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Paleontology

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Paleontology

Paleontology or palaeontology (, or , ) is the scientific study of life existent prior to, but sometimes including, the start of the comparative anatomy, and developed rapidly in the 19th century. The term itself originates from Greek παλαιός, palaios, i.e. "old, ancient", ὄν, on (gen. ontos), i.e. "being, creature" and λόγος, logos, i.e. "speech, thought, study".[1]

Paleontology lies on the border between ecology and environmental history, such as ancient climates.

Body fossils and DNA is in their genomes. Molecular phylogenetics has also been used to estimate the dates when species diverged, but there is controversy about the reliability of the molecular clock on which such estimates depend.

Contents

  • Overview 1
    • A historical science 1.1
    • Related sciences 1.2
    • Subdivisions 1.3
  • Sources of evidence 2
    • Body fossils 2.1
    • Trace fossils 2.2
    • Geochemical observations 2.3
  • Classifying ancient organisms 3
  • Estimating the dates of organisms 4
  • Overview of the history of life 5
    • Mass extinctions 5.1
  • History of paleontology 6
  • See also 7
  • Notes 8
  • References 9
  • External links 10

Overview

A palaeontologist at work at John Day Fossil Beds National Monument
The preparation of the fossilized bones of Europasaurus holgeri

The simplest definition is "the study of ancient life".[2] Paleontology seeks information about several aspects of past organisms: "their identity and origin, their environment and evolution, and what they can tell us about the Earth's organic and inorganic past".[3]

A historical science

Paleontology is one of the historical sciences, along with archaeology, geology, astronomy, cosmology, philology and history itself.[4] This means that it aims to describe phenomena of the past and reconstruct their causes.[5] Hence it has three main elements: description of the phenomena; developing a general theory about the causes of various types of change; and applying those theories to specific facts.[4]

When trying to explain past phenomena, paleontologists and other historical scientists often construct a set of hypotheses about the causes and then look for a smoking gun, a piece of evidence that indicates that one hypotheses is a better explanation than others. Sometimes the smoking gun is discovered by a fortunate accident during other research. For example, the discovery by Luis Alvarez and Walter Alvarez of an iridium-rich layer at the CretaceousTertiary boundary made asteroid impact and volcanism the most favored explanations for the Cretaceous–Paleogene extinction event.[5]

The other main type of science is experimental science, which is often said to work by conducting experiments to disprove hypotheses about the workings and causes of natural phenomena – note that this approach cannot confirm a hypothesis is correct, since some later experiment may disprove it. However, when confronted with totally unexpected phenomena, such as the first evidence for invisible radiation, experimental scientists often use the same approach as historical scientists: construct a set of hypotheses about the causes and then look for a "smoking gun".[5]

Related sciences

Paleontology lies on the boundary between biology and geology since paleontology focuses on the record of past life but its main source of evidence is fossils, which are found in rocks.[6] For historical reasons paleontology is part of the geology departments of many universities, because in the 19th century and early 20th century geology departments found paleontological evidence important for estimating the ages of rocks while biology departments showed little interest.[7]

Paleontology also has some overlap with archaeological site, to discover what the people who lived there ate; or they might analyze the climate at the time when the site was inhabited by humans.[8]

Analyses using engineering techniques show that Tyrannosaurus had a devastating bite, but raise doubts about how fast it could move.

In addition paleontology often uses techniques derived from other sciences, including biology, Tyrannosaurus could move and how powerful its bite was.[12][13]

A combination of paleontology, biology, and archaeology, paleoneurology is the study of endocranial casts (or endocasts) of species related to humans to learn about the evolution of human brains.[14]

Paleontology even contributes to astrobiology, the investigation of possible life on other planets, by developing models of how life may have arisen and by providing techniques for detecting evidence of life.[15]

Subdivisions

As knowledge has increased, paleontology has developed specialised subdivisions.[16]

  • Smithsonian's Paleobiology website
  • University of California Museum of Paleontology FAQ About Paleontology
  • The Paleontological Society
  • The Palaeontological Association
  • The Paleontology Portal

External links

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  4. ^ a b Laudan, R. (1992). "What's so Special about the Past?". In Nitecki, M.H., and Nitecki, D.V. History and Evolution. SUNY Press. p. 58.  
  5. ^ a b c Cleland, C.E. (September 2002). "Methodological and Epistemic Differences between Historical Science and Experimental Science" (PDF). Philosophy of Science 69 (3): 474–496.  
  6. ^ McGraw-Hill Encyclopedia of Science & Technology. McGraw-Hill. 2002. p. 58.  
  7. ^ Laudan, R. (1992). "What's so Special about the Past?". In Nitecki, M.H., and Nitecki, D.V. History and Evolution. SUNY Press. p. 57.  
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References

Notes

See also

Increasing awareness of Gregor Mendel's pioneering work in genetics led first to the development of population genetics and then in the mid-20th century to the modern evolutionary synthesis, which explains evolution as the outcome of events such as mutations and horizontal gene transfer, which provide genetic variation, with genetic drift and natural selection driving changes in this variation over time.[106] Within the next few years the role and operation of DNA in genetic inheritance were discovered, leading to what is now known as the "Central Dogma" of molecular biology.[107] In the 1960s molecular phylogenetics, the investigation of evolutionary "family trees" by techniques derived from biochemistry, began to make an impact, particularly when it was proposed that the human lineage had diverged from apes much more recently than was generally thought at the time.[108] Although this early study compared proteins from apes and humans, most molecular phylogenetics research is now based on comparisons of RNA and DNA.[109]

The last half of the 19th century saw a tremendous expansion in paleontological activity, especially in North America.[103] The trend continued in the 20th century with additional regions of the Earth being opened to systematic fossil collection. Fossils found in China near the end of the 20th century have been particularly important as they have provided new information about the earliest evolution of animals, early fish, dinosaurs and the evolution of birds.[104] The last few decades of the 20th century saw a renewed interest in mass extinctions and their role in the evolution of life on Earth.[105] There was also a renewed interest in the Cambrian explosion that apparently saw the development of the body plans of most animal phyla. The discovery of fossils of the Ediacaran biota and developments in paleobiology extended knowledge about the history of life back far before the Cambrian.[60]

Haikouichthys, from about in China, may be the earliest known fish.[102]

[101], and evolutionary theory.human evolution paths, including evolutionary in 1859, much of the focus of paleontology shifted to understanding Origin of Species published Charles Darwin After [101].transmutation of species As knowledge of life's history continued to improve, it became increasingly obvious that there had been some kind of successive order to the development of life. This encouraged early evolutionary theories on the [100] This contributed to a rapid increase in knowledge about the history of life on Earth and to progress in the definition of the

The first half of the 19th century saw geological and paleontological activity become increasingly well organized with the growth of geologic societies and museums[97][98] and an increasing number of professional geologists and fossil specialists. Interest increased for reasons that were not purely scientific, as geology and paleontology helped industrialists to find and exploit natural resources such as coal.[99]

In comparative anatomy as a scientific discipline and, by proving that some fossil animals resembled no living ones, demonstrated that animals could become extinct, leading to the emergence of paleontology.[95] The expanding knowledge of the fossil record also played an increasing role in the development of geology, particularly stratigraphy.[96]

Although paleontology became established around 1800, earlier thinkers had noticed aspects of the fossil record. The ancient Greek philosopher Xenophanes (570–480 BC) concluded from fossil sea shells that some areas of land were once under water.[92] During the Middle Ages the Persian naturalist Ibn Sina, known as Avicenna in Europe, discussed fossils and proposed a theory of petrifying fluids on which Albert of Saxony elaborated in the 14th century.[93] The Chinese naturalist Shen Kuo (1031–1095) proposed a theory of climate change based on the presence of petrified bamboo in regions that in his time were too dry for bamboo.[94]

This illustration of an Cuvier's 1796 paper on living and fossil elephants.

History of paleontology

shows a different trend: a fairly swift rise from , a slight decline from , in which the devastating Permian–Triassic extinction event is an important factor, and a swift rise from to the present.[91]

"the number of distinct genera alive at any given time; that is, those whose first occurrence predates and whose last occurrence postdates that time"[91]

Biodiversity in the fossil record, which is

All genera
"Well-defined" genera
Trend line
"Big Five" mass extinctions
Other mass extinctions
Million years ago
Thousands of genera
  • Reasonably complete [87]
  • The oceans may have become more hospitable to life over the last 500 million years and less vulnerable to mass extinctions: dissolved oxygen became more widespread and penetrated to greater depths; the development of life on land reduced the run-off of nutrients and hence the risk of eutrophication and anoxic events; marine ecosystems became more diversified so that food chains were less likely to be disrupted.[88][89]

The fossil record appears to show that the rate of extinction is slowing down, with both the gaps between mass extinctions becoming longer and the average and background rates of extinction decreasing. However, it is not certain whether the actual rate of extinction has altered, since both of these observations could be explained in several ways:[87]

Life on earth has suffered occasional mass extinctions at least since . Although they are disasters at the time, mass extinctions have sometimes accelerated the evolution of [85][86]

Marine extinction intensity during the Phanerozoic
%
Millions of years ago
(H)
Apparent extinction intensity, i.e. the fraction of genera going extinct at any given time, as reconstructed from the fossil record (graph not meant to include recent epoch of Holocene extinction event)

Mass extinctions

Humans evolved from a lineage of upright-walking apes whose earliest fossils date from over .[82] Although early members of this lineage had chimp-sized brains, about 25% as big as modern humans', there are signs of a steady increase in brain size after about .[83] There is a long-running debate about whether modern humans are descendants of a single small population in Africa, which then migrated all over the world less than 200,000 years ago and replaced previous hominine species, or arose worldwide at the same time as a result of interbreeding.[84]

Fossil evidence indicates that flowering plants appeared and rapidly diversified in the Early Cretaceous, between and .[79] Their rapid rise to dominance of terrestrial ecosystems is thought to have been propelled by coevolution with pollinating insects.[80] Social insects appeared around the same time and, although they account for only small parts of the insect "family tree", now form over 50% of the total mass of all insects.[81]

A modern social insect collects pollen from a modern flowering plant.

During the Permian period synapsids, including the ancestors of mammals, may have dominated land environments,[71] but the Permian–Triassic extinction event came very close to wiping out complex life.[72] The extinctions were apparently fairly sudden, at least among vertebrates.[73] During the slow recovery from this catastrophe a previously obscure group, archosaurs, became the most abundant and diverse terrestrial vertebrates. One archosaur group, the dinosaurs, were the dominant land vertebrates for the rest of the Mesozoic,[74] and birds evolved from one group of dinosaurs.[70] During this time mammals' ancestors survived only as small, mainly nocturnal insectivores, but this apparent set-back may have accelerated the development of mammalian traits such as endothermy and hair.[75] After the Cretaceous–Paleogene extinction event killed off the non-avian dinosaurs – birds are the only surviving dinosaurs – mammals increased rapidly in size and diversity, and some took to the air and the sea.[76][77][78]

Birds are the last surviving dinosaurs.[70]
At about 13 centimetres (5.1 in) the Early Cretaceous Yanoconodon was longer than the average mammal of the time.[69]

The spread of life from water to land required organisms to solve several problems, including protection against drying out and supporting themselves against gravity.[63][64][65] The earliest evidence of land plants and land invertebrates date back to about and respectively.[64][66] The lineage that produced land vertebrates evolved later but very rapidly between and ;[67] recent discoveries have overturned earlier ideas about the history and driving forces behind their evolution.[68] Land plants were so successful that they caused an ecological crisis in the Late Devonian, until the evolution and spread of fungi that could digest dead wood.[22]

The earliest known animals are cnidarians from about , but these are so modern-looking that the earliest animals must have appeared before then.[58] Early fossils of animals are rare because they did not develop mineralized hard parts that fossilize easily until about .[59] The earliest modern-looking bilaterian animals appear in the Early Cambrian, along with several "weird wonders" that bear little obvious resemblance to any modern animals. There is a long-running debate about whether this Cambrian explosion was truly a very rapid period of evolutionary experimentation; alternative views are that modern-looking animals began evolving earlier but fossils of their precursors have not yet been found, or that the "weird wonders" are evolutionary "aunts" and "cousins" of modern groups.[60] Vertebrates remained an obscure group until the first fish with jaws appeared in the Late Ordovician.[61][62]

Opabinia made the largest single contribution to modern interest in the Cambrian explosion.

[57][56]

[20]For about 2,000 million years

This wrinkled "elephant skin" texture is a trace fossil of a non-stromatolite microbial mat.
The image shows the location, in the Burgsvik beds of Sweden, where the texture was first identified as evidence of a microbial mat.[50]

The evolutionary history of life stretches back to over , possibly as far as . Earth formed about and, after a collision that formed the Moon about 40 million years later, may have cooled quickly enough to have oceans and an atmosphere about .[45] However there is evidence on the Moon of a Late Heavy Bombardment from . If, as seem likely, such a bombardment struck Earth at the same time, the first atmosphere and oceans may have been stripped away.[46] The oldest clear evidence of life on Earth dates to , although there have been reports, often disputed, of fossil bacteria from and of geochemical evidence for the presence of life .[9][47] Some scientists have proposed that life on Earth was "seeded" from elsewhere,[48] but most research concentrates on various explanations of how life could have arisen independently on Earth.[49]

Overview of the history of life

It is also possible to estimate how long ago two living clades diverged – i.e. 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 a very approximate timing: for example, they are not sufficiently precise and reliable for estimating when the groups that feature in the Cambrian explosion first evolved,[44] and estimates produced by different techniques may vary by a factor of two.[11]

Family-tree relationships may 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 more than X million years ago.

Consequently, paleontologists must usually rely on stratigraphy to date fossils. Stratigraphy is the science of deciphering the "layer-cake" that is the sedimentary record, and has been compared to a jigsaw puzzle.[39] 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 must lie between the two known ages.[40] 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 next to one another. However, fossils of species that survived for a relatively short time can be used to link up isolated rocks: this technique is called biostratigraphy. For instance, the conodont Eoplacognathus pseudoplanus has a short range in the Middle Ordovician period.[41] If rocks of unknown age are found to have traces of E. pseudoplanus, they must have a mid-Ordovician age. Such index fossils must be distinctive, be globally distributed and have a short time range to be useful. However, misleading results are produced if the index fossils turn out to have longer fossil ranges than first thought.[42] 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 up rocks of the same age across different continents.[43]

Paleontology seeks to map out how living things have changed through time. A substantial hurdle to this aim is the difficulty of working out how old fossils are. 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.[38] Although radiometric dating requires very 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 the element into which it decays 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 a few volcanic ash layers.[38]

Estimating the dates of organisms

Evolutionary developmental biology, commonly abbreviated to "Evo Devo", also helps paleontologists to produce "family trees". For example the embryological development of some modern brachiopods suggests that brachiopods may be descendants of the halkieriids, which became extinct in the Cambrian period.[37]

[34] – this must be taken into account in analyses.convergently, evolved more than once, camera eyes. The result of a successful analysis is a hierarchy of clades – groups that share a common ancestor. Ideally the "family tree" has only two branches leading from each node ("junction"), but sometimes there is too little information to achieve this and paleontologists have to make do with junctions that have several branches. The cladistic technique is sometimes fallible, as some features, such as wings or proteins or DNA, by comparing sequences of molecular, or notochord, such as the presence of a anatomical It works by the logic that, if groups B and C have more similarities to each other than either has to group A, then B and C are more closely related to each other than either is to A. Characters that are compared may be [35] Paleontologists generally use approaches based on

Naming groups of organisms in a way that is clear and widely agreed is important, as some disputes in paleontology have been based just on misunderstandings over names.[35] genus or family or order; this is important since the Linnean rules for naming groups are tied to their levels, and hence if a group is moved to a different level it must be renamed.[36]

Levels in the Linnean taxonomy
Tetrapods

Amphibians

Amniotes
Synapsids

Extinct Synapsids

    Mammals


Reptiles

Extinct reptiles


Lizards and snakes

Archosaurs
 ? 

Extinct
Archosaurs


Crocodilians

Dinosaurs
 ? 

Extinct
Dinosaurs


 ? 
Birds






Simple example cladogram
    Warm-bloodedness evolved somewhere in the
synapsid–mammal transition.
 ?  Warm-bloodedness must also have evolved at one of
these points – an example of convergent evolution.[34]

Classifying ancient organisms

[10].Permian–Triassic extinction event may help to explain major transitions such as the isotope ratios carbon Analyses of [33] and may provide evidence of the presence of [9] Geochemical observations may help to deduce the global level of biological activity, or the affinity of a certain fossil. For example geochemical features of rocks may reveal when life first arose on Earth,

Geochemical observations

[31]).earthworms 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 [32]

Trace fossils

Occasionally, unusual environments may preserve soft tissues. These Signor-Lipps effect.[30]

Fossils of organisms' bodies are usually the most informative type of evidence. The most common types are wood, bones, and shells.[25] Fossilisation is a rare event, and most fossils are destroyed by mineralised are usually preserved, such as the shells of molluscs. Since most animal species are soft-bodied, they decay before they can become fossilised. As a result, although there are 30-plus phyla of living animals, two-thirds have never been found as fossils.[28]

This Marrella specimen illustrates how clear and detailed the fossils from the Burgess Shale lagerstätte are.

Body fossils

Sources of evidence

[24]

Paleoclimatology, although sometimes treated as part of paleoecology,[17] focuses more on the history of Earth's climate and the mechanisms that have changed it[21] – which have sometimes included evolutionary developments, for example the rapid expansion of land plants in the Devonian period removed more carbon dioxide from the atmosphere, reducing the greenhouse effect and thus helping to cause an ice age in the Carboniferous period.[22]

[20] Instead of focusing on individual organisms,

[17]

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