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Artist's reconstruction of Waptia fieldensis.
The evolutionary history of life on Earth traces the processes by which living and fossil organisms have evolved since life on the planet first originated until the present day. Earth formed about 4.5 Ga (billion years ago) and life appeared on its surface within one billion years. Microbial mats of coexisting bacteria and archaea were the dominant form of life in the early Archean. The evolution of oxygenic photosynthesis, around 3.5 Ga, eventually led to the oxygenation of the atmosphere, beginning around 2.4 Ga. The earliest evidence of eukaryotes (complex cells with organelles), dates from 1.85 Ga, and while they may have been present earlier, their diversification accelerated when they started using oxygen in their metabolism. Later, around 1.7 Ga, multicellular organisms began to appear, with differentiated cells performing specialised functions. The earliest land plants date back to around 450 Ma (million years ago), although evidence suggests that algal scum formed on the land as early as 1.2 Ga. Land plants were so successful that they are thought to have contributed to the late Devonian extinction event. Invertebrate animals appear during the Vendian period, while vertebrates originated about525 Ma during the Cambrian explosion. During the Permian period, synapsids, including the ancestors of mammals, dominated the land, but the Permian–Triassic extinction event251 Ma came close to wiping out all complex life. (see more...)

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The Ediacaran biota were ancient lifeforms representing the earliest known complex multicellular organisms. They appeared soon after the Earth thawed from the Cryogenian period's extensive glaciers, and largely disappeared soon before the rapid appearance of biodiversity known as the Cambrian explosion, which saw the first appearance in the fossil record of the basic patterns and body-plans that would go on to form the basis of modern animals. Little of the diversity of the Ediacaran biota would be incorporated in this new scheme, with a distinct Cambrian biota arising and usurping the organisms that dominated the Ediacaran fossil record.

Some Ediacaran organisms might have been closely related to groups that would rise to prominence later, but most non-microscopic fossils are morphologically distinct from later lifeforms, resembling discs, mud-filled bags, or quilted mattresses. Classification is difficult, and the assignment of some species even at the level of kingdom — animal, fungus, protist or otherwise — is uncertain: one paleontologist has even gained support for a separate kingdom Vendobionta. Their strange form and apparent disconnectedness from later organisms have led some to consider them a "failed experiment" in multicellular life, with later multicellular life independently re-evolving from unrelated single-celled organisms. (more...)

Psittacosaurus skeleton.
Psittacosaurus (meaning 'parrot lizard') was a genus of psittacosaurid ceratopsian dinosaur from the Early Cretaceous Period of what is now Asia, about 130 to 100 million years ago. It is notable for being the most species-rich dinosaur genus; At least eight extinct species are recognized from fossils found in different regions of China and Mongolia, with possible additional species from Thailand and Russia.

All species of Psittacosaurus were gazelle-sized bipedal herbivores characterized by a high, powerful beak on the upper jaw. At least one species had long, quill-like structures on its tail and lower back, possibly serving a display function. Psittacosaurs were extremely early ceratopsians and, while they developed many novel adaptations of their own, they also shared many anatomical features with later ceratopsians, such as Protoceratops and the elephant-sized Triceratops.

Psittacosaurus is not as familiar to the general public as its distant relative Triceratops but it is one of the most completely known dinosaur genera. Fossils of over 150 individuals have been collected so far, including many near-complete skeletons. Most different age classes are represented, from nestling through to adult, which has allowed several detailed studies of Psittacosaurus growth rates and reproductive biology. The abundance of this dinosaur in the fossil record has led to its use as an index fossil for Early Cretaceous sediments of central Asia. (see more...)

Triceratops at the Smithsonian.
Triceratops (meaning 'three-horned face') was a herbivorous genus of ceratopsid dinosaur that lived during the late Maastrichtian stage of the Late Cretaceous Period, around 68 to 65 million years ago (mya) in what is now North America. It was one of the last dinosaurs to appear before the great Cretaceous–Paleogene extinction event. Bearing a large bony frill and three horns on its large four-legged body and conjuring similarities with the modern rhinoceros, Triceratops is one of the most recognizable of all dinosaurs. Though it shared the landscape with, and was preyed upon by the fearsome Tyrannosaurus, it is unclear whether the two battled the way they are commonly depicted in movies and children's dinosaur books.

Although no complete skeleton has been found, Triceratops is well-known from numerous partial specimens collected since the introduction of the genus in 1887. The function of their frills and three distinctive facial horns has long inspired debate. Although traditionally viewed as defensive weapons against predators, the latest theories explain how these features were probably primarily used in display for courtship and dominance, much like the antlers and horns of modern reindeer, mountain goats or rhinoceros beetles.

Triceratops is the best-known of the ceratopsids, though the genus' exact placement within the group has been a point of contention amongst paleontologists. Two species, T. horridus and T. prorsus, are considered valid, although many other species have been named. (see more...)

Close up of Compsognathus replica's head.
Compsognathus (meaning 'elegant jaw') was a small, bipedal, carnivorous theropod dinosaur. The animal was the size of a chicken and lived around 150 million years ago, during the early Tithonian stage of the late Jurassic Period, in what is now Europe. Paleontologists have found two well-preserved fossils, one in Germany in the 1850s and the second in France more than a century later. Compsognathus is one of the few dinosaurs for which the diet is known with certainty: the remains of small, agile lizards are preserved in the bellies of both specimens. Teeth discovered in Portugal may be further fossil remains of the genus. Although not recognized as such at the time of its discovery, Compsognathus is the first dinosaur known from a reasonably complete skeleton. Today, C. longipes is the only recognized species, although the larger specimen discovered in France in the 1970s was once thought to belong to a separate species, C. corallestris. Until the 1980s and 1990s, Compsognathus was the smallest known dinosaur and the closest supposed relative of the early bird Archaeopteryx. Thus, the genus is one of the few dinosaur genera to be well known outside of paleontological circles. (see more...)

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Velociraptor (meaning 'swift thief') was a genus of dromaeosaurid theropod dinosaur that existed approximately 83 to 70 million years ago during the later part of the Cretaceous Period. There is only one currently recognized species, V. mongoliensis, although others have been assigned in the past. Fossils of this species have been found in central Asia, from both Inner and Outer Mongolia.

Smaller than other dromaeosaurids like Deinonychus and Achillobator, the turkey-sized Velociraptor nevertheless shared many of the same anatomical features. It was a bipedal carnivore with a long, stiffened tail and had an enlarged, sickle-shaped claw on each hindfoot, which is thought to have been used to kill its prey. Velociraptor can be distinguished from other dromaeosaurids by its long and low skull, with an upturned snout.

Due in large part to its prominent role in Michael Crichton's novel Jurassic Park and the subsequent motion picture series, Velociraptor (commonly shortened to 'raptor') is one of the dinosaur genera most familiar to the general public. It is also well-known to paleontologists, with over a dozen recovered fossil skeletons — the most of any dromaeosaurid. One particularly famous specimen shows a Velociraptor locked in combat with a Protoceratops. (see more...)

Stegosaurus statue at Bałtów Jurassic Park, Bałtów, Poland.
Stegosaurus (meaning 'roof-lizard') is a genus of stegosaurid armoured dinosaur from the Late Jurassic period (Kimmeridgian to Early Tithonian) in what is now western North America. However, in 2006 a specimen of Stegosaurus was announced from Portugal, suggesting that they were present in Europe as well. Due to its distinctive tail spikes and plates, Stegosaurus is one of the most recognisable dinosaurs, along with Tyrannosaurus, Triceratops and Apatosaurus. At least three species have been identified in the upper Morrison Formation and are known from the remains of about 80 individuals. They lived some 155 to 145 million years ago, in an environment and time dominated by the giant sauropods Diplodocus, Camarasaurus and Apatosaurus. A large, heavily built and herbivorous quadruped, Stegosaurus had a distinctive and unusual posture, with a heavily arched back, short forelimbs, head held low to the ground and a stiffened tail held high in the air. Its array of plates and spikes have been the subject of much speculation. The spikes were most likely used for defence, while the plates have also been proposed as a defensive mechanism, as well as having display and thermoregulatory (heat control) functions. Stegosaurus was the largest of all the stegosaurians (bigger than genera such as Kentrosaurus and Huayangosaurus) and, although roughly bus-sized, it nonetheless shared many anatomical features (including the tail spines and plates) with the other stegosaurian genera. (see more...)

Modern depiction of Diplodocus carnegiei.

Diplodocus (meaning 'double bar') is a genus of diplodocid sauropod dinosaur whose fossilised skeleton was first discovered in 1878. The generic name refers to its double-beamed chevron bones (Greek diplos/διπλος meaning 'double' and dokos/δοκος meaning 'wooden beam' or 'bar') located in the underside of the tail. They were initially believed to be unique to Diplodocus; however, they have since then been discovered in other diplodocids. It lived in what is now western North America at the end of the Jurassic Period. Diplodocus was one of the more common dinosaurs found in the Upper Morrison Formation, about 150 to 147 million years ago, in what is now termed the Kimmeridgian and Tithonian stages. This was an environment and time dominated by gigantic sauropod dinosaurs such as Camarasaurus, Barosaurus, Apatosaurus and Brachiosaurus. Diplodocus is among the most easily identifiable dinosaurs, with its classic dinosaur shape, long neck and tail and four sturdy legs. For many years, it was the longest dinosaur known. Its great size may have been a deterrent to the predators Allosaurus and Ceratosaurus: their remains have been found in the same strata, which suggests they coexisted with Diplodocus. (see more...)

Iguanodon (meaning 'iguana tooth') is a genus of ornithopod dinosaur that lived roughly halfway between the first of the swift bipedal hypsilophodontids and the ornithopods' culmination in the duck-billed dinosaurs. Most Iguanodon species lived between 140 and 120 million years ago, in the Valanginian to Barremian ages of the Early Cretaceous Period of Europe, although possible remains are known from North America, Asia, and Africa. Iguanodon's most distinctive features were its large thumb spikes, which were possibly used for defence against predators.

Discovered in 1822 and described three years later by English geologist Gideon Mantell, Iguanodon was the second dinosaur formally named, after Megalosaurus. Together with Megalosaurus and Hylaeosaurus, it was one of the three genera originally used to define Dinosauria. A large, bulky herbivore, Iguanodon is a member of Iguanodontia, along with the duck-billed hadrosaurs. The taxonomy of this genus continues to be a topic of study as new species are named or long-standing ones reassigned to other genera.

Scientific understanding of Iguanodon has evolved over time as new information has been obtained from the fossils. The numerous specimens of this genus, including nearly complete skeletons from two well-known bonebeds, have allowed researchers to make informed hypotheses regarding many aspects of the living animal, including feeding, movement, and social behaviour. As one of the first scientifically well-known dinosaurs, Iguanodon has occupied a small but notable place in the public's perception of dinosaurs, its artistic representation changing significantly in response to new interpretations of its remains. (see more...)

T. rex skull.
Tyrannosaurus (meaning 'tyrant lizard') is a genus of tyrannosaurid theropod dinosaur. The species Tyrannosaurus rex, commonly abbreviated to T. rex, is one of the dinosaurs most often featured in popular culture around the world. It hails from what is now western North America.

Like other tyrannosaurids, Tyrannosaurus was a bipedal carnivore with a massive skull balanced by a long, heavy tail. Relative to the large and powerful hindlimbs, Tyrannosaurus forelimbs were small and retained only two digits. Although other theropods rivaled or exceeded T. rex in size, it was the largest known tyrannosaurid and one of the largest known land predators, measuring over 13 metres (43 ft) in length and up to 7.5 tons in weight.

Fossils of some T. rex have been found in North American rock formations dating to the very end of the Cretaceous Period (late Maastrichtian stage, 65 million years ago); it was among the last dinosaurs to exist prior to the Cretaceous–Paleogene extinction event. More than 30 specimens of T. rex have now been identified, some nearly complete, which has allowed significant research into many aspects of its biology, including its life history and biomechanics. The feeding habits and potential speed of T. rex remain controversial. (see more...)

A model of Archaeopteryx lithographica on display at the Oxford University Museum of Natural History.
Archaeopteryx (meaning 'ancient feather') is the earliest and most primitive known avian to date. It lived in the late Jurassic Period around 155-150 million years ago in what is now southern Germany. At the time Archaeopteryx lived, Europe was an archipelago of islands in a shallow warm tropical sea, much closer to the equator than it is now. Archaeopteryx lived during the time of the dinosaurs, yet was set apart from them because of the inclusion of both avian and theropod dinosaur features. Similar in size and shape to a European Magpie, it bore broad, rounded wings and a long tail. Archaeopteryx could grow to about half a metre, or 1.6 feet in length. Its feathers resembled those of modern birds but Archaeopteryx was rather different from any bird known today, in that it had jaws lined with sharp teeth, three 'fingers' ending in curved claws and a long bony tail. These features, which are consistent with theropod dinosaurs, have made the Archaeopteryx a hot topic in the debate on evolution. Indeed, in 1862 the description of the first intact specimen of Archaeopteryx, just two years after Charles Darwin published The Origin of Species, set off a firestorm of debate about evolution and the role of transitional fossils that endures to this day. The eleven fossils currently classified as Archaeopteryx are the oldest evidence of feathers on the planet and the only ones dated from Jurassic times. Furthermore, their advanced nature and placement suggest their origins must have been even earlier. All remains have been regarded by most as a single species, though this has been debated. (see more...)

Thescelosaurus neglectus drawing.
Thescelosaurus (meaning 'wondrous lizard') was a genus of small ornithopod dinosaur that appeared at the very end of the Late Cretaceous period in North America and was a member of the last dinosaurian fauna before the Cretaceous–Paleogene extinction event around 65.5 million years ago. The preservation and completeness of many of its specimens indicate that it may have preferred to live near stream channels.

This bipedal ornithopod is known from several partial skeletons and skulls that indicate it grew to between 2.5 and 4.0 meters (8.2 to 13.1 feet) in length on average. It had sturdy hind limbs, small wide hands, a head with an elongate pointed snout, and possibly small armor scutes along the midline of the back. This genus of dinosaur is regarded as a specialized hypsilophodont and a herbivore. Several species have been suggested for this genus, but only one, T. neglectus, is currently recognized; the others have been given their own genera, or are believed to be the same as T. neglectus.

The genus attracted media attention in 2000, when a specimen unearthed in 1993 in South Dakota was interpreted as including a fossilized heart. There was much discussion over whether the remains were actually of a heart. Many scientists now doubt the identification of the object and the implications of such an identification. (see more...)

Model Styracosaurus on display at Bałtów Jurassic Park, Poland.
Styracosaurus (meaning 'spiked lizard') was a genus of herbivorous ceratopsian dinosaur from the Cretaceous Period (Campanian stage), about 74 to 72 million years ago. It had four to six long horns extending from its neck frill, a smaller horn on each of its cheeks and a single horn protruding from its nose, which was around 60 centimeters (2 ft) long and 15 centimeters (6 in) wide. The function or functions of the horns and frills have been the subject of debate for many years. Styracosaurus was a large dinosaur, reaching lengths of 5.5 meters (18 ft) and weighing nearly 3 tons. It stood about 1.8 meters (6 ft) tall. Styracosaurus possessed four short legs and a bulky body. Its tail was rather short. It also had a beak and flat cheek teeth, indicating that its diet was herbivorous. Like other ceratopsians, this dinosaur may have been a herd animal, traveling in large groups, as suggested by bonebeds. Named by Lawrence Lambe in 1913, Styracosaurus is a member of the Centrosaurinae. Three species, S. albertensis, S. ovatus, and S. parksi are currently assigned to Styracosaurus, though the last species may be synonymous with S. albertensis. Other species assigned to the genus have since been reassigned elsewhere. (see more...)

Daspletosaurus skeleton at the Field Museum in Chicago.
Daspletosaurus (meaning 'frightful lizard') was a genus of tyrannosaurid theropod dinosaur that lived in western North America between 80 and 73 million years ago, during the Late Cretaceous Period. Fossils of the only named species (D. torosus) were found in Alberta, although other possible species from Alberta, Montana and New Mexico await description. Including these undescribed species, Daspletosaurus is the most species-rich genus of tyrannosaur.

Daspletosaurus is closely related to the much larger and more recent Tyrannosaurus. Like most known tyrannosaurids, it was a multi-ton bipedal predator equipped with dozens of large, sharp teeth. Daspletosaurus had the small forelimbs typical of tyrannosaurids, although they were proportionately longer than in other genera. It was probably similar in weight to a modern white rhinoceros or a small elephant.

As an apex predator, Daspletosaurus was at the top of the food chain, probably preying on large dinosaurs like the ceratopsid Centrosaurus and the hadrosaur Hypacrosaurus. In some areas, Daspletosaurus coexisted with another tyrannosaurid, Gorgosaurus, though there is some evidence of niche differentiation between the two. While Daspletosaurus fossils are rarer than other tyrannosaurids, the available specimens allow some analysis of the biology of these animals, including social behavior, diet and life history. (see more...)

Deinonychus skeleton.
Deinonychus (meaning 'terrible claw') was a genus of carnivorous dromaeosaurid dinosaur. This 3-4 metre (10-13 ft) long dinosaur lived during the early Cretaceous Period (Aptian - Albian stages, 121 to 98.9 million years ago). Fossils of the only named species (D. antirrhopus) have been recovered from Montana, Wyoming and Oklahoma, though teeth that may belong to Deinonychus have been found much farther east in Maryland.

Its name refers to the unusually large, sickle-shaped talon on the second toe of each hind foot, which was probably held retracted while the dinosaur walked on the third and fourth toes. It was commonly thought that Deinonychus would kick with the sickle claw to slash at its prey but recent tests on reconstructions of similar Velociraptor talons suggest that the claw was used to stab, not slash. As in other dromaeosaurids, the tail was stiffened by a series of elongated bones and bone processes. This might have given Deinonychus greater balance and turning ability. In both the Cloverly and Antlers Formation, Deinonychus remains have been found closely associated with those of the ornithopod Tenontosaurus. Teeth discovered associated with Tenontosaurus specimens imply it was hunted or at least scavenged upon by Deinonychus.

Paleontologist John Ostrom's study of Deinonychus in the late 1960s revolutionized the way scientists thought about dinosaurs, igniting the debate on whether or not dinosaurs were warm-blooded. Before this, the popular conception of dinosaurs had been one of plodding, reptilian giants. Ostrom noted lightweight bones and stiffened tendons which revealed an active, agile predator. (see more...)

Head of Majungasaurus.
Majungasaurus (meaning 'Mahajanga lizard') is a genus of abelisaurid theropod dinosaur that lived in Madagascar from 70 to 65 million years ago, at the end of the Cretaceous Period. Only one species (M. crenatissimus) has been identified. This dinosaur was briefly called Majungatholus, a name which is now considered a junior synonym of Majungasaurus.

Like other abelisaurids, Majungasaurus was a bipedal predator with a short snout. Although the forelimbs are not completely known, they were very short, while the hindlimbs were longer and very stocky. It can be distinguished from other abelisaurids by its wider skull, the very rough texture and thickened bone on the top of its snout, and the single rounded horn on the roof of its skull, which was originally mistaken for the dome of a pachycephalosaur. It also had more teeth in both upper and lower jaws than most abelisaurids.

Known from several well-preserved skulls and abundant skeletal material, Majungasaurus has recently become one of the best-studied theropod dinosaurs from the Southern Hemisphere. It appears to be most closely related to abelisaurids from India rather than South America or continental Africa, a fact which has important biogeographical implications. Majungasaurus was the apex predator in its ecosystem, mainly preying on sauropods like Rapetosaurus, and is also the only dinosaur for which direct evidence of cannibalism is known. (see more...)

Artist's rendition of Parasaurolophus walkeri.
Parasaurolophus (meaning 'near crested lizard') was a genus of ornithopod dinosaur from the Late Cretaceous Period of what is now North America, about 76-73 million years ago. It was a herbivore that walked both as a biped and a quadruped. Three species are recognized: P. walkeri (the type species), P. tubicen, and the short-crested P. cyrtocristatus. Remains are known from Alberta (Canada), and New Mexico and Utah (United States). It was first described in 1922 by William Parks from a skull and partial skeleton in Alberta. Parasaurolophus is a hadrosaur, part of a diverse family of Cretaceous dinosaurs known for their range of bizarre head adornments. This genus is known for its large, elaborate cranial crest, which at its largest forms a long curved tube projecting upwards and back from the skull. A similar crest is found on Charonosaurus from China, which may have been its closest relative. The crest has been much discussed by scientists; the consensus is that major functions included visual recognition of both species and gender, acoustic resonance, and thermoregulation. It is one of the rarer duckbills, known from only a handful of good specimens. (see more...)

An asteroid impacting on Earth.
The Cretaceous–Paleogene extinction event, often referred to as the Cretaceous–Tertiary extinction event, was the large-scale mass extinction of animal and plant species in a geologically short period of time, approximately 66 million years ago (mya). The K–Pg extinction event is associated with a geological signature, usually a thin band dated to that time and found in various parts of the world, known as the K–Pg boundary. K is the traditional abbreviation for the Cretaceous Period, and Pg is the abbreviation for the Paleogene Period. The event marks the end of the Mesozoic Era, and the beginning of the Cenozoic Era.

Non-avian dinosaur fossils are only found below the K–Pg boundary and became extinct immediately before or during the event. A very small number of dinosaur fossils have been found above the K-Pg boundary, but they have been explained as reworked, that is, fossils that have been eroded from their original locations then preserved in later sedimentary layers. Mosasaurs, plesiosaurs, pterosaurs and many species of plants and invertebrates also became extinct. Mammalian and bird clades passed through the boundary with few extinctions, and radiation from those Maastrichtian clades occurred well past the boundary. Rates of extinction and radiation varied across different clades of organisms.

Many scientists theorize that the Cretaceous–Paleogene extinctions were caused by one or more catastrophic geological events such as massive asteroid impacts or increased volcanic activity. Several impact craters and massive volcanic activity in the Deccan traps have been dated to the approximate time of the extinction event. These geological events may have reduced sunlight and hindered photosynthesis, leading to a massive disruption in Earth's ecology. Other researchers believe the extinction was more gradual, resulting from slower changes in sea level or climate. (see more...)

Lambeosaurus sketch.
Lambeosaurus (meaning 'Lambe's lizard') is a genus of hadrosaurid dinosaur that lived about 76 to 75 million years ago, in the Late Cretaceous Period (Campanian) of North America. This bipedal/quadrupedal, herbivorous dinosaur is known for its distinctive hollow cranial crest, which in the best-known species resembled a hatchet. Several possible species have been named, from Alberta (Canada), Montana (United States), and Baja California (Mexico), but only the two Canadian species are currently well known. At about 15 meters (50 ft) long, the Mexican species L. laticaudus was one of the longest ornithischians. The other species were more modestly sized. Lambeosaurus was belatedly described in 1923 by William Parks, over twenty years after the first material was studied by Lawrence Lambe. The genus has had a complicated taxonomic history, in part because small-bodied crested hadrosaurids now recognized as juveniles were once thought to belong to their own genera and species. Currently, the various skulls assigned to the type species L. lambei are interpreted as showing age differences and sexual dimorphism. Lambeosaurus was closely related to the better known Corythosaurus, which is found in slightly older rocks, as well as the less well-known genera Hypacrosaurus and Olorotitan. All had unusual crests, which are now generally assumed to have served social functions like noisemaking and recognition. (see more...)

Illustration of trilobite fossils by Joachim Barrande.
The history of paleontology traces the history of the effort to study the fossil record left behind by ancient life forms. Although fossils had been studied by scholars since ancient times, the nature of fossils and their relationship to life in the past became better understood during the 17th and 18th centuries. At the end of the 18th century the work of Georges Cuvier ended a long running debate about the reality of extinction and led to the emergence of paleontology as a scientific discipline.

The first half of the 19th century saw paleontological activity become increasingly well organized. This contributed to a rapid increase in knowledge about the history of life on Earth, and progress towards definition of the geologic time scale. 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. After Charles Darwin published Origin of Species in 1859, much of the focus of paleontology shifted to understanding evolutionary paths.

The last half of the 19th century saw a tremendous expansion in paleontological activity, especially in North America. The trend continued in the 20th century with additional regions of the Earth being opened to systematic fossil collection, as demonstrated by a series of important discoveries in China near the end of the 20th century. There was also a renewed interest in the Cambrian explosion that saw the development of the body plans of most animal phyla. (see more...)

A Marella fossil.
Paleontology or palaeontology (/ˌpliɒnˈtɒləi, ˌpæli-, -ən-/) is the scientific study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments (their paleoecology). As a "historical science" it attempts to explain causes rather than conduct experiments to observe effects. 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) meaning "old, ancient," ὄν, ὀντ- (on, ont-), meaning "being, creature" and λόγος (logos), meaning "speech, thought, study". Paleontology lies on the border between biology and geology. 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 3,800 million years ago. As knowledge has increased, paleontology has developed specialized 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 fossils. (see more...)

Skeletal mount of Edmontosaurus
Edmontosaurus is a genus of crestless duck-billed dinosaur. The fossils of this animal have been found in rocks of western North America that date from the late Campanian stage to the end of the Maastrichtian stage of the Cretaceous Period, between 73 and 65.5 million years ago. It was one of the last non-avian dinosaurs, and lived shortly before the Cretaceous–Paleogene extinction event. Edmontosaurus was one of the largest hadrosaurids, measuring up to 13 meters (43 ft) long and weighing around 4.0 metric tons (4.4 short tons). It is known from several well-preserved specimens that include not only bones, but in some cases extensive skin impressions and possible gut contents.

Edmontosaurus has a lengthy and complicated taxonomic history dating to the late 19th century. The type species, E. regalis, was named by Lawrence Lambe in 1917, although several other species that are now classified in Edmontosaurus were named earlier. The best known of these is E. annectens, originally named by Othniel Charles Marsh in 1892.

Edmontosaurus was widely distributed across western North America. The distribution of Edmontosaurus fossils suggests that it preferred coasts and coastal plains. It was an herbivore that could move on both two legs and four. Because it is known from several bone beds, Edmontosaurus is thought to have lived in groups, and may have been migratory as well.

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Albertosaurus skeleton.
Albertosaurus (meaning 'Alberta lizard') was a genus of tyrannosaurid theropod dinosaur that lived in western North America during the Late Cretaceous Period, more than 70 million years ago. The type species, A. sarcophagus, was restricted in range to the modern-day Canadian province of Alberta, after which the genus is named. Scientists disagree on the number of species represented in the genus, recognizing either one or two species. As a tyrannosaurid, Albertosaurus was a bipedal predator with a massive head, jaws lined with dozens of large teeth and tiny, two-fingered 'hands' and it may have been at the top of the food chain in its local ecosystem. Although relatively large for a theropod, Albertosaurus was much smaller than its more famous relative Tyrannosaurus, probably weighing only as much as a modern black rhinoceros. Fossils of more than twenty individuals have been recovered, providing scientists with a more detailed knowledge of Albertosaurus anatomy than is available for other tyrannosaurids. The discovery of ten individuals at one site provides evidence of pack behavior and allows studies of developmental biology which are impossible with lesser-known animals. (see more...)

Radar topography of the Chicxulub Crater (Image courtesy NASA/JPL-Caltech).
Chicxulub Crater is an ancient impact crater buried underneath the Yucatán Peninsula, with its center located near the town of Chicxulub, Yucatán, Mexico. The crater is over 180 kilometers (110 mi) in diameter, making the feature one of the largest confirmed impact structures in the world; the asteroid or comet whose impact formed the crater was at least 10 km (6 mi) in diameter. The crater was named for the nearby town, as well as for the literal Maya translation of the name: "tail of the devil." The crater was discovered by Glen Penfield, a geophysicist who had been working in the Yucatán while looking for oil during the late 1970s. The presence of tektites, shocked quartz and gravity anomalies, as well as the age of the rocks and isotope analysis, show that this impact structure dates from the late Cretaceous Period, roughly 65 million years ago. The impact associated with the crater is implicated in causing the extinction of the dinosaurs as suggested by the Cretaceous–Paleogene boundary, although some critics disagree that the impact was the sole reason and also debate whether there was a single impact or whether the Chicxulub impactor was one of several that may have struck the Earth at around the same time. Recent evidence suggests that the impactor was a piece of a much larger asteroid which broke up in a collision more than 160 million years ago.(see more...)

Artist's depiction of Allosaurus.
Allosaurus (meaning 'strange lizard') is a genus of large theropod dinosaur that lived 155 to 145 million years ago, in the late Jurassic period. The first remains that can definitely be ascribed to this genus were described in 1877 by Othniel Charles Marsh. As one of the first well-known theropod dinosaurs, it has long attracted attention outside of paleontological circles, and has been a lead dinosaur in several films and documentaries.

Allosaurus was a large bipedal predator with a large skull, equipped with dozens of large, sharp teeth. It averaged 8.5 meters (30 ft) in length, though fragmentary remains suggest it could have reached over 12 meters (39 ft). Relative to the large and powerful hindlimbs, its three-fingered forelimbs were small, and the body was balanced by a long, heavy tail. It is classified as an allosaurid, a type of carnosaurian theropod dinosaur. The genus has a complicated taxonomy, and includes an uncertain number of valid species, the best known of which is A. fragilis. The bulk of Allosaurus remains have come from North America's Morrison Formation, with material also from Portugal and possibly Tanzania. It was known for over half of the 20th century as Antrodemus, but study of the copious remains from the Cleveland-Lloyd Dinosaur Quarry brought the name Allosaurus back to prominence, and established it as one of the best-known dinosaurs.

As the prominent large predator in the Morrison Formation, Allosaurus was at the top of the food chain, probably preying on contemporaneous large herbivorous dinosaurs. Potential prey included ornithopods, stegosaurids, and sauropods. While it is often thought of as preying on sauropod dinosaurs in groups, there is little evidence for cooperative social behavior in this genus, and individuals may have been aggressive toward each other instead. It may have attacked large prey by ambush, using its upper jaws like a hatchet. (see more...)

Artist's impression of Massospondylus depicting the animal as bipedal.
Massospondylus (meaning 'elongated vertebra') is a genus of prosauropod dinosaur from the early Jurassic Period (Hettangian to Pliensbachian ages, ca. 200–183 million years ago). It was described by Sir Richard Owen in 1854 from remains found in South Africa, and is thus one of the first dinosaurs to have been named. Fossils have since been found in Lesotho, Zimbabwe, and other parts of South Africa. Further material from Arizona's Kayenta Formation, India, and Argentina has been assigned to this genus, but may not belong to Massospondylus.

The type, and only universally recognized species, is M. carinatus, although six other species have been named during the past 150 years. Prosauropod systematics have undergone numerous revisions during the last several years, and many scientists disagree where exactly Massospondylus lies on the dinosaur evolutionary tree. The family name Massospondylidae was once coined for the genus, but because knowledge of prosauropod relationships is in a state of flux, it is unclear which other dinosaurs—if any—belong in a natural grouping of massospondylids; several 2007 papers support the family's validity.

Although Massospondylus was long depicted as quadrupedal, a 2007 study found it to be bipedal. It was probably a herbivore, although it is speculated that the prosauropods may have been omnivorous. This animal, 4–6 meters (13–20 ft) long, had a long neck and tail, with a small head and slender body. On each of its forefeet, it bore a sharp thumb claw that was used in defense or feeding. Recent studies indicate Massospondylus grew steadily throughout its lifespan, possessed air sacs similar to those of birds, and may have cared for its young. (see more...)

Tarbosaurus skull.

Tarbosaurus (meaning 'terrifying lizard') is a genus of tyrannosaurid theropod dinosaur that flourished in Asia between 70 and 65 million years ago, near the end of the Late Cretaceous Period. Fossils have been recovered in Mongolia with more fragmentary remains found further afield in parts of China. Although many species have been named, modern paleontologists recognize only one, T bataar, as valid. Some experts contend that this species is actually an Asian representative of the North American genus Tyrannosaurus; if true, this would invalidate the genus Tarbosaurus altogether.

Tarbosaurus and Tyrannosaurus are considered closely related genera, even if they are not synonymous. Alioramus, also from Mongolia, is thought by some authorities to be the closest relative of Tarbosaurus. Like most known tyrannosaurids, Tarbosaurus was a large bipedal predator, weighing more than a ton and equipped with dozens of large, sharp teeth. It had a unique locking mechanism in its lower jaw and the smallest forelimbs relative to body size of all tyrannosaurids, renowned for their disproportionately tiny, two-fingered forelimbs.

Tarbosaurus lived in a humid floodplain criss-crossed by river channels. In this environment, it was an apex predator at the top of the food chain, probably preying on other large dinosaurs like the hadrosaur Saurolophus or the sauropod Nemegtosaurus. Tarbosaurus is very well-represented in the fossil record, known from dozens of specimens, including several complete skulls and skeletons. These remains have allowed scientific studies focusing on its phylogeny, skull mechanics, and brain structure. (see more...)

Life restoration of Gorgosaurus.

Gorgosaurus (meaning 'fierce lizard') is a genus of tyrannosaurid theropod dinosaur that lived in western North America during the Late Cretaceous Period, between about 77 and 74 million years ago. Fossil remains have been found in the Canadian province of Alberta and possibly the U.S. state of Montana. Paleontologists recognize only the type species, G. libratus, although other species have been erroneously referred to the genus.

Like most known tyrannosaurids, Gorgosaurus was a bipedal predator weighing more than a metric ton as an adult; dozens of large, sharp teeth lined its jaws, while its two-fingered forelimbs were comparatively small. Gorgosaurus was most closely related to Albertosaurus, and more distantly related to the larger Tyrannosaurus. Gorgosaurus and Albertosaurus are extremely similar, distinguished mainly by subtle differences in the teeth and skull bones. Some experts consider G. libratus to be a species of Albertosaurus; this would make Gorgosaurus a junior synonym of that genus.

Gorgosaurus lived in a lush floodplain environment along the edge of an inland sea. An apex predator, it was at the top of the food chain, preying upon abundant ceratopsids and hadrosaurs. In some areas, Gorgosaurus coexisted with another tyrannosaurid, Daspletosaurus. Though these animals were roughly the same size, there is some evidence of niche differentiation between the two. Gorgosaurus is the best-represented tyrannosaurid in the fossil record, known from dozens of specimens. These plentiful remains have allowed scientists to investigate its ontogeny, life history and other aspects of its biology. (see more...)

A coal ball

Coal balls, despite their name, are calcium-rich masses of permineralised life forms, generally having a round shape. Coal balls were formed roughly 300 million years ago, during the Carboniferous Period. They are exceptional at preserving organic matter, which makes them useful to scientists, who cut and peel the coal balls to research the geological past of the Earth.

In 1855, two English scientists, Joseph Dalton Hooker and Edward William Binney, discovered coal balls in England, and the initial research on coal balls was carried out in Europe. It was not until 1922 that coal balls were discovered and identified in North America. Since then, coal balls have been discovered in other countries and they have led to the discovery of over 300 species and 130 genera.

Coal balls can be found in coal seams across North America and Eurasia. North American coal balls are relatively widespread, both stratigraphically and geologically, as compared to coal balls from Europe. The oldest known coal balls were found in Germany and the former Czechoslovakia.