dinosaurthe common name given to a group of reptiles, often very large, that first appeared roughly 245 million years ago (at near the beginning of the Middle Triassic Epoch) and thrived worldwide for some nearly 180 million years. Most died out by the end of the Cretaceous Period, about 65.5 66 million years ago, but many lines of evidence now show that one lineage evolved into birds about 150 million years ago.

The name dinosaur comes from the Greek words deinos (“terrible” or “fearfully great”) and sauros (“reptile” or “lizard”). The English anatomist Richard Owen proposed the formal term Dinosauria in 1842 to include three giant extinct animals (Megalosaurus, Iguanodon, and Hylaeosaurus) represented by large fossilized bones that had been unearthed at several locations in southern England during the early part of the 19th century. Owen recognized that these reptiles were far different from other known reptiles of the present and the past for three reasons: they were large yet obviously terrestrial, unlike the aquatic ichthyosaurs and plesiosaurs that were already known; they had five vertebrae in their hips, whereas most known reptiles have only two; and, rather than holding their limbs sprawled out to the side in the manner of lizards, dinosaurs held their limbs under the body in columnar fashion, like elephants and other large mammals.

Originally applied to just a handful of incomplete specimens, the category Dinosauria now encompasses more than 800 generic names and at least 1,000 species, with new names being added to the roster every year as the result of scientific explorations around the world. Not all of these names are valid taxa, however. A great many of them have been based on fragmentary or incomplete material that may actually have come from two or more different dinosaurs. In addition, bones have sometimes been misidentified as dinosaurian when they are not from dinosaurs at all. Nevertheless, dinosaurs are well documented by abundant fossil remains recovered from every continent on Earth, and the number of known dinosaurian taxa is estimated to be 10–25 percent of actual past diversity.

The extensive fossil record of genera and species is testimony that dinosaurs were diverse animals, with widely varying lifestyles and adaptations. Their remains are found in sedimentary rock layers (strata) dating to the Late Triassic Epoch (229 approximately 237 million to 200 201.3 million years ago). The abundance of their fossilized bones is substantive proof that dinosaurs were the dominant form of terrestrial animal life during the Mesozoic Era (251 about 252.2 million to 65.5 66 million years ago). It is likely that the known remains represent a very small fraction (probably less than 0.0001 percent) of all the individual dinosaurs that once lived.

The search for dinosaurs
The first finds

Before Richard Owen introduced the term Dinosauria in 1842, there was no concept of anything even like a dinosaur. Large fossilized bones quite probably had been observed long before that time, but there is little record—and no existing specimens—of such findings much before 1818. In any case, people could not have been expected to understand what dinosaurs were even if they found their remains. For example, some classical scholars now conclude that the Greco-Roman legends of griffins from the 7th century bce were inspired by discoveries of protoceratopsian dinosaurs in the Altai region of Mongolia. In 1676 Robert Plot of the University of Oxford included, in a work of natural history, a drawing of what was apparently the knee-end of the thigh bone thighbone of a dinosaur, which he thought might have come from an elephant taken to Britain in Roman times. Fossil bones of what were undoubtedly dinosaurs were discovered in New Jersey in the late 1700s and were probably discussed at the meetings of the American Philosophical Society in Philadelphia. Soon thereafter, Lewis and Clark’s expedition encountered dinosaur fossils in the western United States.

The earliest verifiable published record of dinosaur remains that still exists is a note in the 1820 American Journal of Science and Arts by Nathan Smith. The bones described had been found in 1818 by Solomon Ellsworth, Jr., while he was digging a well at his homestead in Windsor, Connecticut. At the time, the bones were thought to be human, but much later they were identified as Anchisaurus. Even earlier (1800), large birdlike footprints had been noticed on sandstone slabs in Massachusetts. Pliny Moody, who discovered these tracks, attributed them to “Noah’s raven,” and Edward Hitchcock of Amherst College, who began collecting them in 1835, considered them to be those of some giant extinct bird. The tracks are now recognized as having been made by several different kinds of dinosaurs, and such tracks are still commonplace in the Connecticut River Valley valley today.

Better known are the finds in southern England during the early 1820s by William Buckland (a clergyman) and Gideon Mantell (a physician), who described Megalosaurus and Iguanodon, respectively. In 1824 Buckland published a description of Megalosaurus, fossils of which consisted mainly of a lower jawbone with a few teeth. The following year Mantell published his “Notice on the Iguanodon, a Newly Discovered Fossil Reptile, from the Sandstone of Tilgate Forest, in Sussex,” on the basis of several teeth and some leg bones. Both men collected fossils as an avocation and are credited with the earliest published announcements in England of what later would be recognized as dinosaurs. In both cases their finds were too fragmentary to permit a clear image of either animal. In 1834 a partial skeleton was found near Brighton that corresponded with Mantell’s fragments from Tilgate Forest. It became known as the Maidstone Iguanodon, after the village where it was discovered. The Maidstone skeleton provided the first glimpse of what these creatures might have looked like.

Two years before the Maidstone Iguanodon came to light, a different kind of skeleton was found in the Weald of southern England. It was described and named Hylaeosaurus by Mantell in 1832 and later proved to be one of the armoured dinosaurs. Other fossil bones began turning up in Europe: fragments described and named as Thecodontosaurus and Palaeosaurus by two English students, Henry Riley and Samuel Stutchbury, and the first of many skeletons named Plateosaurus by the naturalist Hermann von Meyer in 1837. Richard Owen identified two additional dinosaurs, albeit from fragmentary evidence: Cladeiodon, which was based on a single large tooth, and Cetiosaurus, which he named from an incomplete skeleton composed of very large bones. Having carefully studied most of these fossil specimens, Owen recognized that all of these bones represented a group of large reptiles that were unlike any living varieties. In a report to the British Association for the Advancement of Science in 1841, he described these animals, and the word Dinosauria was first published in the association’s proceedings in 1842.

Reconstruction and classification

During the decades that followed Owen’s announcement, many other kinds of dinosaurs were discovered and named in England and Europe: Massospondylus in 1854, Scelidosaurus in 1859, Bothriospondylus in 1875, and Omosaurus in 1877. Popular fascination with the giant reptiles grew, reaching a peak in the 1850s with the first attempts to reconstruct the three animals on which Owen based Dinosauria—Iguanodon, Megalosaurus, and Hylaeosaurus—for the first world exposition, the Great Exhibition of 1851 in London’s Crystal Palace. A sculptor under Owen’s direction (Waterhouse Hawkins) created life-size models of these two genera, and in 1854 they were displayed together with models of other extinct and living reptiles, such as plesiosaurs, ichthyosaurs, and crocodiles.

By the 1850s it had become evident that the reptile fauna of the Mesozoic Era was far more diverse and complex than it is today. The first important attempt to establish an informative classification of the dinosaurs was made by the English biologist T.H. Huxley as early as 1868. Because he observed that these animals had legs similar to birds as well as other birdlike features, he established a new order called Ornithoscelida. He divided the order into two suborders. Dinosauria was the first and included the iguanodonts, the large carnivores (or megalosaurids), and the armoured forms (including Scelidosaurus). Compsognatha was the second order, named for the very small birdlike carnivore Compsognathus.

Huxley’s classification was replaced by a radically new scheme proposed in 1887 by his fellow Englishman H.G. Seeley, who noticed that all dinosaurs possessed one of two distinctive pelvic designs, one like that of birds and the other like that of reptiles. Accordingly, he divided the dinosaurs into the orders Ornithischia (having a birdlike pelvis) and Saurischia (having a reptilian pelvis). Ornithischia included four suborders: Ornithopoda (Iguanodon and similar herbivores), Stegosauria (plated forms), Ankylosauria (Hylaeosaurus and other armoured forms), and Ceratopsia (horned dinosaurs, just then being discovered in North America). Seeley’s second order, the Saurischia, included all the carnivorous dinosaurs, such as Megalosaurus and Compsognathus, as well as the giant herbivorous sauropods, including Cetiosaurus and several immense “brontosaur” types that were turning up in North America. In erecting Saurischia and Ornithischia, Seeley cast doubt on the idea that Dinosauria was a natural grouping of these animals. This uncertainty persisted for a century thereafter, but it is now understood that the two groups share unique features that indeed make the Dinosauria a natural group.

In 1878 a spectacular discovery was made in the town of Bernissart, Belgium, where several dozen complete articulated skeletons of Iguanodon were accidentally uncovered in a coal mine during the course of mining operations. Under the direction of the Royal Institute of Natural Science of Belgium, thousands of bones were retrieved and carefully restored over a period of many years. The first skeleton was placed on exhibit in 1883, and today the public can view an impressive herd of Iguanodon. The discovery of these multiple remains gave the first hint that at least some dinosaurs may have traveled in groups and showed clearly that some dinosaurs were bipedal (walking on two legs). The supervisor of this extraordinary project was Louis Dollo, a zoologist who was to spend most of his life studying Iguanodon, working out its structure, and speculating on its living habits.f

American hunting expeditions

England and Europe produced most of the early discoveries and students of dinosaurs, but North America soon began to contribute a large share of both. One leading student of fossils was Joseph Leidy of the Academy of Natural Sciences in Philadelphia, who named some of the earliest dinosaurs found in America, including Palaeoscincus, Trachodon, Troodon, and Deinodon. Unfortunately, some names given by Leidy are no longer used, because they were based on such fragmentary and undiagnostic material. Leidy is perhaps best known for his study and description of the first dinosaur skeleton to be recognized in North America, that of a duckbill, or hadrosaur, found at Haddonfield, New Jersey, in 1858, which he named Hadrosaurus foulkii. Leidy’s inference that this animal was probably amphibious influenced views of dinosaur life for the next century.

Two Americans whose work during the second half of the 19th century had worldwide impact on the science of paleontology in general, and the growing knowledge of dinosaurs in particular, were O.C. Marsh of Yale College and E.D. Cope of Haverford College, the University of Pennsylvania, and the Academy of Natural Sciences in Philadelphia. All previous dinosaur remains had been discovered by accident in well-populated regions with temperate, moist climates, but Cope and Marsh astutely focused their attention on the wide arid expanses of bare exposed rock in western North America. In their intense quest to find and name new dinosaurs, these scientific pioneers became fierce and unfriendly rivals.

Marsh’s field parties explored widely, exploiting dozens of now famous areas, among them Yale’s sites at Morrison and Canon City, Colorado, and, most important, Como Bluff in southeastern Wyoming. The discovery of Como Bluff in 1877 was a momentous event in the history of paleontology that generated a burst of exploration and study as well as widespread public enthusiasm for dinosaurs. Como Bluff brought to light one of the greatest assemblages of dinosaurs, both small and gigantic, ever found. For decades the site went on producing the first known specimens of Late Jurassic Period Epoch (161 163.5 million to 145 .5 million years ago) dinosaurs such as Stegosaurus, Camptosaurus, Camarasaurus, Laosaurus, Coelurus, and others. From the Morrison site came the original specimens of Allosaurus, Diplodocus, Atlantosaurus, and Brontosaurus (later renamed Apatosaurus). Canon City provided bones of a host of dinosaurs, including Stegosaurus, Brachiosaurus, Allosaurus, and Camptosaurus.

Another major historic site was the Lance Creek area of northeastern Wyoming, where J.B. Hatcher discovered and collected dozens of Late Cretaceous horned dinosaur remains for Marsh and for Yale College, among them the first specimens of Triceratops and Torosaurus. Marsh was aided in his work at these and other localities by the skills and efforts of many other collaborators like Hatcher—William Reed, Benjamin Mudge, Arthur Lakes, William Phelps, and Samuel Wendell Williston, to name a few. Marsh’s specimens now form the core of the Mesozoic collections at the National Museum of Natural History of the Smithsonian Institution and the Peabody Museum of Natural History at Yale University.

Cope’s dinosaur explorations ranged as far as, or farther than, Marsh’s, and his interests encompassed a wider variety of fossils. Owing to a number of circumstances, however, Cope’s dinosaur discoveries were fewer and his collections far less complete than those of Marsh. Perhaps his most notable achievement was finding and proposing the names for Coelophysis and Monoclonius. Cope’s dinosaur explorations began in the eastern badlands of Montana, where he discovered Monoclonius in the Judith River Formation of the Late Cretaceous Period Epoch (100 million to 65.5 million to 66 million years ago). Accompanying him there was a talented young assistant, Charles H. Sternberg. Later Sternberg and his three sons went on to recover countless dinosaur skeletons from the Oldman and Edmonton formations of the Late Cretaceous along the Red Deer River of Alberta, Canada.

Dinosaur ancestors

During the early decades of dinosaur discoveries, little thought was given to their evolutionary ancestry. Not only were the few specimens known unlike any living animal, but they were so different from any other reptiles that it was difficult to discern much about their relationships. Early on it was recognized that, as a group, dinosaurs appear to be most closely allied to crocodilians, though T.H. Huxley had proposed in the 1860s that dinosaurs and birds must have had a very close common ancestor in the distant past. Three anatomic features—socketed teeth, a skull with two large holes (diapsid), and another hole in the lower jaw—are present in both crocodiles and dinosaurs. The earliest crocodilians occurred nearly simultaneously with the first known dinosaurs, so neither could have given rise to the other. It was long thought that the most likely ancestry of dinosaurs could be found within a poorly understood group of Triassic reptiles termed thecodontians (“socket-toothed reptiles”). Today it is recognized that “thecodontian” is simply a name for the basal, or most primitive, members of the archosaurs (“ruling reptiles”), a group that is distinguished by the three anatomic features mentioned above and that includes dinosaurs, pterosaurs (flying reptiles), crocodiles, and their extinct relatives.

An early candidate for the ancestor of dinosaurs was a small basal archosaur from the Early Triassic Epoch (251 252.2 million to 246 247.2 million years ago) of South Africa called Euparkeria. New discoveries suggest creatures that are even more dinosaur-like from the Middle Triassic (246 247.2 million to 229 237 million years ago) and from an early portion of the Late Triassic (229 237 million to 200 201.3 million years ago) of South America; these include Lagerpeton, Lagosuchus, Pseudolagosuchus, and Lewisuchus. Other forms, such as Nyasasaurus and Asilisaurus, date from the Middle Triassic of East Africa; Nyasasaurus is considered by some to be the oldest known member of Dinosauria. Other South American forms such as Eoraptor and Herrerasaurus are particularly dinosaurian in appearance and are sometimes considered dinosaurs.

The earliest appearance of “true dinosaurs” is almost impossible to pinpoint, since it can never be known with certainty whether the very first (or last) specimen of any kind of organism has been found. The succession of deposits containing fossils is discontinuous and contains many gaps; even within these deposits, the fossil record of dinosaurs and other creatures contained within is far from complete. Further complicating matters is that evolution from ancestral to descendant form is usually a stepwise process. Consequently, as more and more gaps are filled between the first dinosaurs and other archosaurs, the number of features distinguishing them becomes smaller and smaller. Currently, paleontologists define dinosaurs as Triceratops (representing Ornithischia), birds (the most recent representatives of the Saurischia), and all the descendants of their most recent common ancestor. That common ancestor apparently had a suite of features not present in other dinosaur relatives, including the loss of the prefrontal bone above the eye, a long deltopectoral crest on the humerus, three or fewer joints on the fourth finger of the hand, three or more hip vertebrae, a fully open hip socket, and a cnemial crest on the shin bone (tibia). These features were passed on and modified in the descendants of the first dinosaurs. Compared with most of their contemporaries, dinosaurs had an improved stance and posture with a resulting improved gait and, in several independent lineages, an overall increase in size. They also were more efficient at gathering food and processing it and apparently had higher metabolic rates and cardiovascular nourishment. All these trends, individually or in concert, probably contributed to the collective success of dinosaurs, which resulted in their dominance among the terrestrial animals of the Mesozoic.

Modern studies

During the first century or more of dinosaur awareness, workers in the field more or less concentrated on the search for new specimens and new types. Their discoveries then required detailed description and analysis, followed by comparisons with other known dinosaurs in order to classify the new finds and develop hypotheses about evolutionary relationships. These pursuits continue, but newer methods of exploration and analysis have been adopted. Emphasis has shifted from purely descriptive procedures to analyses of relationships by using the methods of cladistics, which dispenses with the traditional taxonomic hierarchy in favour of “phylogenetic trees” that are more explicit about evolutionary relationships. Phylogenetic analyses also help us to understand how certain features evolved in groups of dinosaurs and give us insight into their possible functions. For example, in the evolution of horned dinosaurs (ceratopsians), it can be seen that the beak evolved first, followed by the frill, and finally the nose and eye horns, which were differently developed in different groups. The hypothesis that the frill was widely used in defense by ceratopsians such as Protoceratops can thus be tested phylogenetically. On this basis, the idea is now generally rejected because the frill was basically just an open rim of bone in nearly all ceratopsians except Triceratops, which is often pictured charging like a rhinoceros.

Functional anatomic studies extensively use analogous traits of present-day animals that, along with both mechanical and theoretical models, make it possible to visualize certain aspects of extinct animals. For example, estimates of normal walking and maximum running speeds can be calculated on the basis of the analysis of trackways, which can then be combined with biomechanical examination of the legs and joints and reconstruction of limb musculature. Similar methods have been applied to jaw mechanisms and tooth wear patterns to obtain a better understanding of feeding habits and capabilities.

The soft parts of dinosaurs are only imperfectly known. Original colours and patterns cannot be known, but skin textures have occasionally been preserved. Most show a knobby or pebbly surface rather than a scaly texture as in most living reptiles. Impressions of internal organs are rarely preserved, but, increasingly, records of filaments and even feathers have been found on some dinosaurs. Gastroliths (“stomach stones”) used for processing food in the gizzard have been recovered from a variety of dinosaurs.


A misconception commonly portrayed in popular books and media is that all the dinosaurs died out at the same time—and apparently quite suddenly—at the end of the Cretaceous Period 65.5 , 66 million years ago. This is not entirely correct, and not only because birds are a living branch of dinosaurian lineage. The best records, which are almost exclusively from North America, show that dinosaurs were already in decline during the latest portion of the Cretaceous. The causes of this decline, as well as the fortunes of other groups at the time, are complex and difficult to attribute to a single source. In order to understand extinction, it is necessary to understand the basic fossil record of dinosaurs.

Faunal changes

During the 160 million years or so of the Mesozoic Era (251 252.2 million to 65.5 66 million years ago) from which dinosaurs are known, there were constant changes in dinosaur communities. Different species evolved rapidly and were quickly replaced by others throughout the Mesozoic; it is rare that any particular type of dinosaur survived from one geologic formation into the next. The fossil evidence shows a moderately rich fauna of plateosaurs and other prosauropods, primitive ornithopods, and theropods during the Late Triassic Period Epoch (229 237 million to 200 201.3 million years ago). Most of these kinds of dinosaurs are also represented in strata of the Early Jurassic Period Epoch (200 201.3 million to 176 174.1 million years ago), but following a poorly known Middle Jurassic, the fauna of the Late Jurassic (161 163.5 million to 145 .5 million years ago) was very different. By this time sauropods, more advanced ornithopods, stegosaurs, and a variety of theropods predominated. The Early Cretaceous (145 .5 million to 100.5 million years ago) then contained a few sauropods (albeit they were all new forms), a few stegosaurian holdovers, new kinds of theropods and ornithopods, and some of the first well-known ankylosaurs. By the Late Cretaceous (100 million to 65.5 million to 66 million years ago), sauropods, which had disappeared from the northern continents through most of the Early early and mid-Cretaceous, had reinvaded the northern continents from the south, and advanced ornithopods (duckbills) had become the dominant browsers. A variety of new theropods of all sizes were widespread; stegosaurs no longer existed; and the ankylosaurs were represented by a collection of new forms that were prominent in the North America and Asia. New groups of dinosaurs, the pachycephalosaurs and ceratopsians, had appeared in Asia and had successfully colonized North America. The overall picture is thus quite clear: throughout Mesozoic time there was a continuous dying out and renewal of dinosaurian life.

It is important to note that extinction is a normal, universal occurrence. Mass extinctions often come to mind when the term extinction is mentioned, but the normal background extinctions that occur throughout geologic time probably account for most losses of biodiversity. Just as new species constantly split from existing ones, existing species are constantly becoming extinct. The speciation rate of a group must, on balance, exceed the extinction rate in the long run, or that group will become extinct. The history of animal and plant life is replete with successions as early forms are replaced by new and often more advanced forms. In most instances the layered (stratigraphic) nature of the fossil record gives too little information to show whether the old forms were actually displaced by the new successors (from the effects of competition, predation, or other ecological processes) or if the new kinds simply expanded into the declining population’s ecological niches.

Because the fossil record is episodic rather than continuous, it is very useful for asking many kinds of questions, but it is not possible to say precisely how long most dinosaur species or genera actually existed. Moreover, because the knowledge of the various dinosaur groups is somewhat incomplete, the duration of any particular dinosaur can be gauged only approximately—usually by stratigraphic boundaries and presumed “first” and “last” occurrences. The latter often coincide with geologic age boundaries; in fact, the absence of particular life-forms has historically defined most geologic boundaries ever since the geologic record was first compiled and analyzed in the late 18th century. The “moments” of apparently high extinction levels among dinosaurs were near the ends of two stages of occurred at two points in the Triassic (about 221 million and 210 million years ago), perhaps at the end of the Jurassic (145 .5 million years ago), and, of course, at the end of the Cretaceous (65.5 66 million years ago). Undoubtedly, there were lesser extinction peaks at other times in between, but there are poor terrestrial records for most of the world in the Middle Triassic, Middle Jurassic, and mid-Cretaceous.

The K–T boundary event

It was not only the dinosaurs that disappeared 65.5 66 million years ago at the Cretaceous–Tertiary, or K–T, boundary (also referred to as the Cretaceous–Paleogene, or K–Pg, boundary). Many other organisms became extinct or were greatly reduced in abundance and diversity, and the extinctions were quite different between, and even among, marine and terrestrial organisms. Land plants did not respond in the same way as land animals, and not all marine organisms showed the same patterns of extinction. Some groups died out well before the K–T boundary, including flying reptiles (pterosaurs) and sea reptiles (plesiosaurs, mosasaurs, and ichthyosaurs). Strangely, turtles, crocodilians, lizards, and snakes were either not affected or affected only slightly. Effects on amphibians and mammals were mild. These patterns seem odd, considering how environmentally sensitive and habitat-restricted many of these groups are today. Many marine groups—such as the molluscan ammonites, the belemnites, and certain bivalves—were decimated. Other greatly affected groups were the moss animals (phylum Bryozoa), the crinoids, and a number of planktonic life-forms such as foraminiferaforaminifers, radiolarians, coccolithophores, and diatoms.

Whatever factors caused it, there was undeniably a major, worldwide biotic change near the end of the Cretaceous. But the extermination of the dinosaurs is the best-known change by far, and it has been a puzzle to paleontologists, geologists, and biologists for two centuries. Many hypotheses have been offered over the years to explain dinosaur extinction, but only a few have received serious consideration. Proposed causes have included everything from disease to heat waves and resulting sterility, freezing cold spells, the rise of egg-eating mammals, and X-rays from a nearby exploding supernova. Since the early 1980s, attention has focused on the so-called asteroid theory put forward by the American geologist Walter Alvarez, his father, physicist Luis Alvarez, and their coworkers. This theory is consistent with the timing and magnitude of some extinctions, especially in the oceans, but it does not fully explain the patterns on land and does not eliminate the possibility that other factors were at work on land as well as in the seas.

One important question is whether the extinctions were simultaneous and instantaneous or whether they were nonsynchronous and spread over a long time. The precision with which geologic time can be measured leaves much to be desired no matter what means are used (radiometric, paleomagnetic, or the more traditional measuring of fossil content of stratigraphic layers). Only rarely does an “instantaneous” event leave a worldwide—or even regional—signature in the geologic record in the way that a volcanic eruption does locally. Attempts to pinpoint the K–T boundary event, even by using the best radiometric dating techniques, result in a margin of error on the order of 50,000 years. Consequently, the actual time involved in this, or any of the preceding or subsequent extinctions, has remained undetermined.

The asteroid theory

The discovery of an abnormally high concentration of the rare metal iridium at, or very close to, the K–T boundary provides what has been recognized as one of those rare instantaneous geologic time markers that seem to be worldwide. This iridium anomaly, or spike, was first found by Walter Alvarez in the Cretaceous–Tertiary stratigraphic sequence at Gubbio, Italy, in the 1970s. The spike has subsequently been detected at hundreds of localities in Denmark and elsewhere, both in rock outcrops on land and in core samples drilled from ocean floors. Iridium normally is a rare substance in rocks of the Earth’s crust (about 0.3 part per billion). At Gubbio the iridium concentration is more than 20 times greater (6.3 parts per billion), and it exceeds this concentration at other sites.

Because the levels of iridium are higher in meteorites than on the Earth, the Gubbio anomaly is thought to have an extraterrestrial explanation. If this is true, such extraterrestrial signatures will have a growing influence on the precision with which geologic time boundaries can be specified. The level of iridium in meteorites has been accepted as representing the average level throughout the solar system and, by extension, the universe. Accordingly, the iridium concentration at the K–T boundary is widely attributed to a collision between the Earth and a huge meteor or asteroid. The size of the object is estimated at about 10 km (6.2 miles) in diameter and one quadrillion metric tons in weight; the velocity at the time of impact is reckoned to have been several hundreds of thousands of kilometres per hour. The crater resulting from such a collision would be some 100 km or more in diameter. Such an impact site (called an astrobleme) is the Chicxulub crater, in the Yucatán Peninsula. A second, smaller impact site, which predates the Chicxulub site by about 2,000 to 5,000 years, appears at Boltysh in Ukraine. Its existence raises the possibility that the K–T boundary event resulted from multiple extraterrestrial impacts.

The Although the amount of iridium dispersed worldwide was more consistent with the impact of a smaller object, such as a comet, the asteroid theory is widely accepted as the most probable explanation of the K–T iridium anomaly, but it . The asteroid theory does not, however, appear to account for all the paleontological data. An impact explosion of this kind would have ejected an enormous volume of terrestrial and asteroid material into the atmosphere, producing a cloud of dust and solid particles that would have encircled the Earth and blocked out sunlight for many months, possibly years. The loss of sunlight could have eliminated photosynthesis and resulted in the death of plants and the subsequent extinction of herbivores, their predators, and scavengers.

The K–T mass extinctions, however, do not seem to be fully explained by this hypothesis. The stratigraphic record is most complete for extinctions of marine life—foraminiferalife—foraminifers, ammonites, coccolithophores, and the like. These apparently died out suddenly and simultaneously, and their extinction accords best with the asteroid theory. The fossil evidence of land dwellers, however, suggests a gradual rather than a sudden decline in dinosaurian diversity (and possibly abundance). Alterations in terrestrial life seem to be best accounted for by environmental factors, such as the consequences of seafloor spreading and continental drift, resulting in continental fragmentation, climatic deterioration, increased seasonality, and perhaps changes in the distributions and compositions of terrestrial communities. But one phenomenon does not preclude another. It is entirely possible that a culmination of ordinary biological changes and some catastrophic events, including increased volcanic activity, took place around the end of the Cretaceous.

Dinosaur descendants

Contrary to the commonly held belief that the dinosaurs left no descendants, Archaeopteryx, which was first discovered in 1861, and Xiaotingia, which was formally classified in 2011, provide compelling evidence that birds (class Aves) evolved from small theropod dinosaurs. Following the principles of genealogy that are applied to humans as much as to other organisms, organisms are classified at a higher level within the groups from which they evolved. Archaeopteryx and Xiaotingia—the oldest birds known—are therefore classified as both dinosaurs and birds, just as humans are both primates and mammals.

The specimens of Archaeopteryx contain particular anatomic features that also are exclusively present in certain theropods (Oviraptor, Velociraptor, Deinonychus, and Troodon, among others). These animals share long arms and hands, a somewhat shorter, stiffened tail, a similar pelvis, and an unusual wrist joint in which the hand is allowed to flex sideways instead of up and down. This wrist motion is virtually identical to the motion used by birds (and bats) in flight, though in these small dinosaurs its initial primary function was probably in catching prey.

Beginning in the 1990s, several specimens of small theropod dinosaurs from the Early Cretaceous of Liaoning province, China, were unearthed. These fossils are remarkably well preserved, and because they include impressions of featherlike, filamentous structures that covered the body, they have shed much light on the relationship between birds and Mesozoic dinosaurs. Such structures are now known in a compsognathid (Sinosauropteryx), a therizinosaurid (Beipiaosaurus), a dromaeosaur (Sinornithosaurus), and an alvarezsaurid (Shuvuuia). The filamentous structures on the skin of Sinosauropteryx are similar to the barbs of feathers, which suggests that feathers evolved from a much simpler structure that probably functioned as an insulator. True feathers of several types, including contour and body feathers, have been found in the 125-million-year-old feathered oviraptorid Caudipteryx and the apparently related Protarchaeopteryx. Because these animals were not birds and did not fly, it is now evident that true feathers neither evolved first in birds nor developed for the purpose of flight. Instead, feathers may have evolved for insulation, display, camouflage, species recognition, or some combination of these functions and only later became adapted for flight. In the case of Caudipteryx, for example, it has been established that these animals not only sat on nests but probably protected the eggs with their feathers.

Until comparatively recent times, the two groups of birds from Cretaceous time that received the most attention because of their strange form were the divers, such as Hesperornis, and the strong-winged Ichthyornis, a more ternlike form. Because they were the first well-known Cretaceous birds, having been described by American paleontologist O.C. Marsh in 1880, they were thought to represent typical Cretaceous birds. Recent discoveries, however, have changed this view. For example, members of one Early Cretaceous bird group, the Confuciusornithidae, showed very little advancement compared with Archaeopteryx and the Enantiornithes (a major group of birds widely distributed around the world through most of the Cretaceous Period). Because representatives of living bird groups have long been known among the fossil species from the Paleocene and Eocene epochs (65.5 66 million to 33.9 million years ago), it has seemed evident that bird groups other than those including Hesperornis and Ichthyornis must have existed during the Cretaceous. Knowledge of these, based on fragments of fossil bone, has slowly come to light, and there is now a fairly definite record from Cretaceous rock strata of other ancestral birds related to the living groups of loons, grebes, flamingos, cranes, parrots, and shorebirds—and thus indication of early avian diversity. Therefore, it is clear that birds did not go through a “bottleneck” of extinction at the end of the Cretaceous that separated the archaic groups from the extant groups. Rather, the living groups were mostly present by the latest Cretaceous, and by this time the archaic groups seem to have died out.