Paleontology or palaeontology is the scientific study of the life of the past, mainly but not exclusively through the study of fossils. Paleontologists use fossils as a means to classify organisms, measure geologic time, and assess the interactions between prehistoric organisms and their natural environment. While paleontological observations are known from at least the 6th century BC, the foundation of paleontology as a science dates back to the work of Georges Cuvier in 1796. Cuvier demonstrated evidence for the concept of extinction and how the life of the past was not necessarily the same as that of the present. The field developed rapidly over the course of the following decades, and the French word paléontologie was introduced for the study in 1822, which was derived from the Ancient Greek word for 'ancient' and words describing relatedness and a field of study. Further advances in the field accompanied the work of Charles Darwin who popularized the concept of evolution. Together, evolution and extinction can be understood as complementary processes that shaped the history of life. Paleontology overlaps the most with the fields of geology and biology. It draws on technology and analysis of a wide range of sciences to apply them to the study of life and environments of the past, particularly for the subdisciplines of paleobiology and paleoecology that are analogous to biology and ecology. Paleontology also contributes to other sciences, being utilized for biostratigraphy to reconstruct the geologic time scale of Earth, or in studies on extinction to establish both external and internal factors that can lead to the disappearance of a species. Much of the history of life is now better understood because of advances in paleontology and the increase in interdisciplinary studies. Several improvements in understanding have occurred from the introduction of theoretical analysis to paleontology in the 1950s and 1960s which led to the rise of more focused fields of paleontology that assess the changing geography and climate of Earth, the phylogenetic relationships between different species, and the analysis of how fossilization occurs and what biases can impact the quality of the fossil record. Paleontology is also one of the most high profile of the sciences, comparable to astrophysics and global health in the amount of attention in mass media. Public attention to paleontology can be traced back to the mythologies of indigenous peoples of many continents and the interpretation of discovered fossils as the bones of dragons or giants. Prehistoric life is used as the inspiration for toys, television and film, computer games, and tourism, with the budgets for these public projects often exceeding the funding within the field of paleontology itself.
Concept Paleontology (also spelled palaeontology) is the study of life of the past, characterized but not defined by the study and interpretation of fossils. It overlaps with the fields of geology and biology especially, but also with ecology, chemistry, physics and mathematics. Paleontology consists of both conceptual theorizing and focused scientific study. Traditionally, the sub-field of invertebrate paleontology has been closely tied to the study of geology, biostratigraphy, and historical geology, which have both commercial and academic drivers, whereas vertebrate paleontology has been more closely tied to biology with limited commercial applications. Both areas of study have broadened over time as a result of developing technology, but the "classical" requirements of fieldwork, laboratory preparation, and study of comparative anatomy remain core components of most sub-fields of paleontology. Paleontological study provides a direct source of information on the anatomy, physiology, ecology, and chronology of life on Earth, and the fossil record can be used to test hypotheses relevant to a range of scientific disciplines including other earth sciences and life sciences. The word paleontology or palaeontology is a compound word formed from the roots "paleo-", "onto-" and "-logy", equivalent to the French word paléontologie or the German word Paläontologie. The spelling paleontology is primarily used in North America, while the spelling palaeontology is preferred in the United Kingdom and was historically spelled as palæontology. Multiple different pronunciations can be found, including (pay-lee-uhn-TOL-uh-jee), (pal-ee-uhn-TOL-uh-jee), and (pay-lee-uhn-TAH-luh-jee). The root word "paleo-" is from the classical Latin or scientific Latin palaeo- and its predecessor Ancient Greek παλαιο- meaning "ancient" or "old", the root noun "onto-" is from the Ancient Greek ὀντ- meaning a sense of relatedness, and the root word "-logy" is from the French ‑logie which derives from the classical Latin ‑logia and the Ancient Greek ‑λογία and in context means a field of study.
Foundation Paleontology includes the study of extinct animals and plants, including both direct observations about their remains and inferences about their behavior and how they interacted with their environment. From the recognition that fossils represented the remains of extinct organisms, paleontology became the zoology, botany, and biology of extinct organisms and therefore an important source for comparative anatomy. It was not always understood that paleontology is an evolutionary science, but over time, instances of evolution were recognized in the fossil record, and the two concepts have been closely linked ever since. The long span of geological time preserved in the fossil record allows very slow evolutionary changes to be observed, and the discovery of extinct organisms has allowed scientists to fill in gaps in the tree of life that cannot be understood through the study of extant organisms. The incorporation of a wider range of life sciences has allowed sub-disciplines like paleobiology and others to emerge.
Fossils
Prior to the 19th century, the word "fossil" was used as a descriptive noun to characterize anything that had been dug out of the ground, including bones, stones, and gems. Early descriptions of what we now understand to be fossils described their appearances alongside and in the context of other minerals, crystals, and rocks. These early publications varied in contents of "fossils" across a wide spectrum of inorganic to organic appearances, including true fossils of differing preservation qualities, inorganic concretions, and structures with a resemblance to organisms. Over time the criteria for separating organic fossils from potentially organic or clearly inorganic materials brought about a change in the etymology of the word "fossil" itself, so accounts before the 19th century may not reflect the same use of the word fossil as modern paleontology. Both inorganic and organic fossils were illustrated in numerous books on the topic throughout the 16th century, with some attributing them to the work of God and other suggesting applications in construction or medicine. Fossils were not believed to have been organic, but instead to have exhibited the same kind of "growth" as crystals. Support for a possibly organic nature of fossils began in the 17th century, though it remained contentious as different quarries or strata yielded different fossils, which the scientists of the time did not have the context to explain. The fact that most fossils came from organisms that had never been observed alive anywhere in the world seemed to imply that these organisms were extinct, which was contrary to the belief of a perfect divine creation. Another compounding factor was that fossils of apparently marine animals were found in parts of the world that were well above sea-level. Some suggested that these fossils had accumulated in horizontal layers under the sea and that subsequent tectonic activity had displaced them from their original positions. As these observations were made over time, it was eventually understood that fossils could be used to make inferences about the history of life from their presence or absence in particular areas over time. The fossil record is the main tool used by scientists to study the history of life and assess the diversification of life over time. Very little is known about the origins of life and the oldest life forms, and this is likely a result of the poor quality of fossil preservation in older rocks. Older rocks preserve less information on average than those deposited closer to the present, and this effect is compounded across the billions of years that life is believed to have existed. Most fossils are made up of the hard parts of an organism that have been recrystallized by minerals, preserving bone, wood, or shells in a material than can be harder or denser than in life. While the hard parts are the most likely to fossilize, soft tissues can also leave impressions on sediment before they fully decompose, allowing non-mineralized parts of an organisms anatomy to be preserved. Even more rarely, a complete organism can be encased in sediment before decomposition, preserving it completely. While most fossils are body fossils (made of the actual body parts of a dead organism), some fossils can also consist of traces of the behavior or life of organisms. This can include preserved burrows, footprints or coprolites, which are grouped together and called trace fossils. However, only a small minority of all dead organisms will ever become fossils. Some things can destroy organisms before or even after fossilization, including scavengers, decomposers, or natural disasters, and fossils can even be destroyed after they are formed by taphonomic processes. Even if a fossil survives burial, it can still be destroyed by weathering if it is exposed and not collected. The habitat of an organism can also impact its chances of fossilization. Seafloors are more likely to fossilize than land, and rivers or lakes are more likely to fossilize than mountains or deserts. Fossilized teeth are very common, but are not always collected when they are found, and more complete fossils may be more likely to be collected, but they are generally rarer in absolute terms. Even after collection, fossils may not be studied for a long time. They may remain in museum storage in crates, be on display, or be otherwise unaccessible to scientists.
Geologic time
The earliest discussions in the field of geology centered around the possible origins of geological features and what implications these had on Christianity. The concept of a history of Earth had existed for a long time, and those who studies rocks of fossils had come to the idea of changes over time. However, in the beginning of the field of geology in the early 19th century, the most common explanation for causes of geological change were that they were the result of sedimentation during the Biblical Flood, rather than slow processes drawn out over millions of years. French naturalist Georges Cuvier and his contemporaries believed that the Earth was not recently created (as in Young Earth Creationism), nor had it been around forever. They instead believed that there was a vast "prehuman" or antediluvian history. Cuvier was not the first to believe in a lengthy but finite age of the Earth, but he was the first to combine this idea with his study of fossils to suggest prehistoric events could be understood through the study of geology and the fossil record. Studies on rocks and their stratigraphy continued, including the development of geological maps highlighting the relative ages of regional geologic formations, and it was still believed that the Biblical Flood was a primary explanation for the formation of these features. English geologist Charles Lyell was among the first to propose that a great flood had not occurred, and this was supported by the existence of overlapping terrestrial and marine sediment layers. He observed that the twisting, uplifting and carving of geological features supported the idea that the crust was moving continuously, and the sea level was also adjusting over time. This interpretation was not only supported by the differing levels of marine strata, but also by the shared commonality of fossils he found within them, even across large distances and at different levels above the sea. Combining these facts with his own previous work led Lyell to suggest some core principles of the history of the Earth. He suggested that here were progressive trends in the history of life, that geological history was continuously changing with periods of calm and chaos, and that the causes of these geological events were as much around in present day as in the deep past. Following the ongoing study of geology, geologic formations, and the establishment of geochronology, the geologic time scale was created to separate and categorize the vast history of Earth into a scale of named geochronologic units, defined and standardized by the International Commission on Stratigraphy. The age and duration of different units has changed over time following further restudy including absolute and relative dating of different sediments, with the current standard recognizing four eons, ten eras, 22 periods, 37 epochs and 96 ages. Present day is recognized as the Meghalayan age, of the Holocene epoch, of the Quaternary period, of the Cenozoic era, of the Phanerozoic eon. These geological time units are correlated globally through combinations of assigned times, index fossils, paleomagnetism, and other methods, with the correlation of taxa with time being termed biochronology. Through biochronology, paleontological events such as the evolution, extinction, or speciation of a taxon can be established at a point in time, and features such as mass extinctions can be identified.
Extinction
Fossils have been documented from at least as far back as Ancient Greece. However, the belief of philosophers including Plato and Aristotle was that anything that existed had existed forever and would exist forever, or was along a continuum of perfection without any gaps. As a result of this fundamental belief, evidence of extinction was ignored or explained away by naturalists for most of recorded history. It was not until the work of Cuvier with the publication of his Recherches sur les ossemens fossiles (or Investigations on fossil bones), that extinction was understood and considered the principal basis for paleontology as a science. By the early-mid 19th century, it was no longer controversial that fossil animals existed in a sequential order and as a result that fauna and flora were changing over time. Cuvier himself denied that there was any direct continuity from any of these fossils to organisms alive in the present day, and thus that all were extinct. However, he also did not believe the idea that any presently extant organisms had been alive in the past. Instead he believed that over time great "revolutions" occurred in which all living organisms went extinct, and new ones arose, which was consistent with belief in the Biblical Flood. It was not until English naturalist Charles Darwin suggested that extinction and evolution both occurred together, that a full explanation could be given for changes of life over time. The fossil record showed that there was not a predetermined length of time for which a particular organism (or group of organisms) existed, and it also gave evidence for periods where a large percentage of organisms went extinct at once, which could be the result of mass extinction events. Extinction can be seen as the final step of evolutionary change for any species. While modern biologists assess rates of extinction can be through the presence or absence of species in nature, paleontologists are limited in their understanding of this by the inherent rarity of fossils and the incompleteness of the fossil record. These difficulties make it more challenging to infer what extinction rates were in the past, and can make it difficult to differentiate between a true extinction and a "pseudoextinction", where one species evolves directly into another. Extinction of a species can occur from a variety of causes, and the intensity of extinction rates vary significantly over time. At least five mass extinction events are recognized to have occurred during the history of Earth, and it is also possible that the Earth is currently undergoing a sixth extinction as a result of human activity. However, mass extinction events only account for a small percentage of total species extinctions. Most extinctions occur as a result of other causes at differing times throughout Earth's history, which is sometimes called the background extinction rate. For most organisms in the fossil record, it is impossible to determine the cause of extinction in particular or even general cases.
Evolution
For most of human history, philosophers, theologians, and other intellectuals believed that the world was perfectly ordered by divine forces and could not have come about from natural processes. There are exceptions such as the Greek philosopher Empedocles who thought that fossils may have come from organic life that had undergone change, but this was the exception. Most religious doctrines, including Christianity and Judaism, taught that the world was created by God as it currently exists, so life could not have progressed and the natural world was instead the product of intelligent design. The evolutionary significance of the fossil record was not initially recognized because individual fossils only show snapshots of evolutionary history. However, recognition of the ability for traits to be passed to later generations was used by French naturalist Jean-Baptiste Lamarck in the 19th century to argue for evolution. Early proponents of evolution initially believed that God had set the world in motion but let it progress naturally, while critics such as Cuvier thought that intermediate forms required would have been unable to survive and so rejected the possibility of evolution outright. Influenced by the writings of Lyell, Charles Darwin studied similarities in organisms during his time aboard HMS Beagle which would eventually became the book On the Origin of Species. In it, Darwin proposed the concept of natural selection which would become fundamental to the later theory of evolution. Darwin also suggested that gaps in the fossil record were the result of incomplete fossilization and that transitional fossils would eventually be found that would corroborate the theory of evolution. Paleontologists cannot use the species concepts of modern biology due to limitations of working on fossils rather than living organisms. However, differences in the morphology of organisms based on their fossil remains can be used to separate phenotypes. Once phenotypic differences in a population of organisms accumulate, they should become genetically isolated and thus separate species. Therefore, the phenotypes observed in fossils can be used as a proxy to infer differences between species throughout deep time. It is possible that these evolutionary and morphological changes occurred slowly and gradually as is hypothesized by phyletic gradualism, or that short bursts of rapid evolution occurred in punctuated equilibrium. Evidence for both methods of macroevolution are present in the fossil record, and the discovery of new fossils continuously helps to fill gaps in our understanding of the evolutionary history of life.
History
Cuvier is generally regarded as the first paleontologist, and the origins of paleontology as a science trace their origins directly to his demonstrations that fossils in stone were traces of organisms that were once alive but had gone extinct. Despite this, he was far from the first to write about fossils or make observations about things found in rock. Isolated comments from writers about fossils can be found going back to classical antiquity. The philosopher Xenophanes (6th century BCE) believed fossil shells represented life from the past, whereas Aristotle instead explained fossils as "vaporous exhalations". Aristotle's belief was later refined into the theory of a petrifying liquid by Arabic philosopher Avicenna and German philosopher Albert of Saxony in the middle ages. Chinese naturalist Shen Kuo also proposed a theory of climate change around this time based on the presence of petrified bamboo in regions that in his time were too dry for bamboo. In unpublished notebooks, the Italian polymath Leonardo da Vinci justified an organic origin for the fossil shells available to him. His notes show observations of living mollusks and their ecology, the processes of sedimentation, and the recognition that the fossil shells had similar features, showed similar growth stages, and had similar pathologies to living mollusks. Da Vinci's study of sedimentation meant he understood why fossils were usually embedded in rocks, and his notes demonstrate a very modern interpretation of the origin of fossils. He rejected the Aristotelian theory of vapors and also did not believe that the Biblical Flood was the primary cause of fossil formation. Da Vinci's notebooks may have inspired others of the time to accept a biologic origin of fossils, but this belief was not accepted by everyone. In addition to his study of body fossils, da Vinci is also credited as the founder of the field of ichnology, which is primarily concerned with trace fossils and how they can provide insights into the behavior of extinct organisms. In the 17th century, naturalists like the Danish scientist Nicolas Steno and the English polymath Robert Hooke provided further discussions on the origins of fossils. The general belief was that fossils were of organic origin, but that they had been fossilized by petrifying liquids and moved into elevation by the Biblical Flood or some other means. Conversely, the English physician Martin Lister completely rejected the possibility of organic fossil origins. The fossils available to Steno, da Vinci, and others mentioned above were primarily the easily-identifiable shells of marine animals, and their organic origin was a relatively straightforward inference. The fossils in England were from rocks dating to the Jurassic or Carboniferous and came from a variety of different organisms that bore no clear resemblance to modern organisms. Many explanations were suggested for the possible inorganic or organic origins of fossils, how they came to be lithified, and how they ended up far above the sea, but the ideas of extinction and deep time had not yet been developed, so an explanation eluded naturalists of the time.
A significant moment in the history of paleontology was the publication of the 1796 paper On the species of living and fossil elephants by Georges Cuvier, which contained detailed evidence for extinction. Cuvier named the fossil taxon Megatherium, based on bones found in Paraguay. The large size of these bones made it unlikely that they were from an extant, but undiscovered, animal. Cuvier reached a similar conclusion regarding the fossils named the mastodon, with the uniqueness of these animals demonstrating that they belonged to species that were no longer alive and thus extinct. To further justify this conclusion, Cuvier extensively studied the fossils of elephants and proved the distinction of mammoths from Siberia and Europe from their living relatives. Presenting this work on the extinction of the megafauna, Cuvier termed the events that led to their disappearance "revolutions", contrasting with the idea of gradual change in the environment and the fauna within it. Of the three possibilities leading to the disappearance, Cuvier supported extinction over migration as well as over evolution as suggested by Lamarck, with his view that extinction and evolution were conflicting explanations. Cuvier also studied the comparative anatomy of both living and fossil organisms and developed a way to assess their morphological characters, which opened the door for developing an understanding of the animals of the past. Developments in the fields of stratigraphy and paleontology following the work of Cuvier became widespread throughout Europe, and the classification of extinct organisms into different groups that included their living relatives also proliferated. While most of Cuvier's early studies had been on mammals, there were some fossils with no close living analogues such as the bird-like fossil reptile he called the Ptero-dactyle or the fish-like marine reptiles that were eventually named ichthyosaurs. It was in 1822 that Henri Marie Ducrotay de Blainville, a former student of Cuvier, introduced the name paléontologie for the study of these ancient beings. He had earlier introduced the names paléozoologie and paléosomiologie for the studies of fossil animals and fossils in general, respectively, but the latter did not see widespread use and paleontology was the name generally adopted for the field by naturalists of the time. Some of the most significant discoveries of this early time in paleontology were made by Mary Anning and her family, who uncovered skeletons from a variety of marine reptiles and other animals in the Lyme Regis region including Ichthyosaurus and Plesiosaurus. These animals were geologically older than the mammals of Cuvier's earlier work, and this relative age became the study of stratigraphy which enabled scientists to date and order animals relative to one another in geologic time. The works of Cuvier and Lamarck on extinction and the history of life, and the works of Lyell and English geologist Adam Sedgwick on geology, were all synthesized by Charles Darwin in his seminal works on the theory of evolution. He suggested that the history of life was full of gradual changes, with the constant presence of extinction acting as the driver evolution through natural selection. This was validated by multiple discoveries soon after Darwin began publishing. The discovery of the theropods Compsognathus and Archaeopteryx demonstrated evidence for the progressive evolution of birds from other reptiles, which shifted paleontological study in the direction of studying the evolution of life.
For a time paleontology was considered a sub-discipline of geology with relatively little study given to the biological aspects of the field, and paleontology was generally not treated as an important field of study of either science. Over the subsequent decades, geology and biology advanced to theory-based analysis while paleontology lagged behind as a field focused primarily on stratigraphy. This changed with the development of paleobiology in the second half of the 20th century. This shift was driven by conceptual changes in the study of evolution and phylogenetics and the emergence of new ways to study geology through biostratigraphy, paleobiogeography, taphonomy and paleoclimatology. Phylogenetics were developed as a way to quantitatively analyze and interpret the evolution and relationships of organisms, providing context and predictability for evolutionary processes and the impacts of mass extinctions and their recoveries. Paleoecology itself has seen the emergence of subdisciplines including the field of taphonomy to study the nature of the fossil record. Emphasis was also given to the analysis of diversity and the distribution of taxa, the study of trace fossils, the understanding of paleoenvironments, and conservation paleobiology. Advancements in technology and the analytical tools of other sciences have also been integrated into paleontology including geochemical analysis, molecular biology, and other computer-aided visualization or analysis techniques. The heyday of paleontology was arguably in the Victorian era, with little substantial change since beyond the notable discoveries of new taxa. These on their own have done little to change our overall understanding of the history of life. However, the history of life is not just the story of evolutionary changes, and paleontology has increasingly broadened to include a wider variety of scientific questions. The sizes of the largest dinosaurs, pterosaurs, or arthropods pose interesting questions to study in the fields of biomechanics, ontogeny, and physiology. Diversification and mass extinction can be predicted and better understood from the study of phylogenetics, and as technologies and precision improve, the depth to which we understand life of the past will increase.
Applications
Paleontology both draws from and contributes to the fields of geology and biology, despite historically being dismissed as an undemanding science. Analysis and description of fossils allows the researchers to illustrate biological, geological, ecological and tectonic changes and phenomena which have implications for our understanding of science in the present. Many disciplines and areas of study interact with paleontology and overlap in some areas with the field. Through this overlap, paleontology has the ability to better our understanding of the origin, diversity and evolution of life, and can be used by other fields to investigate patterns in the fossil record. In the modern day, paleontology is viewed as important by researchers. Its study enables scientists to understand the history of life. It can explain different worlds of the past and the impact of a changing climate and biodiversity, and paleontology helps expand our understanding of both evolution and extinction. Subfields of paleontology also enable geologists to robustly establish the ages of various rock formations.
History of life
Paleontological discoveries have discussed the origins and history of life for centuries, with very little knowledge of life before the Cambrian for a significant amount of time. Fossils from prior to the Cambrian were limited to 2.1 billion year old fossilized algae and possibly "plants" until the discoveries of fauna in the Bitter Springs Group and Apex chert of Australia, the Mistaken Point Formation of Canada, and the Doushantuo Formation of China, all of which have significantly expanded knowledge of the Ediacaran biota that includes a range of life from microscopic single-celled organisms to macroscopic multicellular life. Fossil discoveries have also improved knowledge about the Cambrian explosion with the discoveries of multiple new lagerstätte deposits. The Burgess Shale was one of the first such deposits and has been further explored, and around 40 other Burgess-type localities are now known globally. These localities are filled with soft-bodied taxa that show the decline of the Ediacaran biota and the emergence of other kinds of metazoan life. The refinement of Cambrian stratigraphy will also improve the understanding of these early faunas and how they changed over time. Through advances in paleontology many other evolutionary paths have become better understood even in more recent life. The evolution of birds is now understood to have occurred from gradual evolutionary changes in saurischian dinosaurs up to the point where it is difficult to draw a line between what dinosaurs are or are not birds. The origins of dinosaurs themselves are better understood from the discoveries of multiple near-dinosaur taxa. Discoveries within the Eocene of fossil mammals have allowed for the evolution of whales to be nearly completely understood, with the fully terrestrial mesonychids becoming gradually amphibious before becoming aquatic swimmers. Relatives of modern whales such as Basilosaurus were obligate swimmers, but even then had not developed the bauplan of modern cetaceans that occurred over further gradual evolution. The evolution of reptile groups such as ichthyosaurs and turtles, while still controversial, is much better understood with finds such as the early incompletely-shelled turtle Proganochelys. Human evolution is also much better understood from progress in paleontology, including both the evolution of hominids from basal primates as well as the speciation and origins of humans within the hominids. Fossils of Australopithecus and Ardipithecus show that humans never transitioned through an ape-like stage, instead being bipedal with adaptations for arboreal locomotion. Adripithecus is known from lowland forest environments, and not grasslands, suggesting the origins of humanity within a variable and unpredictable habitat. The evolution of humans within the genus Homo is similarly complex and does not follow a clean linear path as sometimes described. Some species of Homo may have overlapped in time and place with others, but all show that then evolution of the genus was likely in Africa. Advancements and new discoveries have also shown that the neanderthals were a complex society with the use of tools, clothes, and having their own mythology. DNA from neanderthals and humans show substantial differences, but also that there was interbreeding between populations.
Extinction events
The idea of a mass extinction has been around since the beginning of paleontology and is generally accepted as true events that drive the evolution of life. However, the question of what makes an extinction event a "mass extinction" is still uncertain. On the scale of geologic time, mass extinction events happen rapidly, and such rapid events can be caused by both gradual environmental processes and large-scale catastrophes. A notable exception to this rule is the Cretaceous-Paleogene extinction event, which is believed to have been caused by an asteroid impact which caused global wildfires and a disruption of the nutrient cycle in the ocean. If this is the case, it would be an unprecedentedly rapid extinction event, occurring over the course of one or a few years. However, even this extinction's cause is debated. Some have suggested that it was caused by marine regression or volcanism that occurred near or at the same time as an impact. No other extinction events can be linked clearly with an extraterrestrial cause. Glaciation and subsequent global warming has been suggested as a cause for the Late Ordovician mass extinction, and the volcanic activity of the Siberian Traps large igneous province has been suggested as the primary cause for the Permian-Triassic mass extinction. The causes of the Late Devonian mass extinction and the Triassic-Jurassic mass extinction remain mostly uncertain to this day. The period of ecological recovery following a mass extinction is also a significant time for biodiversity and adaptive radiation. The term "disaster species" has been applied to the organisms that follow an ecological disruption, and there are many known from the fossil record. Following the Cretaceous-Paleogene extinction, there is a large spike in the abundance of fossil ferns that is interpreted as an early post-extinction flora that would later be overtaken by different floral communities. There is a similarly rapid diversification of small, generalist mammals for the first 3 million years before more diverse faunal communities evolved. However, not all mass extinctions have similarly rapid diversification events. The recovery period following the Permian-Triassic extinction took up to 10 million years. The recovery of ecosystems from mass extinctions involves the evolution of novel ecological relationships between groups of animals that would not have been possible in the pre-extinction ecosystem.
Biostratigraphy
Fossils have been used for stratigraphic correlation since at least the 18th century. Observed changes in the fossils found through geologic time led to the principle of ecological succession, however this study was not elaborated on until the 1960s. The first and last appearance of a taxon in the fossil record can be used to compare the relative ages of different lithographic sections of sediment. This principle allows for relative ages of different sediments to be determined more precisely. These "index fossils" are combined with measurements of volcanic ash, paleomagnetic reversals, or pre-dated sediments to make precise measurements of geologic time. For example, the Jurassic Period was named and defined based on ten main subdivisions identified through the English and French assemblages of ammonites, some of which are still in use today. Biostratigraphy is also applied to the analysis of stratotype sections and boundaries of geologic time units. It can also use the first or last appearance date of a taxon to establish time periods that are independent of their constituent strata. The geologic time scale is based primarily on the biostratigraphy (correlating strata) and equivalent biochronology (correlating times) of the appearance and disappearance of various fossil taxa. Some factors can introduce uncertainty into this process including the quality or quantity of sampled fossils. Different graphical and numerical methods are used in the construction of the geologic time scale. Even the Ediacaran, which is poorly represented through fossils, can be assessed using biostratigraphy in combination with chemostratigraphy and absolute dating. The biostratigraphy of the Ordovician and Silurian is based primarily on fossils of graptolites and conodonts. Other common groups used in zonation include ammonites, foraminifera, and plant pollen, where it is preserved.
Classification
The foundation of modern taxonomy is the scheme of hierarchy adopted by Carl Linnaeus where taxa were grouped in taxonomic categories. While Linnaean hierarchy was not the first system, it formed the basis for subsequent systems, with seven principle categories arising to classify life: Kingdom, Phylum, Class, Order, Family, Genus, and Species (Division instead of Phylum for botany). Intermediate categories were also developed, though they were not considered mandatory to specify. Following On the Origin of Species in 1859, the concept of taxonomic hierarchy shifted to describe common descent through evolution rather than similarity, rendering some previously-accepted groups non-monophyletic as they excluded descendants. The continued trend of emphasizing evolutionary descent has led to a reduced significance of Linnaean taxonomy because of these inconsistencies. An important development classification in modern paleontology was the adoption of phylogenetic systematics as a tool to study the evolutionary tree of life. The use of phylogenetics allows scientists to quantitatively describe the relatedness of organisms through reconstructions of evolutionary trees. Phylogenetic analysis was first applied to the fields of entomology and ichthyology, after extensive debates within those fields before being adopted by evolutionary biologists more broadly. By using systematics, scientists can test and retest hypotheses about evolutionary relationships, and the results are typically displayed as a cladogram. The widespread use of systematics coincided with the advent of molecular biology, which has allowed scientists to use genetic data in addition to morphological data to study evolutionary relationships. Classification systems in general have also shifted in favor of phylogenetics. The Linnean classification scheme with its well-defined taxonomic ranks has gradually fallen out of use, because it does not generally perform well as a reflection of true evolutionary relationships. Further applications of classification to paleontology include more focused issues such as delineating the distinction between microevolution and macroevolution. Microevolutionary paleontology is the study of how evolutionary pressure impacts the ability of single individuals to survive over others, while macroevolutionary paleontology focuses on the ability of whole species to survive over others. Some scientists have suggested that microevolution and macroevolution are separate processes, with morphological changes originating from speciation rather than gradual anagenesis of a population. Others have argued