Dinosaur

Biology of dinosaurs

Fossils and other evidence of the dinosaurs

Major groups of dinosaurs

Carnivorous dinosaurs

Herbivorous dinosaurs

Other extinct orders of Mesozoic-age reptiles

Theories about the extinction of dinosaurs

Resources

Dinosaurs are a group of now-extinct terrestrial reptiles, order Dinosauria, that lived from about 225 million years ago to 66 million years ago, during the Mesozoic era. Dinosaurs ranged from chickensized creatures such as the 2 lb (1 kg) predator Compsognathus to colossal, herbivorous animals known as sauropods, which were larger than any terrestrial (land-dwelling) animals that have lived before or since. Some dinosaurs were enormous, awesomely fierce predators, while others were mild-mannered herbivores, or plant eaters, that reached an immense size. The word “dinosaur” is derived from two Greek words, meaning “terrible lizard.” The term refers to some of the huge and awesome predatory dinosaurs— the first of these extinct reptiles to be discovered that were initially thought to be lizardlike in appearance and biology. But Richard Owen (1804–1892), the British expert in comparative anatomy, also coined the word in awe of the complexity of this wide variety of creatures that lived so long ago and yet were so well-adapted to their world.

Dinosaurs were remarkable and impressive animals but are rather difficult to define as a zoological group. They were terrestrial animals that had upright legs, rather than legs that sprawled outward from the body. Their skulls had two temporal openings on each side (in addition to the opening for the eyes), as well as other common and distinctive features. The dinosaurs were distinguished from other animals, however, by distinctive aspects of their behavior, physiology, and ecological relationships. Unfortunately, relatively little is known about these traits because we can only learn about dinosaurs using their fossil traces, which are rare and incomplete. It is clear from the available evidence that some species of dinosaurs were large predators, others were immense herbivores, and still others were smaller predators, herbivores, or scavengers. Sufficient information is available to allow paleontologists to assign scientific names to many of these dinosaurs and to speculate about their evolutionary and ecological relationships.

Although they are now extinct, the dinosaurs were among the most successful large animals ever to live on Earth. The dinosaurs arose during the interval of geologic time known as the Mesozoic (middle life) era, often called the “golden age of reptiles” or “the age of dinosaurs.” Radiometric dating of volcanic rocks associated with dinosaur fossils suggests they first evolved 225 million years ago, during the late Triassic period and became extinct 66 million years ago, at the end of the Cretaceous period. Dinosaurs lived for about 160 million years and were the dominant terrestrial animals on Earth throughout the Jurassic and Cretaceous periods—a span of over 100 million years.

Interestingly, mammal-like animals co-existed almost continuously with the dinosaurs and prospered after the last of the dinosaurs became extinct. Although they co-existed with dinosaurs, mammals were clearly subordinate, never developing large forms, for example. It was not until the disappearance of the last dinosaurs that an adaptive radiation of larger species of mammals occurred, and they then became the dominant large animals on Earth.

It is not known exactly what caused the last dinosaurs to become extinct. Many paleontologists think it likely that the large asteroid impact that formed the

112-mile-wide Chicxulub (pronounced CHICKS-aloob) crater in Mexico contributed to their demise. Others argue that large volcanic eruptions in Asia were a factor. It is possible that multiple factors contributed to the extinction of the dinosaurs. It must be stressed, however, that dinosaurs were remarkably successful animals. These creatures were dominant on Earth for an enormously longer length of time than the few tens of thousands of years that humans have been a commanding species, or the few millions that the genus Homo has existed at all. The popular myth that dinosaurs became extinct because their bodies became too large for their brains is false.

Biology of dinosaurs

The distinguishing characteristics of the dinosaurs include the structure of their skull and other bones. Dinosaurs typically had 25 vertebrae, plus three vertebrae that were fused to form their pelvic bones. The dinosaurs displayed an enormous range of forms and functions, however, and they filled a wide array of ecological niches. Some of the dinosaurs were, in fact, quite bizarre in their shape and, undoubtedly, their behavior.

The smallest dinosaurs were chickenlike carnivores that were only about 1 ft (30 cm) long and weighed 5–6 lb (2–3 kg). The largest dinosaurs reached a length of over 100 ft (30 m) and weighed 80 tons (73 metric tons) or more—more than any other terrestrial animal has ever achieved. The largest blue whales can weigh more than this, about 110 tons (100 metric tons), representing the largest animals ever to occur on Earth. The weight of these aquatic animals is partially buoyed by the water that they live in; whales do not have to fully support their immense weight against the forces of gravity, as the dinosaurs did. When compared with the largest living land animal, the African elephant, which weighs as much as 7.5 tons (6.8 metric tons), the large species of dinosaurs were enormous creatures.

Most species of dinosaurs had long tails and long necks, but this was not the case for all species. Most of the dinosaurs walked on all four legs, although some species were bipedal, using only their rear legs for locomotion. Their forelegs were greatly reduced in size and probably were used only for grasping. The tetrapods that walked on four legs were all peaceful herbivores. In contrast, many of the bipedal dinosaurs were fast-running predators.

The teeth of dinosaur species were highly diverse. Many species were exclusively herbivorous, and their teeth were correspondingly adapted for cutting and grinding vegetation. Other dinosaurs were fierce predators, and their teeth were shaped like serrated knives, which seized and stabbed their prey and cut it into smaller pieces that could be swallowed whole.

Until recently, it was widely believed that dinosaurs were rather stupid, slow-moving, cold-blooded (or poikilothermic) creatures. Some scientists now believe, however, that dinosaurs were intelligent, social, quick-moving, and probably warm-blooded (or homoiothermic) animals. This is a controversial topic, and scientific consensus has not been reached on whether or not some of the dinosaurs were able to regulate their body temperature by producing heat through metabolic reactions. It is absolutely undeniable that dinosaurs were extremely capable animals. This should not be a surprise to us, considering the remarkable evolutionary successes that they attained.

Fossils and other evidence of the dinosaurs

Humans never co-existed with dinosaurs, yet a surprising amount is known about these remarkable reptiles. Evidence about the existence and nature of dinosaurs is entirely indirect; it has been gleaned from fossilized traces that these animals left in sediment deposits.

The first indications suggesting the existence of the huge, extinct creatures that we now know as dinosaurs were traces of their ancient footprints in sedimentary rocks. Dinosaurs left their footprints in soft mud as they moved along marine shores or riverbanks. That mud was subsequently covered over as a new layer of sediment accumulated, and it later solidified into rock. Under very rare circumstances, this process preserved traces of the footprints of dinosaurs. Interestingly, the footprints were initially attributed to giant birds because of their superficial resemblance to tracks made by the largest of the living birds, such as the ostrich and emu.

The first fossilized skeletal remains to be identified as those of giant, extinct reptiles were discovered by miners in western Europe. These first discoveries were initially presumed to be astonishingly gigantic, extinct lizards. Several naturalists recognized substantial anatomical differences between the fossil bones and those of living reptiles, however, and so the dinosaurs were “discovered.” The first of these finds consisted of bones of a 35–50 ft (10–15 m) long carnivore named Megalosaurus ; this was the first dinosaur to be named scientifically. A large herbivore named Iguanodon was found at about the same time in sedimentary rocks in mines in England, Belgium, and France.

Discoveries of fantastic, extinct mega-reptiles in Europe were soon followed by even more exciting finds of dinosaur fossils in North America and elsewhere. These events captured the fascination of both naturalists and the general public. Museums started to develop extraordinary displays of re-assembled dinosaur skeletons, and artists prepared equally extraordinary depictions of dinosaurs and their hypothesized appearances and habitats.

This initial hey-day of dinosaur fossil discoveries occurred in the late nineteenth and early twentieth centuries. During this period, many of the most important finds were made by North American paleontologists who discovered and began to mine rich deposits of fossils in the prairies. There was intense scientific interest in these American discoveries of fossilized bones of gargantuan, seemingly preposterous animals, such as the awesome predator Tyrannosaurus and the immense herbivore Apatosaurus (initially known as Brontosaurus ). Unfortunately, the excitement and scientific frenzy led to competition among some of the paleontologists, who wanted to be known for discovering the biggest, fiercest, or weirdest dinosaurs. The most famous rivals were two American scientists, Othniel C. Marsh and Edwin Drinker Cope.

Other famous discoveries of fossilized dinosaur bones were made in the Gobi Desert of eastern Asia. Some of the finds include nests with eggs that contain fossilized embryos used to study dinosaur development. Some nests contain hatchlings, suggesting that dinosaur parents cared for their young. In addition, the clustering of the nests of some dinosaurs suggests social behavior including communal nesting, possibly for mutual protection against marauding predatory dinosaurs. In the valley of Ukhaa Tolgod, Mongolia, the skeleton of an adult oviraptor was found hunched over her nest of eggs, just like any incubating bird.

A find of dinosaur eggs in an Argentinian desert in 1998 is one of the largest collections ever discovered. It consists of hundreds of 6 in (15 cm) eggs of Titanosaurs, 45 ft (13.7 m) long relatives of Apatosaurus. The eggs were laid 70–90 million years ago, and skeletons of about 36 15-in (38-cm) long babies were also found in the mudstone. The paleontologists named the site “Auca Maheuvo,” after a local volcano and the Spanish words for “more eggs.” They hope to assemble an “ontological series” of eggs and embryos from the fossils to show all the stages of baby dinosaur development. Other scientists have speculated that this type of dinosaur gave birth to live young, and the discovery of the egg bonanza resolves that question.

Fossilized dinosaur bones have been discovered on all continents. Discoveries of fossils in the Arctic and in Antarctica suggest that the climate was much warmer when dinosaurs roamed Earth. It is also likely that polar dinosaurs were migratory, probably traveling to high latitudes to feed and breed during the summer and returning to lower latitudes during the winter. These migrations may have occurred mostly in response to the lack of sunlight during the long polar winters, rather than the cooler temperatures.

Although the most important fossil records of dinosaurs involve their bones, there is other evidence as well. In addition to footprints, eggs, and nests, there have also been finds of imprints of dinosaur skin, feces (known as coprolites), rounded gizzard stones (known as gastroliths), and even possible stomach contents. Fossilized imprints of feathers associated with dinosaurs called Sinosauropteryx and Protarchaeiopteryx found in the Liaoning Province of China show not only long flight and tail feathers but downy under feathers. In addition, fossilized plant remains are sometimes associated with deposits of dinosaur fossils, and these can be used to infer something about the habitats of these animals. Inferences can also be based on the geological context of the locations of fossils, such as their proximity to ocean shores or geographical position for-polar dinosaurs. These types of information have been studied and used to infer the shape, physiology, behavior, and ecological relationships of extinct dinosaurs.

Major groups of dinosaurs

There is only incomplete knowledge of the evolutionary relationships of dinosaurs with each other and with other major groups of reptiles. This results from the fact that dinosaurs, like any other extinct organism, can only be studied through their fossilized remains, which are often rare and fragmentary. Nevertheless, some dinosaur species bear clear resemblances to each other but are also obviously distinct from certain other dinosaurs.

The dinosaurs evolved from a group of early reptiles known as thecodonts, which arose during the Permian period (290–250 million years ago) and were dominant throughout the Triassic (250–208 million years ago). It appears that two major groups of dinosaurs evolved from the thecodonts, the ornithischian (bird hips) dinosaurs and the saurischian (lizard hips) dinosaurs. These two groups are distinguished largely on the basis of the anatomical structure of their pelvic or hip bones.

Both of these dinosaur lineages originated at about the same time. Both evolved into many species that were ecologically important and persisted until about 66 million years ago. Both groups included quadrupeds that walked on all four legs, as well as bipeds that walked erect on their much-larger hind legs. All of the ornithischians had birdlike beaks on their lower jaws and all were herbivores. Most of the carnivorous, or predatory, dinosaurs were saurischians, as were some of the herbivorous species. Interestingly, despite some resemblance between ornithischian dinosaur and bird physiology, it appears that the first birds actually evolved from saurischians.

Carnivorous dinosaurs

The carnosaurs were a group of saurischian predators, or theropods, that grew large and had enormous hind limbs but tiny fore limbs. Tyrannosaurus rex was the largest carnivore that has ever stalked Earth; its scientific name is derived from Greek words for “tyrant reptile king.” This fearsome, bipedal predator of the Late Cretaceous could grow to a length of 45 ft (14 m) and may have weighed as much as 9 tons (8.2 metric tons). Tyrannosaurus rex (T. rex ) had a massive head and a mouth full of about 60 dagger-shaped, 6-in long (15-cm-long), very sharp, serrated teeth, which were renewed throughout the life of the animal. This predator probably ran in a lumbering fashion using its powerful hind legs, which may also have been wielded as sharp-clawed, kicking weapons. It is thought that T. rex may have initially attacked its prey with powerful head-butts and then torn the animal apart with its enormous, 3 ft long (1 m long) jaws. Alternatively, T. rex may have been a scavenger of dead dinosaurs. The relatively tiny fore legs of T. rex probably only had little use. The long and heavy tail of T. rex was used as a counter-balance for the animal while it was running and as a stabilizing prop while it was standing.

Albertosaurus was also a large theropod of the Late Cretaceous. Albertosaurus was similar to Tyrannosaurus, but it was a less massively built animal at about 25 ft (8 m) long and 2 tons (1.8 metric tons) in weight. Albertosaurus probably moved considerably faster than Tyrannosaurus.

Allosaurus was a gigantic, bipedal predator of the Late Jurassic. Allosaurus could grow to a length of 36 ft (12 m) and a weight of 2 tons (1.8 metric tons). The jaws of Allosaurus were loosely hinged, and they could detach to swallow large chunks of prey.

Spinosaurus was a “fin-back” (or “sail-back”) dinosaur of the Late Cretaceous period that was distantly related to Allosaurus. Spinosaurus had long, erect, skin-covered, bony projections from its verte-brae that may have been used to regulate body temperature or perhaps for behavioral displays to impress others or attract a mate. Spinosaurus could have achieved a length of 40 ft (13 m) and a weight of 7 tons (6.3 metric tons). These animals had small, sharp teeth and were probably carnivores. Dimetrodon and Edaphosaurus, early Permian pelycosaurs (mammal-like reptiles, not dinosaurs), are sometimes confused with Spinosaurus as they also had sail-like back spines.

Not all of the fearsome dinosaurian predators or theropods were enormous. Deinonychus, for example, was an Early Cretaceous dinosaur that grew to about 10 ft (3 m) and weighed around 220 lb (100 kg). Deinonychus was one of the so-called running lizards, which were fast, agile predators that likely hunted in packs. As a result, Deinonychus was probably a fearsome predator of animals much larger than itself. Deinonychus had one of its hind claws enlarged into a sharp, sickle-like, slashing weapon, which was wielded by jumping on its prey and then kicking, slashing, and disemboweling the victim. The scientific name of Deinonychus is derived from the Greek words for terrible claw.

The most infamous small theropod is Velociraptor, or “swift plunderer,” a 6-ft-long (2-m-long) animal of the Late Cretaceous. Restorations of this fearsome, highly intelligent, pack-hunting, killing machine were used in the movie Jurassic Park.

Oviraptosaurs (egg-stealing reptiles) were relatively small, probably highly intelligent theropods that were fast-running hunters of small animals, and some are believed to have also been specialized predators of the nests of other dinosaurs. The best known of these animals is Late Cretaceous Oviraptor. Ingenia, a somewhat smaller oviraptorsaur, was about 6 ft (2 m) long, weighed about 55 lb (25 kg), and also lived during the Late Cretaceous. Microvenator of the early Cretaceous was less than 3 ft (1 m) long and weighed about 12 lb (6 kg).

Herbivorous dinosaurs

The sauropods were a group of large saurischian herbivores that included the world’s largest-ever terrestrial animals. This group rumbled along on four, enormous, pillar-like, roughly equal-sized legs, with a long tail trailing behind. Sauropods also had very long necks, and their heads were relatively small, at least in comparison with the overall mass of these immense animals. The teeth were peglike and were mostly used for grazing, rather than for chewing their diet of plant matter. Digestion was probably aided by large stones in an enormous gizzard, in much the same way that modern, seed-eating birds grind their food. The sauropods were most abundant during the Late Jurassic. They declined afterwards and were replaced as dominant herbivores by different types of dinosaurs, especially the hadrosaurs.

Apatosaurus was a large sauropod that lived during the Late Jurassic and reached a length of 65 ft (20 m) and a weight of 30 tons (27 metric tons). Diplodocus was a related animal of the Late Jurassic, but it was much longer in its overall body shape. A remarkably complete skeleton of Diplodocus was found that was 90 ft (27 m) long overall, with a 25 ft (8 m) neck and a 45 ft (14 m) tail, and an estimated body weight of 11 tons (10 metric tons). In comparison, the stouter-bodied Apatosaurus was slightly shorter but considerably heavier. Brachiosaurus also lived during the Late Jurassic and was an even bigger herbivore, with a length as great as 100 ft (30 m) and an astonishing weight that may have reached 80 tons (73 metric tons), although conservative estimates are closer to 55 tons (50 metric tons). Supersaurus and Ultrasaurus were similarly large. Seismosaurus may have been longer than 160 ft (50 m), and Argentinosaurus (recently discovered in Patagonia, South America) may set a new weight record of 100 tons (91 metric tons).

Stegosaurus was a 30 ft long (9 m long), Late Jurassic tetrapod with a distinctive row of triangular, erect, bony plates running along its back. These may have been used to regulate heat. Stegosaurus had sharp-spiked projections at the end of its tail, which were lashed at predators as a means of defense. Dacentrurus was a 13-ft-long (4-m-long), Jurassic-age animal related to Stegosaurus, but it had a double row of large spikes along the entire top of its body, from the end of the tail to the back of the head.

The ceratopsians were various types of “horned” dinosaurs. Triceratops was a three-horned dinosaur and was as long as 33 ft (10 m) and weighed 6 tons (5.4 metric tons). Triceratops lived in the late Cretaceous, and it had a large bony shield behind the head with three horns projecting from the forehead and face, which were used as defensive weapons. Anchiceratops was a 7-ton (6.3-metric-ton) animal that lived somewhat later. It was one of the last of the dinosaurs and became extinct 66 million years ago at the end of the Cretaceous period. There were also rhinoceros-like, single-horned dinosaurs, such as the 20-ft-long (6-m-long), 2-ton (1.8-metric-ton) Centrosaurus of the late Cretaceous. Fossilized skeletons of this animal have been found in groups, suggesting that it was a herding dinosaur. The horned dinosaurs were herbivores, and they had parrot-like beaks useful for eating vegetation.

Ankylosaurus was a late Cretaceous animal that was as long as 36 ft (11 m) and weighed 5 tons (4.5 metric tons). Ankylosaurus was a stout, short-legged, lumbering herbivore. This animal had very heavy and spiky body armor and a large bony club at the end of its tail that was used to defend itself against predators.

The duck-billed dinosaurs or hadrosaurs included many herbivorous species of the Cretaceous period. Hadrosaurs are sometimes divided into groups based on aspects of their head structure; they could have a flattish head, a solid crest on the top of their head, or an unusual, hollow crest. Hadrosaurs were the most successful of the late Cretaceous dinosaurs in terms of their relative abundance and wide distribution.

Hadrosaurs apparently were social animals; they lived in herds for at least part of the year and migrated seasonally in some places. Hadrosaurs appear to have nested communally, incubated their eggs, and brooded their young. Hadrosaurs had large hind legs and could walk on all four legs or bipedally if more speed was required; these animals were probably very fast runners.

Hadrosaurus was a 5-ton (4.5-metric-ton), late Cretaceous animal and was the first dinosaur to be discovered and named in North America—in 1858 from fossils found in New Jersey. Corythosaurus was a 36 ft-long (11 m-long), 4 ton (3.6 metric ton), Late Cretaceous herbivore that had a large, hollow, helmet-like crest on the top of its head. Parasaurolophus of the late Cretaceous was similar in size, but it had a curved, hollow crest that swept back as far as 10 ft (3 m) from the back of the head. It has been suggested that this exaggerated helmet may have worked like a snorkel when this animal was feeding underwater on aquatic plants; however, more likely uses of the swept-back helmet were in species recognition and resonating the loud sounds made by these hadrosaurs. Edmontosaurus was a large, non-helmeted hadrosaur that lived in the Great Plains during the late Cretaceous and was as long as 40 ft (13 m) and weighed 3 tons (2.7 metric tons). Anatosaurus was a 3 ton (2.7 metric-ton) hadrosaur that lived as recently as 66 million years ago and was among the last of the dinosaurs to become extinct. The hadrosaurs probably were a favorite prey for some of the large theropods, such as Tyrannosaurus rex.

Other extinct orders of Mesozoic-age reptiles

Several other orders of large reptiles lived at the same time as the dinosaurs and are also now extinct. The pterosaurs (order Pterosauria) were large, flying reptiles that lived from the late Triassic to the late Cretaceous. Some species of pterosaurs had wingspans as great as 40 ft (12 m), much wider than any other flying animal has ever managed to achieve. Functional biologists studying the superficially awkward designs of these animals have long wondered how they flew. Some species of pterosaurs are thought to have fed on fish, which were scooped up as the pterosaur glided just above the water surface.

The ichthyosaurs (Ichthyosauria), plesiosaurs (Plesiosauria), and mosasaurs (Mososauria) were orders of carnivorous marine reptiles that became extinct in the Late Cretaceous. The ichthyosaurs were shark-like in form, except that their vertebral column extended into the lower part of their caudal (or tail) fin, rather than into the upper part like the sharks. Of course, ichthyosaurs also had well-developed, bony skeletons, whereas sharks have a skeleton composed entirely of cartilage rather than bone. The plesiosaurs were large animals reaching a length as great as 45 ft (14 m). These marine reptiles had paddle-shaped limbs, and some species had very long necks. Mosasaurs were large lizards that had fin-shaped limbs and looked something like a cross between a crocodile and an eel; but they grew to lengths of more than 30 ft (9 m).

Theories about the extinction of dinosaurs

There are many theories about what caused the extinction of the last of the dinosaurs, which occurred at the end of the Cretaceous period, about 66 million years ago. Some of the more interesting ideas include: the intolerance of these animals to rapid climate change, the emergence of new species of dominant plants that contained toxic chemicals the herbivorous dinosaurs could not tolerate, an inability to compete successfully with the rapidly evolving mammals, insatiable destruction of dinosaur nests and eggs by mammalian predators, and widespread disease to which dinosaurs were not able to develop immunity. All of these hypotheses

KEY TERMS

Adaptive radiation— An evolutionary phenomenon in which a single, relatively uniform population gives rise to numerous, reproductively isolated species. Adaptive radiation occurs in response to natural selection, in environments in which there are diverse ecological opportunities, and little competition to filling them.

Homoiothermic— Refers to “warm-blooded” animals that regulate their body temperature independently of the ambient, environmental temperature.

Mass extinction— The extinction of an unusually large number of species in a geologically short period of time.

Poikilothermic— Refers to animals that do not have a physiological mechanism to control their internal body temperature and so adopt the temperature of the ambient environment. “Cold-blooded” animals.

are interesting, but the supporting evidence for any one of them is not enough to convince most paleontologists.

Interestingly, at the time of the extinction of the last of the dinosaurs, there were also apparently mass extinctions of other groups of organisms. These included the reptilian order Pterosauria, along with many groups of plants and invertebrates. In total, perhaps three quarters of all species and one half of all genera may have become extinct at the end of the Cretaceous. A popular hypothesis for the cause of this catastrophic, biological event was the impact of a meteor hitting Earth. The impact of an estimated 6 mi-wide (10 km-wide) meteorite could have spewed an enormous quantity of fine dust into the atmosphere, which could have caused climate changes that most large animals and other organisms could not tolerate. As with the other theories about the end of the dinosaurs, this one is controversial. Other scientists believe the extinctions of the last dinosaurs were more gradual and were not caused by—or were not caused solely by—the shorter-term effects of a rogue meteorite.

Dinosaurs share many anatomical characteristics with Aves, the birds, a group that is now known to have evolved from a dinosaur ancestor. In fact, there are excellent fossil remains of evolutionary links between birds and dinosaurs. The 3 ft-long (1 m-long), Late Jurassic fossil organism Archaeopteryx looked remarkably like Compsognathus but had a feathered body and could fly or glide. Fossils of feathered (but flightless) dinosaur species have been discovered in China since 2000, adding weight to the view that feathers evolved first as a temperature-regulation mechanism—as they still are by birds, including flightless birds—and only later were used for flight. Moreover, some of the living, flightless birds such as emus and ostriches and recently extinct birds such as elephant birds and moas bear a remarkable resemblance to certain types of dinosaurs. Because of the apparent continuity of anatomical characteristics between dinosaurs and birds, some paleontologists believe that the dinosaurs did not actually become extinct. Instead, the dinosaur lineage survives today in a substantially modified form as the group Aves, the birds.

See also Evolution; Fossil and fossilization; Paleontology.

Resources

BOOKS

Fastovsky, David E., et al. The Evolution and Extinction of the Dinosaurs. Cambridge, UK: Cambridge University Press, 2005.

Rogers, Kristina and Jeffrey Wilson. The Sauropods: Evolution and Paleobiology. Berkeley, CA: University of California Press, 2005.

Tidwell, Virginia and Kenneth Carpenter. Thunder-lizards: The Sauropodomorph Dinosaurs. Bloomington, IN: Indiana University Press, 2005.

Weishampel, David B., et al., eds. The Dinosauria. Berkeley, CA: University of California Press, 2004.

Bill Freedman

Dinosaur

Dinosaurs are a group of now-extinct, terrestrial reptiles in the order Dinosauria that lived from about 225 million years ago to 66 million years ago, during the Mesozoic era. Species of dinosaurs ranged from chicken-sized creatures such as the 2 lb (1 kg) predator Compsognathus to colossal, herbivorous animals known as "sauropods," which were larger than any terrestrial animals that lived before or since. Some dinosaurs were enormous, awesomely fierce predators, while others were mild-mannered herbivores, or plant eaters, that reached an immense size. The word "dinosaur" is derived from two Greek words, meaning "terrible lizard." The term refers to some of the huge and awesome predatory dinosaurs—the first of these extinct reptiles to be discovered that were initially thought to be lizard-like in appearance and biology . But Richard Owen (1804-1892), the British expert in comparative anatomy , also coined the word in awe of the complexity of this wide variety of creatures that lived so long ago and yet were so well-adapted to their world.

Dinosaurs were remarkable and impressive animals but are rather difficult to define as a zoological group. They were terrestrial animals that had upright legs, rather than legs that sprawled outward from the body. Their skulls had two temporal openings on each side (in addition to the opening for the eyes), as well as other common and distinctive features. The dinosaurs were distinguished from other animals, however, by distinctive aspects of their behavior , physiology , and ecological relationships. Unfortunately, relatively little is known about these traits because we can only learn about dinosaurs using their fossil traces, which are rare and incomplete. It is clear from the available evidence that some species of dinosaurs were large predators, others were immense herbivores, and still others were smaller predators, herbivores, or scavengers. Sufficient information is available to allow paleontologists to assign scientific names to many of these dinosaurs and to speculate about their evolutionary and ecological relationships.

Although they are now extinct, the dinosaurs were among the most successful large animals ever to live on Earth . The dinosaurs arose during the interval of geologic time known as the Mesozoic (middle life) era, often called the "golden age of reptiles" or "the age of dinosaurs." Radiometric dating of volcanic rocks associated with dinosaur fossils suggests they first evolved 225 million years ago, during the late Triassic Period and became extinct 66 million years ago, at the end of the Cretaceous period. Dinosaurs lived for about 160 million years and were the dominant terrestrial animals on Earth throughout the Jurassic and Cretaceous periods—a span of over 100 million years.

Interestingly, mammal-like animals co-existed almost continuously with the dinosaurs and obviously prospered after the last of the dinosaurs became extinct. Although they co-existed in time with dinosaurs, mammals were clearly subordinate to these reptiles. It was not until the disappearance of the last dinosaurs that an adaptive radiation of larger species of mammals occurred, and they then became the dominant large animals on Earth.

It is not known exactly what caused the last dinosaurs to become extinct. It must be stressed, however, that dinosaurs were remarkably successful animals. These creatures were dominant on Earth for an enormously longer length of time than the few tens of thousands of years that humans have been a commanding species.

Biology of dinosaurs

The distinguishing characteristics of the dinosaurs include the structure of their skull and other bones. Dinosaurs typically had 25 vertebrae, plus three vertebrae that were fused to form their pelvic bones. The dinosaurs displayed an enormous range of forms and functions, however, and they filled a wide array of ecological niches. Some of the dinosaurs were, in fact, quite bizarre in their shape and, undoubtedly, their behavior.

The smallest dinosaurs were chicken-like carnivores that were only about 1 ft (30 cm) long and weighed 5-6 lb (2-3 kg). The largest dinosaurs reached a length of over 100 ft (30 m) and weighed 80 tons (73 metric tons) or more—more than any other terrestrial animal has ever achieved. The largest blue whales can weigh more than this, about 110 tons (100 metric tons), representing the largest animals ever to occur on Earth. The weight of these aquatic animals is partially buoyed by the water that they live in; whales do not have to fully support their immense weight against the forces of gravity, as the dinosaurs did. When compared with the largest living land animal, the African elephant , which weighs as much as 7.5 tons (6.8 metric tons), the large species of dinosaurs were enormous creatures.

Most species of dinosaurs had long tails and long necks, but this was not the case for all species. Most of the dinosaurs walked on all four legs, although some species were bipedal, using only their rear legs for locomotion. Their forelegs were greatly reduced in size and probably were used only for grasping. The tetrapods that walked on four legs were all peaceful herbivores. In contrast, many of the bipedal dinosaurs were fast-running predators.

The teeth of dinosaur species were highly diverse. Many species were exclusively herbivorous, and their teeth were correspondingly adapted for cutting and grinding vegetation. Other dinosaurs were fierce predators, and their teeth were shaped like serrated knives, which seized and stabbed their prey and cut it into smaller pieces that could be swallowed whole.

Until recently, it was widely believed that dinosaurs were rather stupid, slow-moving, cold-blooded (or poikilothermic) creatures. Some scientists now believe, however, that dinosaurs were intelligent, social, quick-moving, and probably warm-blooded (or homoiothermic) animals. This is a controversial topic, and scientific consensus has not been reached on whether or not some of the dinosaurs were able to regulate their body temperature by producing heat through metabolic reactions. It is absolutely undeniable that dinosaurs were extremely capable animals. This should not be a surprise to us, considering the remarkable evolutionary successes that they attained.

Fossils and other evidence of the dinosaurs

Humans never co-existed with dinosaurs, yet a surprising amount is known about these remarkable reptiles. Evidence about the existence and nature of dinosaurs is entirely indirect; it has been gleaned from fossilized traces that these animals left in sediment deposits.

The first indications suggesting the existence of the huge, extinct creatures that we now know as dinosaurs were traces of their ancient footprints in sedimentary rocks. Dinosaurs left their footprints in soft mud as they moved along marine shores or riverbanks. That mud was subsequently covered over as a new layer of sediment accumulated, and it later solidified into rock. Under very rare circumstances, this process preserved traces of the footprints of dinosaurs. Interestingly, the footprints were initially attributed to giant birds because of their superficial resemblance to tracks made by the largest of the living birds, such as the ostrich and emu.

The first fossilized skeletal remains to be identified as those of giant, extinct reptiles were discovered by miners in western Europe . These first discoveries were initially presumed to be astonishingly gigantic, extinct lizards. Several naturalists recognized substantial anatomical differences between the fossil bones and those of living reptiles, however, and so the dinosaurs were "discovered." The first of these finds consisted of bones of a 35-50 ft (10-15 m) long carnivore named Megalosaurus; this was the first dinosaur to be named scientifically. A large herbivore named Iguanodon was found at about the same time in sedimentary rocks in mines in England, Belgium, and France.

Discoveries of fantastic, extinct mega-reptiles in Europe were soon followed by even more exciting finds of dinosaur fossils in North America and elsewhere. These events captured the fascination of both naturalists and the general public. Museums started to develop extraordinary displays of re-assembled dinosaur skeletons, and artists prepared equally extraordinary depictions of dinosaurs and their hypothesized appearances and habitats.

This initial hey-day of dinosaur fossil discoveries occurred in the late nineteenth and early twentieth centuries. During this period, many of the most important finds were made by North American paleontologists who discovered and began to mine rich deposits of fossils in the prairies. There was intense scientific interest in these American discoveries of fossilized bones of gargantuan, seemingly preposterous animals, such as the awesome predator Tyrannosaurus and the immense herbivore Apatosaurus (initially known as Brontosaurus). Unfortunately, the excitement and scientific frenzy led to competition among some of the paleontologists, who wanted to be known for discovering the biggest, fiercest, or weirdest dinosaurs. The most famous rivals were two American scientists, Othniel C. Marsh and Edwin Drinker Cope.

Other famous discoveries of fossilized dinosaur bones were made in the Gobi Desert of eastern Asia . Some of those finds include nests with eggs that contain fossilized embryos used to study dinosaur development. Some nests contain hatchlings, suggesting that dinosaur parents cared for their young. In addition, the clustering of the nests of some dinosaurs suggests social behavior including communal nesting, possibly for mutual protection against marauding predatory dinosaurs. In the valley of Ukhaa Tolgod, Mongolia, the skeleton of an adult oviraptor was found hunched over her nest of eggs, just like any incubating bird.

A find of dinosaur eggs in an Argentinian desert in 1998 is one of the largest collections ever discovered. It consists of hundreds of 6 in (15 cm) eggs of Titanosaurs, 45 ft (13.7 m) long relatives of the Brontosaurus. The eggs were laid 70-90 million years ago, and skeletons of about 36 15-in (38-cm) long babies were also found in the mudstone. The paleontologists named the site "Auca Maheuvo," after a local volcano and the Spanish words for "more eggs." They hope to assemble an "ontological series" of eggs and embryos from the fossils to show all the stages of baby dinosaur development. Other scientists have speculated that this type of dinosaur gave birth to live young, and the discovery of the egg bonanza resolves that question.

Fossilized dinosaur bones have been discovered on all continents. Discoveries of fossils in the Arctic and in Antarctica suggest that the climate was much warmer when dinosaurs roamed Earth. It is also likely that polar dinosaurs were migratory, probably traveling to high latitudes to feed and breed during the summer and returning to lower latitudes during the winter. These migrations may have occurred mostly in response to the lack of sunlight during the long polar winters, rather than the cooler temperatures.

Although the most important fossil records of dinosaurs involve their bones, there is other evidence as well. In addition to footprints, eggs, and nests, there have also been finds of imprints of dinosaur skin, feces (known as coprolites), rounded gizzard stones (known as gastroliths), and even possible stomach contents. Fossilized imprints of feathers associated with dinosaurs called Sinosauropteryx and Protarchaeiopteryx found in the Liaoning Province of China show not only long flight and tail feathers but downy under feathers. In addition, fossilized plant remains are sometimes associated with deposits of dinosaur fossils, and these can be used to infer something about the habitats of these animals. Inferences can also be based on the geological context of the locations of fossils, such as their proximity to ocean shores or geographical position for polar dinosaurs. These types of information have been studied and used to infer the shape, physiology, behavior, and ecological relationships of extinct dinosaurs.

Major groups of dinosaurs

There is only incomplete knowledge of the evolutionary relationships of dinosaurs with each other and with other major groups of reptiles. This results from the fact that dinosaurs, like any other extinct organism , can only be studied through their fossilized remains, which are often rare and fragmentary. Nevertheless, some dinosaur species bear clear resemblances to each other but are also obviously distinct from certain other dinosaurs.

The dinosaurs evolved from a group of early reptiles known as thecodonts, which arose during the Permian period (290-250 million years ago) and were dominant throughout the Triassic (250-208 million years ago). It appears that two major groups of dinosaurs evolved from the thecodonts, the ornithischian (bird hips) dinosaurs and the saurischian (lizard hips) dinosaurs. These two groups are distinguished largely on the basis of the anatomical structure of their pelvic or hip bones.

Both of these dinosaur lineages originated at about the same time. Both evolved into many species that were ecologically important and persisted until about 66 million years ago. Both groups included quadrupeds that walked on all four legs, as well as bipeds that walked erect on their much-larger hind legs. All of the ornithischians had bird-like beaks on their lower jaws and all were herbivores. Most of the carnivorous, or predatory, dinosaurs were saurischians, as were some of the herbivorous species. Interestingly, despite some resemblance between ornithischian dinosaur and bird physiology, it appears that the first birds actually evolved from saurischians.

Carnivorous dinosaurs

The carnosaurs were a group of saurischian predators, or theropods, that grew large and had enormous hind limbs but tiny fore limbs. Tyrannosaurus rex was the largest carnivore that has ever stalked Earth; its scientific name is derived from Greek words for "tyrant reptile king." This fearsome, bipedal predator of the Late Cretaceous could grow to a length of 45 ft (14 m) and may have weighed as much as 9 tons (8.2 metric tons). Tyrannosaurus rex (T. rex) had a massive head and a mouth full of about 60 dagger-shaped, 6-in-long (15-cm-long), very sharp, serrated teeth, which were renewed throughout the life of the animal. This predator probably ran in a lumbering fashion using its powerful hind legs, which may also have been wielded as sharp-clawed, kicking weapons. It is thought that T. rex may have initially attacked its prey with powerful head-butts and then torn the animal apart with its enormous, 3 ft long (1 m long) jaws. Alternatively, T. rex may have been a scavenger of dead dinosaurs. The relatively tiny fore legs of T. rex probably only had little use. The long and heavy tail of T. rex was used as a counter-balance for the animal while it was running and as a stabilizing prop while it was standing.

Albertosaurus was also a large theropod of the Late Cretaceous. Albertosaurus was similar to Tyrannosaurus, but it was a less massively built animal at about 25 ft (8 m) long and 2 tons (1.8 metric tons) in weight. Albertosaurus probably moved considerably faster than Tyrannosaurus.

Allosaurus was a gigantic, bipedal predator of the Late Jurassic. Allosaurus could grow to a length of 36 ft (12 m) and a weight of 2 tons (1.8 metric tons). The jaws of Allosaurus were loosely hinged, and they could detach to swallow large chunks of prey.

Spinosaurus was a "fin-back" (or "sail-back") dinosaur of the Late Cretaceous period that was distantly related to Allosaurus. Spinosaurus had long, erect, skin-covered, bony projections from its vertebrae that may have been used to regulate body temperature or perhaps for behavioral displays to impress others or attract a mate. Spinosaurus could have achieved a length of 40 ft (13 m) and a weight of 7 tons (6.3 metric tons). These animals had small, sharp teeth and were probably carnivores. Dimetrodon and Edaphosaurus, early Permian pelycosaurs (mammal-like reptiles, not dinosaurs), are sometimes confused with Spinosaurus as they also had sail-like back spines.

Not all of the fearsome dinosaurian predators or theropods were enormous. Deinonychus, for example, was an Early Cretaceous dinosaur that grew to about 10 ft (3 m) and weighed around 220 lb (100 kg). Deinonychus was one of the so-called running lizards, which were fast, agile predators that likely hunted in packs. As a result, Deinonychus was probably a fearsome predator of animals much larger than itself. Deinonychus had one of its hind claws enlarged into a sharp, sickle-like, slashing weapon, which was wielded by jumping on its prey and then kicking, slashing, and disemboweling the victim. The scientific name of Deinonychus is derived from the Greek words for terrible claw.

The most infamous small theropod is Velociraptor, or "swift plunderer," a 6-ft-long (2-m-long) animal of the Late Cretaceous. Restorations of this fearsome, highly intelligent, pack-hunting, killing machine were used in the movie Jurassic Park.

Oviraptosaurs (egg-stealing reptiles) were relatively small, probably highly intelligent theropods that were fast-running hunters of small animals, and some are believed to have also been specialized predators of the nests of other dinosaurs. The best known of these animals is Late Cretaceous Oviraptor. Ingenia, a somewhat smaller oviraptorsaur, was about 6 ft (2 m) long, weighed about 55 lb (25 kg), and also lived during the Late Cretaceous. Microvenator of the early Cretaceous was less than 3 ft (1 m) long and weighed about 12 lb (6 kg).

Herbivorous dinosaurs

The sauropods were a group of large saurischian herbivores that included the world's largest-ever terrestrial animals. This group rumbled along on four, enormous, pillar-like, roughly equal-sized legs, with a long tail trailing behind. Sauropods also had very long necks, and their heads were relatively small, at least in comparison with the overall mass of these immense animals. The teeth were peg-like and were mostly used for grazing, rather than for chewing their diet of plant matter . Digestion was probably aided by large stones in an enormous gizzard, in much the same way that modern, seed-eating birds grind their food. The sauropods were most abundant during the Late Jurassic. They declined afterwards and were replaced as dominant herbivores by different types of dinosaurs, especially the hadrosaurs.

Apatosaurus (previously known as Brontosaurus or the ground-shaking "thunder lizard") was a large sauropod that lived during the Late Jurassic and reached a length of 65 ft (20 m) and a weight of 30 tons (27 metric tons). Diplodocus was a related animal of the Late Jurassic, but it was much longer in its overall body shape. A remarkably complete skeleton of Diplodocus was found that was 90 ft (27 m) long overall, with a 25 ft (8 m) neck and a 45 ft (14 m) tail, and an estimated body weight of 11 tons (10 metric tons). In comparison, the stouter-bodied Apatosaurus was slightly shorter but considerably heavier. Brachiosaurus also lived during the Late Jurassic and was an even bigger herbivore, with a length as great as 100 ft (30 m) and an astonishing weight that may have reached 80 tons (73 metric tons), although conservative estimates are closer to 55 tons (50 metric tons). Supersaurus and Ultrasaurus were similarly large. Seismosaurus may have been longer than 160 ft (50 m), and Argentinosaurus (recently discovered in Patagonia, South America ) may set a new weight record of 100 tons (91 metric tons).

Stegosaurus was a 30 ft long (9 m long), Late Jurassic tetrapod with a distinctive row of triangular, erect, bony plates running along its back. These may have been used to regulate heat. Stegosaurus had sharp-spiked projections at the end of its tail, which were lashed at predators as a means of defense. Dacentrurus was a 13-ft-long (4-m-long), Jurassic-age animal related to Stegosaurus, but it had a double row of large spikes along the entire top of its body, from the end of the tail to the back of the head.

The ceratopsians were various types of "horned" dinosaurs. Triceratops was a three-horned dinosaur and was as long as 33 ft (10 m) and weighed 6 tons (5.4 metric tons). Triceratops lived in the late Cretaceous, and it had a large bony shield behind the head with three horns projecting from the forehead and face, which were used as defensive weapons. Anchiceratops was a 7-ton (6.3-metric-ton) animal that lived somewhat later. It was one of the last of the dinosaurs and became extinct 66 million years ago at the end of the Cretaceous period. There were also rhinoceros-like, single-horned dinosaurs, such as the 20-ft-long (6-m-long), 2-ton (1.8-metric-ton) Centrosaurus of the late Cretaceous. Fossilized skeletons of this animal have been found in groups, suggesting that it was a herding dinosaur. The horned dinosaurs were herbivores, and they had parrot-like beaks useful for eating vegetation.

Ankylosaurus was a late Cretaceous animal that was as long as 36 ft (11 m) and weighed 5 tons (4.5 metric tons). Ankylosaurus was a stout, short-legged, lumbering herbivore. This animal had very heavy and spiky body armor and a large bony club at the end of its tail that was used to defend itself against predators.

The duck-billed dinosaurs or hadrosaurs included many herbivorous species of the Cretaceous period. Hadrosaurs are sometimes divided into groups based on aspects of their head structure; they could have a flattish head, a solid crest on the top of their head, or an unusual, hollow crest. Hadrosaurs were the most successful of the late Cretaceous dinosaurs in terms of their relative abundance and wide distribution.

Hadrosaurs apparently were social animals; they lived in herds for at least part of the year and migrated seasonally in some places. Hadrosaurs appear to have nested communally, incubated their eggs, and brooded their young. Hadrosaurs had large hind legs and could walk on all four legs or bipedally if more speed was required—these animals were probably very fast runners.

Hadrosaurus was a 5-ton (4.5-metric-ton), late Cretaceous animal and was the first dinosaur to be discovered and named in North America-in 1858 from fossils found in New Jersey. Corythosaurus was a 36 ft-long (11 m-long), 4 ton (3.6 metric ton), Late Cretaceous herbivore that had a large, hollow, helmet-like crest on the top of its head. Parasaurolophus of the late Cretaceous was similar in size, but it had a curved, hollow crest that swept back as far as 10 ft (3 m) from the back of the head. It has been suggested that this exaggerated helmet may have worked like a snorkel when this animal was feeding underwater on aquatic plants; however, more likely uses of the sweptback helmet were in species recognition and resonating the loud sounds made by these hadrosaurs. Edmontosaurus was a large, non-helmeted hadrosaur that lived in the Great Plains during the late Cretaceous and was as long as 40 ft (13 m) and weighed 3 tons (2.7 metric tons). Anatosaurus was a 3 ton (2.7 metric-ton) hadrosaur that lived as recently as 66 million years ago and was among the last of the dinosaurs to become extinct. The hadrosaurs probably were a favorite prey for some of the large theropods, such as Tyrannosaurus rex.

Other extinct orders of Mesozoic-age reptiles

Several other orders of large reptiles lived at the same time as the dinosaurs and are also now extinct. The pterosaurs (order Pterosauria) were large, flying reptiles that lived from the late Triassic to the late Cretaceous. Some species of pterosaurs had wingspans as great as 40 ft (12 m), much wider than any other flying animal has ever managed to achieve. Functional biologists studying the superficially awkward designs of these animals have long wondered how they flew. Some species of pterosaurs are thought to have fed on fish , which were scooped up as the pterosaur glided just above the water surface.

The ichthyosaurs (Ichthyosauria), plesiosaurs (Plesiosauria), and mosasaurs (Mososauria) were orders of carnivorous marine reptiles that became extinct in the Late Cretaceous. The ichthyosaurs were shark-like in form, except that their vertebral column extended into the lower part of their caudal (or tail) fin, rather than into the upper part like the sharks . Of course, ichthyosaurs also had well-developed, bony skeletons, whereas sharks have a skeleton composed entirely of cartilage rather than bone. The plesiosaurs were large animals reaching a length as great as 45 ft (14 m). These marine reptiles had paddle-shaped limbs, and some species had very long necks. Mosasaurs were large lizards that had fin-shaped limbs and looked something like a cross between a crocodile and an eel; but they grew to lengths of more than 30 ft (9 m).

Theories about the extinction of dinosaurs

There are many theories about what caused the extinction of the last of the dinosaurs, which occurred at the end of the Cretaceous period, about 66 million years ago. Some of the more interesting ideas include: the intolerance of these animals to rapid climate change, the emergence of new species of dominant plants that contained toxic chemicals the herbivorous dinosaurs could not tolerate, an inability to compete successfully with the rapidly evolving mammals, insatiable destruction of dinosaur nests and eggs by mammalian predators, and widespread disease to which dinosaurs were not able to develop immunity. All of these hypotheses are interesting, but the supporting evidence for any one of them is not enough to convince most paleontologists.

Interestingly, at the time of the extinction of the last of the dinosaurs, there were also apparently mass extinctions of other groups of organisms. These included the reptilian order Pterosauria, along with many groups of plants and invertebrates . In total, perhaps three quarters of all species and one half of all genera may have become extinct at the end of the Cretaceous. A popular hypothesis for the cause of this catastrophic, biological event was the impact of a meteor hitting Earth. The impact of an estimated 6 mi-wide (10 km-wide) meteorite could have spewed an enormous quantity of fine dust into the atmosphere, which could have caused climate changes that most large animals and other organisms could not tolerate. As with the other theories about the end of the dinosaurs, this one is controversial. Many scientists believe the extinctions of the last dinosaurs were more gradual and were not caused by the shorter-term effects of a rogue meteorite.

Another interesting concept concerns the fact that dinosaurs share many anatomical characteristics with Aves, the birds, a group that clearly evolved from a dinosaur ancestor. In fact, there are excellent fossil remains of an evolutionary link between birds and dinosaurs. The 3 ft-long (1 m-long), Late Jurassic fossil organism Archaeopteryx looked remarkably like Compsognathus but had a feathered body and could fly or glide. Moreover, some of the living, flightless birds such as emus and ostriches and recently extinct birds such as elephant birds and moas bear a remarkable resemblance to certain types of dinosaurs. Because of the apparent continuity of anatomical characteristics between dinosaurs and birds, some paleontologists believe that the dinosaurs did not actually become extinct. Instead, the dinosaur lineage survives today in a substantially modified form, as the group Aves, the birds.

See also Evolution; Fossil and fossilization; Paleontology.

Resources

books

Carpenter, K., and P.J. Currie. Dinosaur Systematics. Approaches and Perspectives. Cambridge, UK: Cambridge University Press, 1990.

Cowen, R. History of Life. London: Blackwell Scientific Publishing, 1995.

Palmer, Douglas. The Marshall Illustrated Encyclopedia of Dinosaurs & Prehistoric Animals: A Comprehensive Color Guide to over 500 Species. New York: Todtri, 2002.

Prothero, Donald R. Bringing Fossils To Life: An IntroductionTo Paleobiology. Columbus: McGraw-Hill Science/Engineering/Math, 1997.

Weishampel, D.B., ed. The Dinosauria. Berkeley, CA: University of California Press, 1990.

Bill Freedman

KEY TERMS

Adaptive radiation

—An evolutionary phenomenon in which a single, relatively uniform population gives rise to numerous, reproductively isolated species. Adaptive radiation occurs in response to natural selection, in environments in which there are diverse ecological opportunities, and little competition to filling them.

Homoiothermic

—Refers to "warm-blooded" animals that regulate their body temperature independently of the ambient, environmental temperature.

Mass extinction

—The extinction of an unusually large number of species in a geologically short period of time.

Poikilothermic

—Refers to animals that do not have a physiological mechanism to control their internal body temperature and so adopt the temperature of the ambient environment. "Cold-blooded" animals.

Dinosaur

Dinosaurs are a group of now-extinct, terrestrial reptiles in the order Dinosauria. They lived during the Mesozoic Era, from about 225 million years ago to 66 million years ago. Species of dinosaurs ranged from chicken-sized creatures such as the 2-pound (1-kilogram) predator Compsognathus to colossal, herbivorous animals known as sauropods weighing more than 80 tons (72 metric tons). The sauropods were larger than any terrestrial animals that lived before or since.

Some dinosaurs were enormous, awesomely fierce predators, while others were mild-mannered plant eaters. The word dinosaur is derived from two Greek words meaning "terrible lizard." The name comes from the fact that the remains of the earliest dinosaurs discovered were very large and showed they had a lizardlike appearance.

Biology of the dinosaurs

The dinosaurs shared some common physical characteristics, such as the presence of two openings on opposite sides of their skulls and 25 vertebrae. However, the dinosaurs also differed from each other in many important ways. They displayed an enormous range of forms and functions, and they filled a wide array of ecological niches. Some of the dinosaurs were, in fact, quite bizarre in their shape and, undoubtedly, their behavior.

Most species of dinosaurs had a long tail and long neck, but this was not the case for all species. Most of the dinosaurs walked on their four legs, although some species were bipedal, using only their rear legs for locomotion. Their forelegs were greatly reduced in size and probably used only for grasping. The species that walked on four legs were all peaceful herbivores. In contrast, many of the bipedal dinosaurs were fast-running predators.

The teeth of dinosaur species were highly diverse. Many species were exclusively herbivorous, and their teeth were correspondingly adapted for cutting and grinding vegetation. Other dinosaurs were fierce predators, and their teeth were shaped like serrated (notched) knives. These teeth were undoubtedly used to seize and stab their prey, cutting it into smaller pieces that could be swallowed whole.

Until recently, it was widely believed that dinosaurs were rather stupid, slow-moving, cold-blooded creatures. However, some scientists now believe that dinosaurs were intelligent, social, quick-moving, and probably warm-blooded animals. This question is still rather controversial. Scientists have not yet reached agreement as to whether at least some of the dinosaurs were able to regulate their body temperature by producing heat through metabolic reactions.

Evidence for the existence of dinosaurs

Humans have never lived at the same time as dinosaurs on Earth. Yet, a surprising amount is known about these remarkable reptiles. Evidence about the existence and nature of dinosaurs has been obtained from fossilized traces left by these animals in sediment deposits.

Words to Know

Bipedal: Walking on two feet.

Carnivore (carnivorous): Meat-eating.

Embryo: The earliest stage of animal development in the uterus before the animal is considered a fetus.

Extinct: No longer alive on Earth.

Fossil: Evidence of plant or animal life preserved in earth, usually in rocks.

Herbivore (herbivorous): Plant-eating.

Ornithischian dinosaurs: Dinosaurs with birdlike characteristics.

Predator: An animal that eats other animals.

Saurischian dinosaurs: Dinosaurs with reptilelike characteristics.

Sauropods: A group of large saurischian herbivores.

Terrestrial: Relating to the land.

Thecodonts: Early reptiles regarded as ancestors of the dinosaurs.

The first evidence suggesting the existence of dinosaurs was the discovery of traces of their ancient footprints in sedimentary rocks. Sedimentary rocks are formed when sand, silt, clay and other materials are packed together under great pressure. Dinosaurs left their footprints in soft mud as they moved along a marine shore or riverbank. That mud was subsequently covered over as a new layer of sediment accumulated, and later solidified into rock. Under very rare circumstances, this process preserved traces of the footprints of dinosaurs. Interestingly, the footprints were initially attributed to giant birds. They were somewhat similar to tracks made by the largest of the living birds, such as the ostrich and emu.

The first fossilized skeletal remains to be identified as those of giant, extinct reptiles were discovered by miners in western Europe. These first discoveries were initially presumed to be astonishingly gigantic, extinct lizards. However, several naturalists recognized substantial anatomical differences between the fossil bones and those of living reptiles. The first of these finds were bones of a 35- to 50-foot-long (10 to 15 meters) carnivore named Megalosaurus and a large herbivore named Iguanodon. Fossils of both were found in sedimentary rocks in mines in England, Belgium, and France.

Discoveries of fantastic, extinct oversized reptiles in Europe were soon followed by even more exciting finds of dinosaur fossils in North America and elsewhere. These events captured the fascination of both naturalists and the general public. Museums started to develop extraordinary displays of reassembled dinosaur skeletons.

This initial period of discoveries occurred in the late nineteenth and early twentieth centuries. During this period many of the most important finds were made by North American paleontologists (scientists who study fossils). An intense scientific interest grew over these American discoveries of fossilized bones of gargantuan, seemingly preposterous animals. Unfortunately, the excitement and scientific frenzy led to a rather passionate competition among some paleontologists, who wanted to be known for discovering the biggest, or the fiercest, or the weirdest dinosaurs.

Other famous discoveries of fossilized dinosaur bones have been made in the Gobi Desert of eastern Asia. Some of those finds include nests with eggs that contain fossilized embryos (the earliest stage of development). The embryos have been used to study dinosaur development. Some nests contain hatchlings, suggesting that dinosaur parents cared for their young. In addition, the clustering of the nests of some dinosaurs suggests that the animals had led a social life. They may have nested together, for example, for mutual protection against predatory dinosaurs.

By now, fossilized dinosaur bones have been discovered on all continents. Discoveries of fossils in the high Arctic and in Antarctica suggest that the climate there was much warmer when dinosaurs roamed Earth. It also is likely that polar dinosaurs were migratory, traveling to high latitudes to feed and breed during the summer and returning to lower latitudes during the winter.

Although the most important fossil records of dinosaurs involve their bones, other sorts of evidence exist as well. In addition to footprints, eggs, and nests, imprints of dinosaur skin, feces, rounded gizzard stones, and even possible stomach contents have been found. In early 2000, paleontologists announced they had discovered the fossilized heart of a dinosaur that had died some 66 million years ago. Uncovered in South Dakota, the heart was encased in a natural sarcophagus of stone in the chest cavity of a dinosaur's fossil skeleton.

Fossilized plant remains are sometimes associated with deposits of dinosaur fossils. These finds allow scientists to make inferences as to the habitats of these animals. Inferences also can be based on the geological context of the locations of fossils, for example, their nearness to a marine shore, or geographical position, as is the case of polar dinosaurs. All of these types of information have been studied and used to infer the shape, physiology, behavior, and ecological relationships of extinct dinosaurs.

Major groups of dinosaurs

Scientists have only incomplete knowledge of the way in which dinosaurs were related to each other and to other major groups of reptiles. The reason for this fact, of course, is that dinosaurs can be studied only through their fossilized remains. These remains are often rare and fragmentary, especially those that are millions or hundreds of millions of years old. Nevertheless, some dinosaur species bear clear resemblances to each other, while also being obviously distinct from certain other dinosaurs.

The dinosaurs evolved from a group of early reptiles known as thecodonts, which arose during the Permian period (290 million to 250 million years ago) and were dominant throughout the Triassic (250 million to 208 million years ago). It appears that two major groups of dinosaurs evolved from the thecodonts, the ornithischian ("bird hips") dinosaurs and the saurischian ("lizard hips") dinosaurs. These two groups are distinguished largely on the basis of the anatomical structure of their pelvic or hip bones. In general, dinosaurs can be classified as carnivorous (meat-eating) or herbivorous (plant-eating).

Carnivorous dinosaurs. The carnosaurs were a group of saurischian predators that grew large and had enormous hind limbs but tiny fore limbs. Tyrannosaurus rex, perhaps the best known of all dinosaurs, was once considered the largest carnivore that ever stalked Earth's landscape. Its scientific name is derived from Greek words for "absolute ruler lizard." This fearsome predator reached a maximum length of 40 feet (12 meters), and may have weighed as much as 7 to 9 tons (6.5 to 8 metric tons). Tyrannosaurus rex had a massive head and a mouth full of about 60 dagger-shaped, sharp, serrated teeth. Those teeth grew to a length of 6 inches (15 centimeters) and were renewed throughout the life of the animal. This predator probably ran in a lumbering fashion on its powerful hind legs. The hind legs also may have been used as sharp-clawed, kicking weapons. Scientists think that Tyrannosaurus rex may have initially attacked its prey with powerful head-butts and then torn the animal apart with its enormous jaws. Alternatively, Tyrannosaurus rex may have been primarily a scavenger of dead dinosaurs. The relatively tiny fore legs of Tyrannosaurus rex probably only had minor uses. The long and heavy tail of the dinosaur was used as a counterbalance for the animal while it was running and as a stabilizing prop while it was standing.

Tyrannosaurus rex 's distinction as the largest carnivore was taken away in 2000 when a team of scientists announced they had discovered the fossilized bones of a previously unknown dinosaur species that had lived about 100 million years ago. The bones of six of the dinosaurs were unearthed in Patagonia, a barren region on the eastern slopes of the Andes Mountains in South America. The scientists estimated that the needle-nosed, razor-toothed, meat-eating giant measured up to 45 feet (14 meters) in length. Like Tyrannosaurus rex, it had a tail and short front legs, but it was heavier and had slightly shorter back legs. It also was probably more terrifying than Tyrannosaurus rex.

Not all of the dinosaurian predators were enormous. Deinonychus, for example, was a dinosaur that grew to about 10 feet (3 meters) and weighed about 220 pounds (100 kilograms). Deinonychus was one of the so-called "running lizards." These dinosaurs were fast, agile predators that probably hunted in packs. As a result, Deinonychus was probably a fearsome predator of animals much larger than itself. One of Deinonychus's hind claws was enlarged into a sharp, sicklelike, slashing weapon. The claw was probably used to slash and tear apart its victim.

The most infamous small carnivorous dinosaur is Velociraptor, or "swift plunderer." Velociraptor attained a length of about 6 feet (2 meters). Restorations of this fearsome, highly intelligent, pack-hunting "killing machine" were used in the popular movie Jurassic Park.

Herbivorous dinosaurs. The sauropods were a group of large saurischian herbivores that included the world's largest-ever terrestrial animals. This group rumbled along on four enormous, pillarlike, roughly equalsized legs, with a long tail trailing behind. Sauropods also had very long necks, and their heads were relatively small. Their teeth were peglike and were used primarily for grazing rather than for chewing their diet of plant matter. Digestion was probably aided by large stones in an enormous gizzard, in much the same way that modern, seed-eating birds grind their food.

Perhaps the most famous of all sauropods was Apatosaurus, previously known as Brontosaurus. (The Apatosaurus was the first of the two to be discovered, and what was thought to be a different dinosaur, the Brontosaurus, was discovered later. In the 1980s it was discovered that they were really the same dinosaur, so they are all now referred to as Apatosaurus.) Apatosaurus achieved a length of 65 feet (20 meters) and a weight of 30 tons (27 metric tons).

Diplodocus was related to Brontosaurus, but had a much longer body. A remarkably complete skeleton of Diplodocus has been found that is 90 feet (27 meters) long overall, with a 25-foot (8-meter) neck, a 45-foot (14-meter) tail, and an estimated body weight of 11 tons (10 metric tons). Brachiosaurus was an even larger herbivore, with a length as great as 100 feet (30 meters) and an astonishing weight that may have reached 80 tons (73 metric tons).

What became of the dinosaurs?

Many theories have been proposed to explain the extinction of the last of the dinosaurs about 65 million years ago. Some of the more interesting ideas include the intolerance of these animals to rapid climate change, the emergence of new species of plants that were toxic to the herbivorous dinosaurs, an inability to compete successfully with the rapidly evolving mammals, destruction of dinosaur nests and eggs by mammalian predators, and some sort of widespread disease to which dinosaurs were not able to develop immunity. All of these hypotheses are interesting, but the supporting evidence for any of them is not enough to convince most paleontologists that the dinosaurs became extinct for any of these reasons.

Perhaps the most widely accepted theory today is based on the belief that a planet-wide catastrophe resulted in the extinction not only of the dinosaurs but also of hundreds of other species. Scientists have now found evidence that such a catastrophe may have occurred when a large asteroid struck Earth 65 million years ago off the northern tip of the Yucatan Peninsula of Mexico. In such an impact, huge amounts of dust and debris would have been thrown into the atmosphere. Carbonates and sulfate rocks would have also been vaporized, releasing chemicals into the atmosphere that produced sulfur and the greenhouse gas carbon dioxide. The dust and rocks would have blocked out sunlight for an extended period of time, perhaps for years, which would have killed off plants in large numbers. Deprived of plants, choking on carbon dioxide, and suffering showers of caustic sulfuric acid rain, the dinosaurs would have died out.

[See also Evolution; Fossil and fossilization; Geologic time; Paleontology ]

DINOSAURS

PHYSICAL CHARACTERISTICS

The dinosaurs are a large group of reptiles that lived from 230 to 65 million years ago. Some, such as the well-known Tyrannosaurus rex, were enormous meat-eating animals. Others, however, were small and timid creatures that nibbled on plants.

Scientists divide the dinosaurs into two orders. One order is Saurischia, which includes the theropods (THAIR-oh-pods) that walked on their two hind legs and were mostly meat-eating dinosaurs and the sauropods (SAWR-oh-pods) that walked on all fours and ate plants. The theropods had more primitive features, including jagged teeth, and some, such as the Ceratosaurus, had hornlike knobs jutting out of their skulls. Tyrannosaurus rex was a theropod. Although it was quite large at 40 to 50 feet (12.2 to 15.2 meters) long, its ancestors only grew to about 10 feet (3 meters) long.

The sauropods looked much different than the theropods. They had very long necks and tiny heads. Some of them were able to lift their front legs off the ground and grab leaves or other things with their hands. Other species, including Brachiosaurus, had longer front legs than hind legs, similar to the arrangement in current-day giraffes. Their tall front legs, combined with their overly long necks, helped them easily reach food even at the tops of most trees.

The other order within the dinosaurs is Ornithischia, which included those dinosaurs that ate plants and had hip bones that looked like those found in present-day birds. Many of them had crests, beaks, horns, or helmets, and some had armor-like plates, called scutes, covering their bodies and occasionally spikes. Stegosaurs are an example of an Orinithischian. These dinosaurs had armor-like spines down the middle of the back and spiked tails. The Ornithischia also includes the duckbill dinosaurs with their wide snouts.

Within these two orders of dinosaurs, the animals are further split into several hundred smaller groups, called genera (jen-AIR-uh). One or more species is grouped into each genus (JEAN-us), which is the singular of genera.

Although the name dinosaur actually means "terrible lizards," dinosaurs are not lizards and are different from all other groups of reptiles. One of the major differences between dinosaurs and other reptiles is in the way they moved. Lizards and crocodiles walk with their legs held out to the side, in the same type of position a person's arms take when doing pushups. A few dinosaurs sprawled their front legs like a lizard, but the vast majority of them walked like a dog or cat—with the legs directly below the hips and shoulders.

Many scientists also now suspect that at least some of the dinosaurs were warm-blooded, instead of cold-blooded like other reptiles. A warm-blooded animal, more properly called an endothermic (EN-doe-THER-mik) animal, uses its own energy to keep its body at a constant temperature. Cold-blooded, or ectothermic (EK-toe-THER-mik), animals get their body heat from an outside source, like the warmth of the sun.

Dinosaurs came in many shapes and sizes. The Seismosaurus, or "earth-shaking dinosaur," may have been the longest at 120 to 150 feet (36.6 to 45.7 meters) long. The heaviest may have been the Argentinosaurus, which grew to 100 to 130 feet (30.5 to 39.6 meters) long and weighed 110 tons (99,800 kilograms). Other enormous dinosaurs include the Supersaurus at 100 feet (30.5 meters) long and about 50 tons (45,000 kilograms) and the Brachiosaurus at 85 feet (25.9 meters) long and about 75 tons (68,000 kilograms). The Tyrannosaurus rex, a name that is often shortened to T. rex, was considerably smaller at 40 to 50 feet (12.2 to 15.2 meters) long and 6 tons (5,400 kilograms) in weight. Since T. rex stood on its hind legs rather than on all fours, it towered over most other dinosaurs. Other similarly sized meat-eating dinosaurs were the Gigantosaurus, Spinosaurus, and Carcharodontosaurus. All dinosaurs were not giants, however. Some, such as the Saltopus and Lesothosaurus, were only 24 to 36 inches (61 to 91 centimeters) long, and the tiny Microraptor's full-grown size may have been only about 16 inches (41 centimeters) long.

GEOGRAPHIC RANGE

The Earth that the early dinosaurs knew looked much different than the Earth does today. The planet had a single, huge land mass, called Pangaea, and the dinosaurs lived over much of this area, particularly in the warmer climates. About 180 million years ago—50 million years after the dinosaurs first evolved—Pangaea began to split up and eventually formed the continents seen on Earth today. Given such huge changes, a fossil found nowadays in Germany, for example, says nothing about the location of the dinosaur that left it 220 million years ago.

HABITAT

Because scientists are studying fossils from many millions of years ago, rather than living animals, they usually cannot tell much about the dinosaur's habitat. They do, however, suspect that none of them lived in the water. Although a few dinosaurs may have been able to keep their bodies afloat for brief periods, or could wade to catch fish, none were full-time swimmers. Some scientists believe that sauropod dinosaurs may have been able to float and, based on footprints left behind, think they pushed themselves along by bouncing their front feet on the bottom of the pond or lake.

DIET

About two-thirds of all genera contain dinosaurs that were plant eaters, and a third of the genera include meat-eating dinosaurs. Scientists can determine whether a dinosaur ate meat or plants by looking at its teeth. The teeth of meat-eaters, also known as carnivores (KAR-nih-voars), are pointed for tearing flesh. The teeth of a plant-eater, or herbivore (ER-bih-voar), are flatter for grinding grasses and leaves. Studies of other dinosaur bones can also reveal information about their diet. One study, for instance, showed that some dinosaurs were cannibals. By looking at teeth marks on the bones of certain dinosaurs and comparing the marks to the teeth of the same species, the scientists figured out that the reptile was eating its own kind. This particular species, a theropod called Majungatholus atopus, grew to 29.5 feet (9 meters) long.

BEHAVIOR AND REPRODUCTION

With almost nothing but fossils to study, scientists can only guess at most dinosaur behavior. For example, although T. rex is often described as a ferocious predator, scientists only know that it had a skeleton that likely supported a strong body, and it had the jaws and teeth necessary to eat large prey animals. It is possible, however, that T. rex never even attacked live animals, but instead ate only animals that were already dead. Recently, scientists think they may have found evidence that some dinosaurs were social animals, which means that they spent time together in groups. They based this idea on a fossil find in Patagonia, where the bones of six, large, carnivorous dinosaurs were found huddled together in one area. The scientists think the dinosaurs, a new species that measures 40 feet (12.2 meters) long and had sharp and bladelike teeth, may have hunted together so they could attack and kill sauropods that grew to at least twice their size. Scientists believe some dinosaurs were social because their bones suggest that they were able to make loud noises. The lambeosaurs, for instance, had sound-producing tubes inside the skull, and scientists suspect that the animals communicated with one another.

Scientists sometimes find dinosaur footprints that have been preserved over time. From these, they can learn how the animal moved. Footprints of ornithomimids, which were ostrich-like dinosaurs, show that they could run at least 25 miles (40 kilometers) an hour, while those of a 3-foot-long (9 meter) meat eater called a Megalosaurus could zip along on its hind legs at speeds of 29 miles (48 kilometers) an hour. By looking at the bones of dinosaurs, scientists can also guess their fastest running speed. A recent study of T. rex bones shows that it probably could run no faster than the much smaller ornithomomids.

Scientists have recently found many dinosaur eggs, some of them with young still inside. A group of Allosaurus eggs found in Portugal provided some clues to the way they were born. The egg shells were covered with tiny holes, called pores, and looked very much like the pore-covered eggs of current-day crocodilians. The pores allow air to flow into the eggs, so the growing babies can breathe. Based on these findings, scientists believe the female dinosaurs of this species laid their eggs in mounds of vegetation or buried them, just as the now-living crocodilians do.

One of the best places in the world to find dinosaur fossils is Mongolia. In 1993, scientists learned that it was also an excellent place to find eggs with developing babies, called embryos (EM-bree-ohs), still inside. Here, they discovered a nest containing the first embryo ever found of a meat-eating dinosaur. It was a theropod, called an oviraptorid, that looked much like an ostrich, and the embryo dated back 70 to 80 million years ago. Interestingly, they also found the skulls of two small velociraptors in the nest. Were the velociraptors there to eat the eggs, or had the mother oviraptorid brought the velociraptors to feed her babies? Scientists do not know for sure. Some even guess that the mother velociraptor may have laid her eggs in the oviraptorid nest. If the oviraptorid mother did not notice the intruders, she would raise them as her own.

DINOSAURS AND PEOPLE

Despite the pictures in some cartoons and science fiction movies that show cave people living at the same time as the dinosaurs, scientists know that this is not true. By dating dinosaur fossils, they can definitely state that dinosaurs lived between 230 to 65 million years ago. Humans did not evolve until about one million years ago. Nonetheless, people nowadays are very interested in these reptiles from the planet's past. Television programs, films, books, web sites, and entire museum wings are devoted to the description or study of these animals.

BRING BACK THE DINOS!

Movies and TV shows sometimes pretend that humans today can bring the extinct dinosaurs back to life by growing them from bits of their DNA found in fossils. In one such film, called "Jurassic Park," a scientist found dinosaur blood in the stomachs of prehistoric blood-sucking insects that had been preserved through the ages in tree sap. The blood contained DNA, which is found in each of a body's cells and holds the instructions for making the animal. In the film, he was able to create a dinosaur from that DNA. Although scientists do sometimes find prehistoric insects, they have yet to find any blood inside, whether from a dinosaur or not. Even if they did, any DNA in the ancient blood would most likely be in such bad shape that it would be useless.

CONSERVATION STATUS

The dinosaurs became extinct 65 million years ago. Their deaths likely resulted from a huge asteroid, a rock from outer space, that slammed into the Earth, probably near the Yucatan Peninsula of Mexico. The impact from the 4 to 9 mile-wide (6 to 15 kilometer) asteroid sent up a thick plume of dust and caused a chain reaction that resulted in a severe change in the planet's climate. For years afterward, the sun was unable to penetrate the dark curtain of dust. Temperatures around the world began to drop. Without sunlight, plants died, and with fewer plants to eat, many herbivores also perished. With fewer and fewer herbivores to eat, the carnivores may have begun to eat each other, until they also disappeared. Scientists believe that one group of dinosaurs survived the great extinction, however. These were the dromaeosaurids that eventually evolved into the birds. For this reason, some books refer to birds as modern-day dinosaurs.

FOR MORE INFORMATION

Books:

Farlow, James O., and M. K. Brett-Surman, eds. The Complete Dinosaur. Bloomington and Indianapolis: Indiana University Press, 1997.

Haines, Tim. Walking with Dinosaurs: A Natural History. New York: Dorling Kindersley, 2000.

Holta, Thomas R., Michael Brett-Surman, and Robert Walters. Jurassic Park Institute (TM) Dinosaur Field Guide. New York: Random House Books for Young Readers, 2001.

Lambert, David, and Steve Hutt. DK Guide to Dinosaurs. New York: Dorling Kindersley, 2000.

Paul, Gregory S., ed. The Scientific American Book of Dinosaurs. New York: St. Martin's Press, 2000.

Weishampel, David B., Peter Dodson, and Halszka Osmólska, eds. The Dinosauria. Berkeley: University of California Press, 1990.

Periodicals:

Adams, Judith. "Footsteps in Time." Faces: People, Places, and Cultures. April 2003, vol. 19: 30.

Hesman, Tina. "Dinosaurs, party of six, meat eating." Science News. April 1, 2000, vol. 157: 223.

Mandel, Peter. "Dino Might! 10 Recent Discoveries That Have Rocked the Dinosaur World." National Geographic Kids. March 2003: 14.

Davy, Emma. "Crash Test: What Wiped Out the Dinosaurs? Scientists Studying an Enormous Crater in Mexico Hope to Find the Answer." Current Science. September 27, 2002, vol. 88: 6.

Perkins, Sid. "Bob, Bob, Bobbin' Along: Dinosaur Buoyancy May Explain Odd Tracks." Science News. October 25, 2003, vol. 164: 262.

Web sites:

"Dinosaur embryo." American Museum of Natural History. http://www.amnh.org/exhibitions/expeditions/treasure_fossil/Treasures/Dinosaur_Embryo/embryo.html?dinos (accessed on December 22, 2004).

"Dinosaurs." BBC. http://www.bbc.co.uk/dinosaurs/ (accessed on December 22, 2004).

"Dinosaurs." EnchantedLearning.com. http://www.enchantedlearning.com/subjects/dinosaurs/ (accessed on December 22, 2004).

"Dinosaurs." KidSites.com. http://www.kidsites.com/sites-edu/dinosaurs.htm (accessed on December 22, 2004).

"Dinosaurs." Scholastic. http://teacher.scholastic.com/researchtools/articlearchives/dinos/general.htm (accessed on December 22, 2004).

"The Science of 'Jurassic Park': Frequently Asked Questions." San Diego Natural History Museum. http://www.sdnhm.org/research/paleontology/jp_qanda.html (accessed on December 22, 2004).

"What is a Dinosaur?" San Diego Natural History Museum. http://www.sdnhm.org/kids/dinosaur/dino.html (accessed on December 22, 2004).

Dinosaurs

The history of the Dinosauria begins with one of the dinosaur's small extinct ancestors called Petrolacosaurus. Around 270 million years ago, this animal was a member of the group of early land vertebrates called the diapsids that had skulls with two openings behind the eye socket (other evolutionary groups, like fish and amphibians, had one opening or none at all). The diapsids are believed to be the ancestors of the lepidosaurs (modern lizards and snakes) as well as of the archosaurs, the group that led to the dinosaurs. The lepidosaurs achieved great evolutionary success. The extra openings in their skulls led to the interesting structures found in modern snakes, including a light and flexible skull that allows them to catch and eat prey larger than their mouths.

Ancestry: Euparkeria

The first known archosaurs appeared in the Permian Period (319 to 286 million years ago) and they were well on their way to becoming large-sized animals by the early Triassic (about 245 million years ago). When discussing dinosaur ancestry, paleontologists prefer to examine an interesting little archosaur known as Euparkeria. Euparkeria had anatomical characteristics of most archosaurs (and eventually the dinosaurs), including deeply rooted, sharp, serrated teeth; two holes behind the eyes; and a broad space in front of the eye sockets. Their jaws had a distinctive opening that was different in shape and position from other tetrapods, and their spine had small bony plates suggestive of the beginning of armor plating. Perhaps the most important feature of Euparkeria is the arrangement of their hipbones.

As the archosaurs evolved, many species developed hipbones that allowed the angle of their hind limbs to change from a sprawling posture, such as a lizard or crocodile has, to an erect one, like a bird. The limbs came under the body instead of being spread out to the side. This leg position provided a firmer basis of support for a larger and heavier body. Dinosaurs became the largest land animals, and part of their ability to become so large was a direct result of this change in posture. When the legs are spread out to the side they can only support so much weight before the joints that attach them to the body give out. With the legs underneath the body, they form a kind of column that can support a great deal more weight. This means a larger animal can move around without being slow and sluggish or breaking its bones. This change in body posture is one of the main reasons dinosaurs were able to become so large.

One important group of archosaurs was the thecodonts, which included Euparkeria. During the Triassic period, thecodonts continued to evolve and undergo changes in body shape. By the end of the Triassic 213 million years ago), there were two groups of thecodonts, the saurischians and the ornithischians. The saurischians ate both plants and meat. Their pelvis was distinctive in that the three bones that made up the hip—the ilium, the ischium, and the pubis—were joined so that they angled away from each other in a triangular shape. There were two distinct groups of saurischians, the theropods ("beast feet") and the sauropods.

The theropods were carnivorous dinosaurs that walked upright on two feet. Some of the most famous dinosaurs we know today are theropods, including Tyrannosaurus rex, Velociraptor, and other swift and dangerous predators. The other group of saurischians, the sauropods, were herbivores and moved about on four legs. Some sauropods, including Brachiosaurus, Camarasaurus, and Ultrasaurus, were the largest animals that ever lived on Earth. The other evolutionary offshoot of the ancestral thecodonts was the ornithischians. The hip structure of the ornithischians differed from that of the saurischians in that their forward-extending pubis bone was turned toward the back. Because this bone structure also occurs in modern birds, the ornithischians are called bird-hipped dinosaurs. (The ornithischians are not ancestors of birds, they just have a similar hip structure.) The legs, feet, and anklebones of ornithischians were similar to those of the saurischians.

A peculiar adaptation of ornithischians was the beaklike covering of the front of the mouth, which is characteristic of the ceratopsians and duck-billed dinosaurs. Ornithischians also had a complex network of bony rods along their spine which supported the spine. All ornithischians were herbivores. The group contains some of the more distinctive-looking dinosaurs, including Stegosaurus, which had a series of large, triangular, horn-covered bony plates along its back and tail; the heavily armored Ankylosaurus; and the ceratopsians, including Triceratops, with its huge bony hood and horns.

The First True Dinosaurs

The first groups of true dinosaurs, the coelurosaurs, appeared about 210 million years ago in the late Triassic period. These dinosaurs were carnivorous theropods. They were agile and lightly built; most species were smaller than an adult human. Some of the best fossil specimens of the coelurosaur known as Coelophysis were discovered in the United States in New Mexico. Other coelurosaurs were the smaller Ornitholestes, a heavily jawed predator with nostrils that faced upward on its skull, and Coelosaurus perhaps the best-known coelorosaur. All coelorosaur feet had three toes pointing forward and a fourth facing back. The fifth toe was greatly reduced. This pattern persisted throughout the history of all the dinosaurs and is one way to document change within the group.

From the remains of dinosaurs that appear in the fossil record, paleontologists can infer what those animals looked like, how they moved, and what and how they ate. But paleontologists today are also asking: What dinosaur behavior can we infer from fossils? The Maiasaura ("Good Mother" dinosaur) eggs and young found in Montana in 1978 are an excellent example of one way paleontologists can hypothesize dinosaur behavior. In this case, the young dinosaurs at the site were too big to fit in the fossil eggs in nests that were found nearby. Paleontologists think that the parent dinosaurs brought food to the baby dinosaurs and protected them from predators. Also, since many nests were discovered together in a small area, the scientists think that these dinosaurs many have lived together in some sort of herd.

The thinking is that if the baby dinosaurs were too big to fit in the eggs, but were still in the nest they probably remained in nest for some time after hatching. They would have to had food brought to them by the parents until they were large enough to forage for food on their own. Many birds care for their young this way by bringing them food until the fledglings are old enough to feed and fly on their own. This is another piece of evidence that links birds and dinosaurs.

At the end of the Cretaceous era (around 64 million years ago), something happened that caused the remaining dinosaurs to die out. No one knows for sure what the event was. Many scientists believe an asteroid hit Earth, causing harsh atmospheric conditions that led to the dinosaurs' extinction. However, very few species of dinosaurs remained by the end of the Cretaceous. Scientists will probably never know what actually happened to the dinosaurs. Most think they are still here as birds. This is not difficult to imagine when you look closely at the characteristics of birds. It is comforting to imagine that these fantastic and lively creatures are still among us, only much smaller.

Recent discoveries of many species of feathered dinosaurs in China have really supported this idea. Most scientists know agree that birds are very closely related to dinosaurs, if not actually dinosaurs themselves. The so-called "missing links" between dinosaurs and birds have been found.

Leslie Hutchinson

Bibliography

Carroll, R. Vertebrate Paleontology and Evolution. New York: W. H. Freeman and Company, 1988.

Czerkas, S., and E. Olsen. Dinosaurs: Past and Present, vols. I and II. Los Angeles: Natural History Museum of Los Angeles County, 1987.

Farlow, J. O., and M. K. Brett-Surman, eds. The Complete Dinosaur. Indiana University Press, 1997.

Horner, John R. Digging Dinosaurs. New York: Workman Publishing, 1988.

Norman, D. Encyclopedia of Dinosaurs. New York: Crescent Books, 1985.

Sloan, Christopher. Feathered Dinosaurs. Washington, D.C.: National Geographic, 2000.