Paleontology

Paleontology, sometimes spelled palaeontology (/ˌpeɪliɒnˈtɒlədʒi, ˌpæli-, -ən-/) is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene Epoch (roughly 11,700 years before present). It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments (their paleoecology). Paleontological observations have been documented as far back as the 5th century BC. The science became established in the 18th century as a result of Georges Cuvier's work on comparative anatomy, and developed rapidly in the 19th century. The term itself originates from Greek παλαιός, palaios, 'old, ancient', ὄν, on (gen. ontos), 'being, creature' and λόγος, logos, 'speech, thought, study'.AmphibiansExtinct SynapsidsMammalsExtinct reptilesLizards and snakesExtinctArchosaursCrocodiliansExtinctDinosaursBirds Paleontology, sometimes spelled palaeontology (/ˌpeɪliɒnˈtɒlədʒi, ˌpæli-, -ən-/) is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene Epoch (roughly 11,700 years before present). It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments (their paleoecology). Paleontological observations have been documented as far back as the 5th century BC. The science became established in the 18th century as a result of Georges Cuvier's work on comparative anatomy, and developed rapidly in the 19th century. The term itself originates from Greek παλαιός, palaios, 'old, ancient', ὄν, on (gen. ontos), 'being, creature' and λόγος, logos, 'speech, thought, study'. Paleontology lies on the border between biology and geology, but differs from archaeology in that it excludes the study of anatomically modern humans. It now uses techniques drawn from a wide range of sciences, including biochemistry, mathematics, and engineering. Use of all these techniques has enabled paleontologists to discover much of the evolutionary history of life, almost all the way back to when Earth became capable of supporting life, about 3.8 billion years ago. As knowledge has increased, paleontology has developed specialised sub-divisions, some of which focus on different types of fossil organisms while others study ecology and environmental history, such as ancient climates. Body fossils and trace fossils are the principal types of evidence about ancient life, and geochemical evidence has helped to decipher the evolution of life before there were organisms large enough to leave body fossils. Estimating the dates of these remains is essential but difficult: sometimes adjacent rock layers allow radiometric dating, which provides absolute dates that are accurate to within 0.5%, but more often paleontologists have to rely on relative dating by solving the 'jigsaw puzzles' of biostratigraphy (arrangement of rock layers from youngest to oldest). Classifying ancient organisms is also difficult, as many do not fit well into the Linnaean taxonomy classifying living organisms, and paleontologists more often use cladistics to draw up evolutionary 'family trees'. The final quarter of the 20th century saw the development of molecular phylogenetics, which investigates how closely organisms are related by measuring the similarity of the DNA 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. The simplest definition of paleontology is 'the study of ancient life'. The field 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'. Paleontology is one of the historical sciences, along with archaeology, geology, astronomy, cosmology, philology and history itself: it aims to describe phenomena of the past and reconstruct their causes. Hence it has three main elements: description of past phenomena; developing a general theory about the causes of various types of change; and applying those theories to specific facts.When trying to explain the past, 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 strongly accords with one hypothesis over the others. Sometimes the smoking gun is discovered by a fortunate accident during other research. For example, the discovery by Luis and Walter Alvarez of iridium, a mainly extraterrestrial metal, in the Cretaceous–Tertiary boundary layer made asteroid impact the most favored explanation for the Cretaceous–Paleogene extinction event, although the contribution of volcanism continues to be debated. 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. This approach cannot prove a hypothesis, since some later experiment may disprove it, but the accumulation of failures to disprove is often compelling evidence in favor. 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'. Paleontology lies between biology and geology since it focuses on the record of past life, but its main source of evidence is fossils in rocks. For historical reasons, paleontology is part of the geology department at many universities: in the 19th and early 20th centuries, geology departments found fossil evidence important for dating rocks, while biology departments showed little interest. Paleontology also has some overlap with archaeology, which primarily works with objects made by humans and with human remains, while paleontologists are interested in the characteristics and evolution of humans as a species. When dealing with evidence about humans, archaeologists and paleontologists may work together – for example paleontologists might identify animal or plant fossils around an archaeological site, to discover what the people who lived there ate; or they might analyze the climate at the time of habitation. In addition, paleontology often borrows techniques from other sciences, including biology, osteology, ecology, chemistry, physics and mathematics. For example, geochemical signatures from rocks may help to discover when life first arose on Earth, and analyses of carbon isotope ratios may help to identify climate changes and even to explain major transitions such as the Permian–Triassic extinction event. A relatively recent discipline, molecular phylogenetics, compares the DNA and RNA of modern organisms to re-construct the 'family trees' of their evolutionary ancestors. It has also been used to estimate the dates of important evolutionary developments, although this approach is controversial because of doubts about the reliability of the 'molecular clock'. Techniques from engineering have been used to analyse how the bodies of ancient organisms might have worked, for example the running speed and bite strength of Tyrannosaurus, or the flight mechanics of Microraptor. It is relatively commonplace to study the internal details of fossils using X-ray microtomography. Paleontology, biology, archaeology, and paleoneurobiology combine to study endocranial casts (endocasts) of species related to humans to clarify the evolution of the human brain.