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Fossil

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   Eocene fossil fish of the genus Knightia
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   Eocene fossil fish of the genus Knightia
   Petrified wood fossil formed through permineralization. The internal
   structure of the tree and bark are maintained in the permineralization
   process.
   Enlarge
   Petrified wood fossil formed through permineralization. The internal
   structure of the tree and bark are maintained in the permineralization
   process.
   Lower Proterozoic Stromatolites from Bolivia, South America
   Enlarge
   Lower Proterozoic Stromatolites from Bolivia, South America

   Fossils (from Latin fossus, literally "having been dug up") are the
   mineralized or otherwise preserved remains or traces (such as
   footprints) of animals, plants, and other organisms. The totality of
   fossils and their placement in fossiliferous (fossil-containing) rock
   formations and sedimentary layers ( strata) is known as the fossil
   record. The study of fossils across geological time, how they were
   formed, and the evolutionary relationships between taxa ( phylogeny)
   are some of the most important functions of the science of
   paleontology.

   While most fossils are several thousands to several billions of years
   old, there is no minimum age for a fossil. Fossils vary in size from
   microscopic, such as single cells, to gigantic, such as dinosaurs. 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 exoskeletons of
   invertebrates. Preservation of soft tissues is exquisitely rare in the
   fossil record. Fossils may also consist of the marks left behind by the
   organism while it was alive, such as the footprint or feces of a
   reptile. These types of fossil are called trace fossils (or
   ichnofossils) as opposed to body fossils. Finally, past life leaves
   some markers that cannot be seen but can be detected in the form of
   chemical signals; these are known as chemical fossils or biomarkers.

   Fossil sites with exceptional preservation, sometimes including
   preserved soft tissues, are known as Lagerstätten. These formations may
   have resulted from carcass burial in an anoxic environment with minimal
   bacteria, thus delaying decomposition. Lagerstätten span geological
   time from the Cambrian. Examples are the Cambrian Maotianshan shales
   and Burgess Shale, the Devonian Hunsrück Slates, the Jurassic Solnhofen
   limestone, and the Carboniferous Mazon Creek localities.

   Earth’s oldest fossils are the stromatolites consisting of rocks built
   from layer upon layer of sediment and precipitants. Based on studies of
   now rare, extant stromatolites, growth of fossil stromatolitic
   structures was biogenetically mediated by mats of microorganisms
   through entrapment of sediments. Abiotic mechanisms for stromatolite
   growth are also known, leading to a decades long and sometimes
   contentious scientific debate regarding biogenesis of certain
   formations, especially those from the lower to middle Archaean. It is
   more widely accepted that stromatolites from the late Archaean and
   through the middle Proterozoic were mostly formed by massive colonies
   of cyanobacteria and that the oxygen byproduct of their photosynthetic
   metabolism first resulted in earth’s massive banded iron formations and
   then oxygenated earth’s atmosphere. Though rare, microstructures
   resembling cells are sometimes found within stromatolites, but are also
   the source of scientific contention. The Gunflint Chert contains
   abundant microfossils widely accepted as a diverse consortium of 2.0
   Bya microbes. In contrast, putative fossil cyanobacteria cells from the
   3.4 Bya Warrawoona Group in Western Australia is in dispute since
   abiotic processes cannot be ruled out. Confirmation of the Warrawoona
   microstructures as cyanobacteria would profoundly impact our
   understanding of when and how early life diversified, pushing important
   evolutionary milestones further back in time (reference). The continued
   study of these oldest fossils is paramount to calibrate complementary
   molecular phylogenetics models.

Developments in interpretation of the fossil record

   Ever since recorded history began, and probably before, people have
   found fossils, pieces of rock and minerals which have replaced the
   remains of biologic organisms or preserved their external form. These
   fossils, and the totality of their occurrence within the sequence of
   Earth's rock strata is referred to as the fossil record.

   The fossil record was one of the early sources of data relevant to the
   study of evolution and continues to be relevant to the history of life
   on Earth. Paleontologists examine the fossil record in order to
   understand the process of evolution and the way particular species have
   evolved. Various explanations have been put forth throughout history to
   explain what fossils are and how they came to be where they were found.
   Many of these explanations relied on folktales or mythologies. In China
   the fossil bones of ancient mammals including Homo erectus were often
   mistaken for “dragon bones” and used as medicine and aphrodisiacs. In
   the West the presence of fossilized sea creatures high up on
   mountainsides was proof of the biblical deluge. More scientific views
   of fossils began to emerge during the Renaissance. For example,
   Leonardo Da Vinci noticed some discrepancies with the biblical account:

                "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..."

   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.

   Smith, who preceded Charles Darwin, was unaware of biological evolution
   and did not know why faunal succession occurred. Biological evolution
   explains why faunal succession exists: as different organisms evolve,
   change and go extinct, they leave behind fossils. Faunal succession was
   one of the chief pieces of evidence cited by Darwin that biological
   evolution had occurred.

   Early naturalists well understood the similarities and differences of
   living species leading Linnaeus to develop a hierarchical
   classification system still in use today. It was Darwin and his
   contemporaries who first linked the hierarchical structure of the great
   tree of life in living organisms 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 Charles Darwin wrote On the Origin of Species, the oldest animal
   fossils were those from the Cambrian Period, now known to be about 540
   million years old. The absence of older fossils worried Darwin about
   the implications for 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.

   Since Darwin's time, the fossil record has been pushed back to 3.5
   billion years before the present. Most of these Precambrian fossils are
   microscopic bacteria or microfossils. However, macroscopic fossils are
   now known from the late Proterozoic. The Ediacaran 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 the fossils
   they contain. 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 carbon-14 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 fossils are over 3.5 billion years old. Various dating methods
   have been used and are used today depending on local geology and
   context, and while there is some variance in the results from these
   dating methods, nearly all of them provide evidence for a very old
   Earth, approximately 4.6 billion years.

   “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 earth’s climate,
   tectonics, atmosphere, oceans, and periodic disasters invoked the
   primary selective pressures on all organisms, which they either adapted
   to, or they perished with or without leaving descendants. Modern
   paleontology has joined with evolutionary biology to share the
   interdisciplinary task of unfolding the tree of life, which inevitably
   leads backwards in time to the microscopic life of the Precambrian when
   cell structure and functions evolved. Earth’s deep time in the
   Proterozoic and deeper still in the Archaean 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 infer taxonomy and evolutionary
   distances among organisms, but with limited statistical confidence. The
   study of fossils, on the other hand, can more specifically pin point
   when and in what organism branching occurred in the tree of life.
   Modern phylogenetics and paleontology work together in the
   clarification of science’s still dim view of the appearance life and
   its evolution during deep time on earth.
   Phacopid trilobite Eldredgeops rana crassituberculata named after Niles
   Eldredge
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   Phacopid trilobite Eldredgeops rana crassituberculata named after Niles
   Eldredge

   Nils Eldredge’s study of the Phacops trilobite 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 fossilised. This and other data led Gould
   and Eldridge to publish the seminal paper on punctuated equilibrium in
   1971.

   An example of modern paleontological progress is the application of
   synchrotron X-ray tomographic techniques to early Cambrian bilaterian
   embryonic microfossils that has recently 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 Ma 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.

   Even with the wealth of information now known about fossils, some
   groups maintain non-scientific beliefs based on the earlier views of
   the fossil record.

Rarity of fossils

   Fossilization is actually a rare occurrence because most components of
   formerly-living things tend to decompose relatively quickly following
   death. In order for an organism to be fossilized, the remains normally
   need to be covered by sediment as soon as possible. However there are
   exceptions to this, such as if an organism becomes frozen, desiccated,
   or comes to rest in an anoxic (oxygen-free) environment such as at the
   bottom of a lake. There are several different types of fossils and
   fossilization processes.

   Due to the combined effect of taphonomic processes and simple
   mathematical chance, fossilization tends to favour organisms with hard
   body parts, those that were widespread, and those that lived for a long
   time. On the other hand, it is very unusual to find fossils of small,
   soft bodied, geographically restricted and geologically ephemeral
   organisms, because of their relative rarity and low likelihood of
   preservation.

   Larger specimens ( macrofossils) are more often observed, dug up and
   displayed, although microscopic remains ( microfossils) are actually
   far more common in the fossil record.

   Some casual observers have been perplexed by the rarity of transitional
   species within the fossil record. The conventional explanation for this
   rarity was given by Darwin, who stated that "the extreme imperfection
   of the geological record," combined with the short duration and narrow
   geographical range of transitional species, made it unlikely that many
   such fossils would be found. Simply put, the conditions under which
   fossilization takes place are quite rare; and it is highly unlikely
   that any given organism will leave behind a fossil. Niles Eldredge and
   Stephen J. Gould developed their theory of punctuated equilibrium in
   part to explain the pattern of stasis and sudden appearance in the
   fossil record.

Permineralization

   A permineralized trilobite, Asaphus kowalewskii
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   A permineralized trilobite, Asaphus kowalewskii

   Permineralization occurs after burial, as the empty spaces within an
   organism (spaces filled with liquid or gas during life) become filled
   with mineral-rich groundwater and the minerals precipitate from the
   groundwater, thus 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 or soon after the initial decaying process.
   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.

Replacement and compression fossils

   In some cases the original remains of the organism have been completely
   dissolved or otherwise destroyed. When all that is left is an
   organism-shaped hole in the rock, it is called a mould fossil or
   typolite. If this hole is later filled with other minerals, it is
   called a cast fossil and is considered a replacement fossil since the
   original materials have been completely replaced by new, unrelated
   ones. In some cases replacement occurs so gradually and at such fine
   scales that no "hole" in the rock can ever be discerned and
   microstructural features are preserved despite the total loss of
   original material.

   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.

   To sum up, fossilization processes proceed differently for different
   kinds of tissues and under different kinds of conditions.

Trace fossils

   Trace fossils are the remains of trackways, burrows, footprints, eggs
   and eggshells, nests, droppings and other types of impressions.
   Fossilized droppings, called coprolites, can give insight into the
   feeding behaviour of animals and can therefore be of great importance.

Microfossils

   '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 cut-off point
   between "micro" and "macro" fossils is 1 mm, although this is only an
   approximate guide. 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.

Resin fossils

   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 Tertiary.
   The excretion of the resin by certain plants is thought to be an
   evolutionary adaptation for protection from insects and to seal wounds
   caused by damage elements. 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.

Pseudofossils

   Example of a pseudofossil: this dendrite looks much like a plant
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   Example of a pseudofossil: this dendrite looks much like a plant

   Pseudofossils are visual patterns in rocks that are produced by
   naturally occurring geologic processes rather than biologic processes.
   They can easily be mistaken for real fossils. Some pseudofossils, such
   as dendrites, 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, round or
   oval-shaped nodules found in some sedimentary strata, were once thought
   to be dinosaur eggs, and are often mistaken for fossils as well.

Living fossils

   Living fossil is a term used for any living species which closely
   resembles a species known from fossils, i.e., as if the fossil had
   "come to life". This can be a species known only from fossils until
   living representatives were discovered, such as the coelacanth and the
   ginkgo tree, or a single living species with no close relatives, or a
   small group of closely related species with no other close relatives,
   such as the horseshoe crabs or the nautilus, that are the sole
   survivors of a once large and widespread group in the fossil record.

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