurus skeleton, sparked both public and scholarly interest.
Linnaeus and Darwin
Early naturalists well understood the similarities and differences of living species leading Linnaeus
to develop a hierarchical classification system still in use today.
Darwin and his contemporaries first linked the hierarchical structure of
the tree of life with the then very sparse fossil record. Darwin
eloquently described a process of descent with modification, or
evolution, whereby organisms either adapt to natural and changing
environmental pressures, or they perish.
When Darwin wrote On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life, the oldest animal fossils were those from the Cambrian
Period, now known to be about 540 million years old. He worried about
the absence of older fossils because of the implications on the validity
of his theories, but he expressed hope that such fossils would be
found, noting that: "only a small portion of the world is known with
accuracy." Darwin also pondered the sudden appearance of many groups
(i.e. phyla) in the oldest known Cambrian fossiliferous strata.
After Darwin
Since Darwin's time, the fossil record has been extended to between 2.3 and 3.5 billion years. Most of these Precambrian fossils are microscopic bacteria or microfossils. However, macroscopic fossils are now known from the late Proterozoic. The Ediacara biota
(also called Vendian biota) dating from 575 million years ago
collectively constitutes a richly diverse assembly of early
multicellular eukaryotes.
The fossil record and faunal succession form the basis of the science of biostratigraphy or determining the age of rocks based on embedded fossils. For the first 150 years of geology, biostratigraphy and superposition were the only means for determining the relative age of rocks. The geologic time scale was developed based on the relative ages of rock strata as determined by the early paleontologists and stratigraphers.
Since the early years of the twentieth century, absolute dating methods, such as radiometric dating (including potassium/argon, argon/argon, uranium series, and, for very recent fossils, radiocarbon dating)
have been used to verify the relative ages obtained by fossils and to
provide absolute ages for many fossils. Radiometric dating has shown
that the earliest known stromatolites are over 3.4 billion years old.
Modern era
The fossil record is life's evolutionary epic that unfolded over
four billion years as environmental conditions and genetic potential
interacted in accordance with natural selection.
The Virtual Fossil Museum
Paleontology has joined with evolutionary biology
to share the interdisciplinary task of outlining the tree of life,
which inevitably leads backwards in time to Precambrian microscopic life
when cell structure and functions evolved. Earth's deep time in the
Proterozoic and deeper still in the Archean is only "recounted by
microscopic fossils and subtle chemical signals." Molecular biologists, using phylogenetics, can compare protein amino acid or nucleotide
sequence homology (i.e., similarity) to evaluate taxonomy and
evolutionary distances among organisms, with limited statistical
confidence. The study of fossils, on the other hand, can more
specifically pinpoint when and in what organism a mutation first
appeared. Phylogenetics and paleontology work together in the
clarification of science's still dim view of the appearance of life and
its evolution.
Niles Eldredge's study of the Phacopstrilobite
genus supported the hypothesis that modifications to the arrangement of
the trilobite's eye lenses proceeded by fits and starts over millions
of years during the Devonian. Eldredge's interpretation of the Phacops
fossil record was that the aftermaths of the lens changes, but not the
rapidly occurring evolutionary process, were fossilized. This and other
data led Stephen Jay Gould and Niles Eldredge to publish their seminal paper on punctuated equilibrium in 1971.
SynchrotronX-raytomographic analysis of early Cambrian bilaterian embryonic microfossils yielded new insights of metazoan
evolution at its earliest stages. The tomography technique provides
previously unattainable three-dimensional resolution at the limits of
fossilization. Fossils of two enigmatic bilaterians, the worm-like Markuelia and a putative, primitive protostome, Pseudooides, provide a peek at germ layer embryonic development. These 543-million-year-old embryos support the emergence of some aspects of arthropod development earlier than previously thought in the late Proterozoic. The preserved embryos from China and Siberia underwent rapid diagenetic phosphatization resulting in exquisite preservation, including cell structures.
This research is a notable example of how knowledge encoded by the
fossil record continues to contribute otherwise unattainable information
on the emergence and development of life on Earth. For example, the
research suggests Markuelia has closest affinity to priapulid worms, and is adjacent to the evolutionary branching of Priapulida, Nematoda and Arthropoda.
Despite significant advances in uncovering and identifying
paleontological specimens, it is generally accepted that the fossil
record is vastly incomplete. Approaches for measuring the completeness of the fossil record have
been developed for numerous subsets of species, including those grouped
taxonomically temporally, environmentally/geographically, or in sum. This encompasses the subfield of taphonomy and the study of biases in the paleontological record.
Art
According to one hypothesis, a Corinthian vase from the 6th century BCE is the oldest artistic record of a vertebrate fossil, perhaps a Miocene giraffe combined with elements from other species.
However, a subsequent study using artificial intelligence and expert
evaluations reject this idea, because mammals do not have the eye bones
shown in the painted monster. Morphologically, the vase painting
correspond to a carnivorous reptile of the Varanidae family that still
lives in regions occupied by the ancient Greek.
Fossil trading is the practice of buying and selling fossils. This is
many times done illegally with artifacts stolen from research sites,
costing many important scientific specimens each year. The problem is quite pronounced in China, where many specimens have been stolen.
Fossil collecting (sometimes, in a non-scientific sense, fossil
hunting) is the collection of fossils for scientific study, hobby, or
profit. Fossil collecting, as practiced by amateurs, is the predecessor
of modern paleontology and many still collect fossils and study fossils
as amateurs. Professionals and amateurs alike collect fossils for their
scientific value.
As medicine
The use of fossils to address health issues is rooted in traditional medicine and include the use of fossils as talismans.
The specific fossil to use to alleviate or cure an illness is often
based on its resemblance to the symptoms or affected organ. The
usefulness of fossils as medicine is almost entirely a placebo effect, though fossil material might conceivably have some antacid activity or supply some essential minerals. The use of dinosaur bones as "dragon bones" has persisted in Traditional Chinese medicine into modern times, with mid-Cretaceous dinosaur bones being used for the purpose in Ruyang County during the early 21st century.
![Carbonized fossil of a cycloneuralian worm that was once misidentified as leech[27] from the Silurian Waukesha Biota of Wisconsin.](https://en.wikipedia.org/wiki/File:Probable_leech_from_the_Waukesha_Biota.jpg)
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