Dinosaurs are a highly distinctive group of vertebrates that played a dynamic role in terrestrial ecosystems for 165 million years of Earth’s history. Fossils of their teeth and bones are found in abundance all over the world, sometimes along with evidence of soft tissues, nests, eggs, feces, and footprints. Collectively these remains comprise an excellent record extending from their emergence in the Triassic period up to the Cretaceous extinction, testifying to the great diversity of dinosaur species. First described in the early 19th century, dinosaurs were once thought to be sluggish, lizard-like reptiles. Our conception of these animals has changed with each new discovery; modern science shows that many dinosaurs were energetic, dynamic creatures more akin to birds and mammals in their activities. Their majesty and variety inspire perennial fascination, and scientific research on the group has vastly improved our knowledge of evolution and survival strategies in ecosystems under pressure.
Dinosaurs arose from archosaurian reptiles during the mid-Triassic period, approximately 230 million years ago. They became a hugely successful group, surviving several mass extinction events and evolving into a vast array of forms throughout their 165-million-year reign. During this time the dinosaurs achieved remarkable feats of specialization. The smallest were about the size and weight of a modern-day crow, while the largest attained lengths over 40 meters and weights up to 60 tons or more, the most gigantic land animals ever known. Some were quadrupeds, others bipeds, and some could move in either mode. Some were frightful, sharp-toothed carnivores; others were herbivores of such voracity that they altered the landscape with their feeding. Their range of forms and adaptability ensured that while not being the most abundant of terrestrial species, they ruled the top of the food chain in whatever niche they occupied and dominated their habitats over an exceptionally long period of life’s history on Earth.
The term dinosaur is derived from the taxonomic group Dinosauria, a term coined by Sir Richard Owen in 1842. The fossils of many large and fantastic animals were being discovered in England at the time, including the carnivorous Megalosaurus, the herbivorous Iguanodon, and the armored Hylaeosaurus. Owen noted that the anatomy of these creatures differed significantly from that of modern and fossil reptiles but felt obligated to respect earlier research classifying the huge beasts as reptiles. He thus invented the name Dinosauria, meaning “fearfully great lizards.”
One modern way of defining groups of organisms is through phylogenetic taxonomy, whereby organisms are related strictly by common ancestry. In this manner, Dinosauria can be defined as all members of the group descended from the most recent common ancestor of Megalosaurus, Iguanodon, and Hylaeosaurus, as these are all quite early forms of the two great dinosaur lineages Ornithischia and Saurischia. Dinosaurs can also be broadly defined as large terrestrial vertebrates that lived only during the Mesozoic era. Contrary to images in movies and popular culture, the prehistoric aquatic reptiles are not dinosaurs, nor are the flying pterosaur reptiles. Dinosaurs were predominantly terrestrial in nature, although fossil traces testify that some dinosaurs swam on occasion. Likewise, although the skeletons and feathers of certain species suggest some sort of flying ability, and theropod dinosaurs did indeed give rise to birds, this entry follows common usage in restricting the term dinosaur to nonavian dinosaurs unless specifically noted.
The earliest known dinosaurs were lightly built, agile bipeds with grasping hands. Although this body plan would later evolve immensely, certain important features defined at the start of the lineage were retained by all dinosaurs. All dinosaurs possessed a perforate acetabulum, that is, a hole in the wall of the hip socket that accepts the ball-like end of the upper thigh bone, the femur. This arrangement allowed dinosaurs to position their hind limbs closer to and underneath the body, allowing a fulltime, “erect” stance compared to the more splayed- leg reptilian posture. The benefit of the more upright stance is that the limbs move within a vertical plane, with flexion and extension aligned to more efficiently convert muscular effort into forward movement. In either bipeDalí or quadrupeDalí mode, this improved mobility; dinosaurs could move around faster and stay active longer than most of their immediate ancestors. The upright stance is also capable of sustaining a greater body weight compared to a sprawled posture, allowing massive future growth potential. Other common characteristics of dinosaurs include an elongated ridge along the top of the upper arm bone and a hinge-like joint in the ankle. This last feature allowed dinosaurs to better adopt a digitigrade stance, where the longer foot bones and ankle are raised from the ground and weight is distributed over the toes and front “balls of the feet.” Early dinosaurs also had “hands” with a semi-opposable thumb, grasping ability, and reduced outermost digits. This adaptation was coeval with their primary shift to bipeDalíism. Later on, when dinosaurs diversified further, some of these defining features were lost in some lineages. Most of these initial traits, however, were retained across all groups.
Dinosaurs are divided into two primary orders, originally based on the shape and form of their hip bones. The Saurischia (“lizard-hipped”) dinosaurs have hip bones with the pubis pointing down and forward, similar to reptiles. The Ornithischia (“birdhipped”) dinosaurs have hip bones with the pubis pointing down but backward, almost parallel to the ischium. As this trait turned out to be a rather weak discriminator for subsequently discovered taxa, Ornithischia and Saurischia are now also defined by a range of skull and skeletal features. The essential division remains valid, however. It should be noted that the most bird-like theropod dinosaurs, while nominally lizard-hipped, in fact have a typical birdshaped hip.
Within these general classifications dinosaurs evolved incredible adaptations, diversified into a wondrous range of sizes and shapes, and grew their formidable teeth and claws—in short, they acquired all those qualities of such fascination to humans. The diversity of their anatomy reflects a similar diversity of habitat; dinosaurs lived in cold polar regions and hot arid plains, by seaside cliffs and highland lakes, and their fossils are found on all continents. Dinosaurs truly dominated their time; new discoveries reveal there was likely a dinosaur in many of the ecological niches that are occupied today by small animals and mammals, including those that could burrow and climb, were active in darkness, and could not only glide but achieve true flight.
Because there are no dinosaurs alive today, our knowledge of them comes exclusively from fossilized remains and traces. Fortunately, the fossil record is abundant and extensive. Several thousand largely complete dinosaur skeletons have been located and collected, not to mention hundreds of thousands of isolated bones and teeth. From these remains, many hundreds of genera and thousands of species of dinosaurs have been classified to date, with additional taxa based on fossilized footprints and trackways.
Nevertheless, it is clear that the fossil record is far from complete. Nearly half of the dinosaur genera are based on a single specimen, and complete skulls and skeletons have been found for only about 20% of the identified species. This is due to the process of fossilization being extremely haphazard, particularly for terrestrial vertebrates; only a minuscule fraction of any population will ever leave fossil remains.
A rare collection of factors is necessary to preserve the body of an animal in fossil form. In even the best environments for fossil formation, such as a river floodplain, most animal remains are destroyed quickly by natural decay—scattered, trampled, and gnawed while being eroded by wind, water, and sun. The coalition of physical and chemical erosion is unrelenting—before long, no trace of the animal is left to join the geological record. The process of destruction may be temporarily halted, however, if a substantial flood breaks the river banks and sweeps over the plain, depositing a layer of sediment over any bones and remains, thereby protecting them from the surface elements. The completeness of the fossil record in this environment is thus largely determined by the frequency of flooding, which can occur at intervals ranging from tens to thousands of years. The resulting fossils form a series of snapshots in time rather than an ideal continuum. In less ideal environments where there is little chance of sedimentary deposition, the record becomes even more scant.
If the remains are deeply buried for a long enough period, the sediment may undergo diagenesis and transform into rock. It is during this process that bones fossilize; the forms of softer tissue are sometimes preserved as well. Later on, massive geological processes such as mountain formation are required to raise fossil-bearing rocks back to the surface. Just enough erosion is then required to expose the fossils, where with luck they may be discovered just prior to their terminal destruction, as weathering again conspires to convert the fossil to dust.
Given all these criteria, it is not surprising that the fossil record of dinosaurs is incomplete. Rather, it is astonishing that the record is so extensive and detailed. By recognizing the rarity of fossil formation, it then becomes possible to appreciate the sheer abundance of dinosaurs over time.
The best-known dinosaur remains are body fossils, which come from the physical remains of the animal itself. These are most commonly isolated bones and teeth, but sometimes articulated parts or even whole skeletons are found. Bumps, grooves, and scars on the fossil bones show sites of muscle attachment and let us make educated guesses regarding their body shape and abilities. The form of the jaws and teeth indicates preferred food types. In very rare cases the contents of a dinosaur’s stomach are still intact, preserving not only its last meal but also a sample of the environment in which it lived. Holes in the skull reveal the size and shape of their brain, ears, eyes, and nostrils. Indeed, the very shape of the braincase allows us to infer some of its functional capability by examining the size and location of specific lobes. For example, scientists might be able to recognize that a part of the brain used for olfactory processing was relatively large. From this evidence we can infer that the dinosaur had a well- developed sense of smell, and we may draw some conclusions concerning its potential behavior.
Occasionally traces of soft tissue are preserved along with the skeletal remains, usually as mineral replacements shaped like the original organic material (pseudomorphs). Spectacular examples of dinosaur fossils may display evidence of muscles and internal organs, skin, and feathers. Thanks to recent technological advances, researchers have even found preserved biomolecules in partially fossilized material. In these exceptionally well-pre- servedspecimens, the moleculesincludebone-related proteins (collagen and osteocalcin) and blood proteins such as hemoglobin. Although much work remains before we fully understand the mechanisms of preservation, it appears that an exciting new area of dinosaur exploration may be opening up on the molecular level. Further progress may eventually lead to the recovery of DNA fragments, which can be used to directly demonstrate evolutionary relationships among taxa.
In addition to body fossils, there are trace fossils that also can provide useful information on dinosaurs and their environs. Footprints and trackways can tell us the animals’ weight, stride length and speed, and the position of legs relative to the body. In some cases they also provide insights into behavior such as herding or predation. Fossilized dinosaur nests and eggs, some containing embryos, have been found on several continents. These provide important evidence on dinosaur reproduction and parental behavior. Coprolites (fossilized dung) are a good source of information on dinosaur diets and, in some cases, their habitat and distribution. Collectively the fossil record of dinosaurs is vast and detailed, allowing us to gain a deeper understanding of how they may have behaved and interacted as well as how they appeared physically.
Emergence and Ascent
Dinosaurs are members of Archosauria (“ruling reptiles”), a significant clade of terrestrial reptilian vertebrates of the Mesozoic that, during the mid-Triassic period, included two significant groups; the Crurotarsi and Ornithodira. Some members of Crurotarsi became quite large, cow or ox sized, and evolved into a range of diverse, mostly carnivorous forms that included phytosaurs, aetosaurs, rauisu- chians, and crocodylomorphs (ancestors of modern crocodiles). The Ornithodira, by contrast, were smaller and acquired a more specialized upright stance, longer rear leg and foot bones, and an extended neck with an S-shaped curve. It is believed that the flying reptile Pterosaurs evolved from this branch of Ornithodira, while another branch further developed the fully erect posture and a new, hinge-like ankle joint, the Dinosauromorphs. From among these, the first dinosaur evolved.
The earliest known dinosaurs are Pisanosaurus, Eoraptor, and Herrerasaurus, whose body fossils are all found in 228 million-year-old (Late Triassic) sedimentary rocks of the Ischigualasto Formation in Argentina. These were lightweight obligate bipeds of small or moderate size. Pisanosaurus and Eoraptor were both about 1 meter in length, while Herrerasaurus was about 5 meters. Other early dinosaurs found slightly later in other parts of the world include Staurikosaurus and Guaibasaurus from Brazil and the ornithischian Lesothosaurus from South Africa.
As the oldest taxa are of similar geological age and lie close to the emergence of dinosaurs within Dinosauromorphs, identifying the first known dinosaur is a bit problematic. Pisanosaurus is regarded as the most basal ornithischian and already showed grinding teeth adapted for herbivory, with inclined tooth-to-tooth wear facets that assisted in slicing and processing plant matter. Herrerasaurus and Eoraptor were contemporaneous predators with adaptations for carnivory such as curved, serrated teeth and elongate hands specialized for grasping prey. They are generally regarded as the most basal saurischians, the great primary dinosaur lineage that later split into the herbivorous sauropodomorphs and the mostly carnivorous theropoda. Although the grasping, raking hands and curved teeth of Eoraptor suggest classification as a saurischian theropod, its lower jaw is not jointed like that of the slightly more derived Herrerasaurus. Because members of both saurischian and ornithischian lineages existed at this earliest time, the two divisions must have evolved from a common ancestor even earlier in the period, likely in the mid-Triassic. But this specific ancestor has not yet been found, if in fact it was ever preserved in geology. As Eoraptor exhibits fewer specializations that define saurischians and ornithischians, it is widely regarded as the first known dinosaur and is thought to be very similar to the original ancestor.
The world over which the first dinosaurs strode was a place of intense competition and major upheavals in the land fauna population. The dinosaurs’ archosaurian relatives had diversified into a wide range of niche-dominating forms: terrestrial quadrupeDalí carnivores, semi- aquatic predators, and armored herbivores. Many of these animals were bigger than the earliest dinosaurs and existed in greater numbers. A huge variety of nonarchosaurs were also present, including early mammals and mammal-like reptiles. By the start of the Late Triassic, dinosaurs were only a small part of the total land fauna.
Yet from this humble start, dinosaurs had become the dominant land animal by the beginning of the Jurassic period 30 million years later. The reasons for their rise to eminence—and why their contemporaries faded away—have been the subject of much debate. Did the dinosaurs evolve some significant advantage that allowed them to overcome their competition, or did they just get lucky during oppressive environmental crises and extinction events?
It has long been thought that, once they had emerged, the dinosaurs swiftly replaced their closest relatives, the dinosauromorphs, and most other archosaurs. However, recent discoveries suggest that the rise of dinosaurs during the Late Triassic was a more gradual process. It now appears that dinosaurs coexisted with dinosauromorphs for a prolonged period, 15 to 20 million years, without establishing clear dominance. Although genetic and environmental factors must have prompted their evolution and emergence, dinosaurs clearly possessed no significant advantage over their contemporary land animals, as under normal environmental conditions they did not overpower their competition.
As the Triassic drew to a close, however, several global scale events occurred that caused mass extinctions of life. As with the more famous Cretaceous-Tertiary mass extinction, which occurred some 140 million years later, it has been suggested that one or more catastrophic events were involved. Global climate change due to sealevel fluctuations, severe volcanic activity, and bolide impacts have all been postulated with evidence for each hypothesis. The event probably best associated with the extinctions is the volcanic activity in the central Atlantic magmatic province. This was a rapid and massive outpouring of basalts and gases related to the rifting and breakup of Pangea during the Late Triassic that may have affected global atmospheric conditions and disrupted food chains.
The nature and timing of these extinction events is still poorly understood. There appear to be two phases of extinction defined by vertebrate loss. The first, around 220 million years ago, defined the end of the Carnian subperiod and wiped out the dicynodonts and basal archosaurs. The dinosaurs then continued to live alongside top predator competitors, such as the ornitho- suchids and rauisuchians, for the remainder of the Triassic. By the end of the period, approximately 200 million years ago, 20% of existing families and about half of all species disappeared, both on land (the basal archosaurs and many mammal-like reptiles) and in the sea (all conodonts, most ammonites, and many bivalves). Indeed, almost all of the archosaurs perished at this time: Only dinosaurs, pterosaurs, and croco- dylomorphs remained. Some aspect of anatomy, physiology, or distribution had contrived to allow Dinosaurian survival (see “Dinosaur Success,” later in this entry), and the environmental bottleneck that depleted many of their competitors had provided a vast open ecospace in which to roam. Taking advantage of their “lucky break,” dinosaurs expanded their territory, occupied new niches, and grew into worldwide terrestrial dominance.
The world that gave rise to and shaped the dinosaurs was quite unlike that of today. The Triassic period, so named because it naturally divides into three periods (from the Latin trias), spanned the period from 251 to 200 million years before the present. The early Triassic world possessed only one landmass, the supercontinent Pangea. While the dinosaurs were emerging, Pangea was splitting into Laurasia (North America, Europe, and Asia) and Gondwanaland (South America, Africa, India, Antarctica, and Australia), but this was not yet complete. The climate was warmer than it is today, with no really cold regions on land. The coasts and certain low mountain ranges hosted marshes and swamps, in which grew an abundance of moisture-loving ferns and horsetails. Pangea was so large, however, that rain struggled to reach its far inland regions. These were occupied by vast, arid deserts and dryland forests, where gymnosperms (“naked seeds”) such as conifers, yews, and ginkgoes grew. Other tree-like plants such as cycads and bennettitaleans were also present at this time. Some evidence indicates that atmospheric oxygen levels were perhaps only 50% of modern levels.
The two main lineages of dinosaur evolution were already established by the Middle to Late Triassic, with representatives of both Saurischia and Ornithischia found in the earliest populations of dinosaurs. The saurischia evolved further into two major clades, the herbivorous sauropodomor- pha (“lizard feet”) and the mostly carnivorous theropoda (“beast feet”). The theropods were the major group of carnivorous dinosaurs for the entire Mesozoicera, although some later subbranches would adapt to be omnivorous or herbivorous. They possessed blade-like, serrated teeth and retained the bipeDalí stance of their ancestors. At this stage of the Triassic, some theropods appear to have dominated the sharp end of the food chain by force of numbers. Their abundance is evidenced by a mass mortality site at Ghost Ranch, New Mexico, containing many hundreds of individual Coelophysis.
Meanwhile the sauropodomorphs, long-necked plant eaters, steadily increased body size while retaining relatively small heads. Their leaf- or pegshaped teeth and skulls indicate an unsophisticated feeding style, coarsely stripping and swallowing vegetation for major processing in the gut. Alongside them the smaller-bodied ornithischian herbivores were evolving a more specialized feeding anatomy, as seen in the primitive ornithischian Eocursor. They would crop vegetation and preprocess it with rows of shearing and grinding teeth, while muscular cheeks kept the food from spilling out. As the Triassic progressed, taxa of both herbivore lineages increased in average size and decreased using their hands for feeding, becoming more and more quadrupeDalí to better support their weight. Eventually the herbivorous dinosaurs would evolve the ground-shaking sizes that characterized the succeeding Jurassic period.
The end of the Triassic was marked by several large extinction events, which severely weakened the archosaurs and decimated the dinosauro- morphs. These catastrophes appear to have had little impact on the dinosaurs, however. This relatively new group survived and then thrived, radiating outward and diversifying, with a trend toward increasing size evident in most lineages. The Jurassic period ranges from 200 to 146 million years before the present and is named after rocks deposited at this time in the French and Swiss Jura Mountains. Pangea was rifting and breaking up, well on its way toward forming the continents we know today. Dinosaurs could still travel between the continents until quite late in the period, however. Geological activity was characterized by the steady deposition of sedimentary rock providing fertile beds for fossilization. The climate was still generally warm, with intermittent periods of extreme dryness. Shallow seas invaded much of North America and Europe, and rains reached lands that had been deserts in the Triassic. Plants grew thickly along the rivers snaking through seasonal floodplains. This period has been called the age of the cycads, but these palm-like plants were actually less successful at the time than their relatives the bennettitaleans. (The latter, however, are now extinct, so clearly the cycads were more successful in the long run.) Both groups flourished along with conifers, ferns, and tree ferns in the moister tropics.
During the Jurassic, dinosaurs underwent a massive rate of diversification within the main lineages laid out in the Triassic. This period is marked by the evolution of gigantism in some dinosaurs, and many early bipeDalí herbivores became obligate quadrupeds with their increasing size; the sauropods, in particular, became the largest land animals ever to have lived. Indeed, they pushed the boundaries of what is physically possible for terrestrial animals. Several clades of sauropods routinely exceeded 20 meters in length and weights over 15 tons, while some clades, such as the argy- rosaurids, antarctosaurids, and brachiosaurids, became supergiants of greater than 30 meters in length and weights exceeding 50 tons.
Like elephants, the Jurassic sauropods possessed columnar limbs and a graviportal gait. They all had relatively long necks to a degree, but although some sauropods such as Brachiosaurus evolved necks set in the iconic upright pose similar to modern-day giraffes, this adaptation was relatively exceptional. Most sauropods had shoulders set lower than their hips and probably carried their necks in a horizontal posture, balanced beam-like by their rearward center of gravity and extensive tails. They would then sweep their heads from side to side, feeding over a radius of predominantly lower vegetation or were able to lean out over boggy ground for riverbank herbage.
Many dinosaur communities appear to have had several sauropod species at the same time, indicating a sort of niche partitioning where herbivores adapted to ground-feeding, mid-level feeding, or high feeding. At the ground to mid-level, sauropods likely competed with a radiating lineage of ornithischia that became very successful in the latter half of the Jurassic. The ornithopoda (“birdfeet”) were herbivores with specialized skulls and jaws for more efficient grinding of foliage compared to other ornithischians at the time. They achieved this by utilizing a pleurokinetic hinge, a joint in the upper jaw and skull that allowed parts of the face to rotate and align both upper and lower tooth rows at the same time. There were no Early Jurassic ornithopods, but in the latter part of the period there were several clades expressed by such genera as the small delicate Hypsilophodon found in Europe, medium-sized Dryosaurus, and the larger-sized Camptosaurus. The ornithopods would go on to become a most successful group of dinosaurs, both abundant and globally distributed, later giving rise to the famous duckbill forms during the Cretaceous.
Another lineage of ornithischians sprouted armored nodes around the outside of the body to provide increased protection from the larger carnivores of the Jurassic. These Thyreophora included the earliest armored dinosaurs, Scutellosaurus and Scelidosaurus, and toward the end of the Jurassic this branch produced the famous plate-backed dinosaurs such as Stegosaurus and the thickly armored, tank-like Ankylosaurus.
It is not surprising that the plant-eaters evolved new ways to deal with carnivores, as the Jurassic marks a great diversification of theropods into a vast range of lineages around the world. Teeth and claws became highly specialized depending on habitat, and while there was an overall trend toward gigantism there was also the evolution of some smaller lineages. So while tiny theropods like Compsognathus or medium-sized Dilophosaurus chased insects and lizards, Ceratosaurus grew up to 8 meters long and Megalosaurus and Allosaurus were large enough to stalk large ornithopods or small sauropods.
The coelurosauria also appeared at this time, evolving larger brain-to-body ratios and longer feet, and some members appeared with protofeathers. This lineage produced a vast range of significantly smaller, hollow-boned theropods, with some groups having well-developed feathers and other characteristics that could be adapted for use in flight. Indeed, they later evolved the ability to glide or fly and eventually would give rise to the clade Aves, modern birds. One famous example of early coelurosauria is the flight-feathered Archaeopteryx, whose fossils are found in the famous Solnhofen limestones of the Late Jurassic. The stage was set for an explosion of small, feathered carnivorous dinosaurs, and the turnover of large dominant herbivores.
Starting 146 million years ago and ending 65.5 million years ago, the Cretaceous was the final and longest period in the Age of Dinosaurs. The landmasses of Pangea had splintered further to form numerous smaller continents. This process
created new seas and expanded the old ones, further increasing the distances between continents. Weather patterns became more dynamic as the smaller landmasses had less stabilizing atmospheric influence. During this period the various dinosaur families became more geographically isolated and suffered from localized habitat fragmentation. This led to more intense competition for resources in some places, perhaps accelerating the process of specialization and encouraging greater diversity in both plants and animals.
By the start of the Cretaceous many of the sau- ropods had become extinct in North America and Europe, replaced as dominant herbivores by successive waves of beaked dinosaurs specialized for low feeding. Although sauropods continued to flourish in the southern hemisphere, where giants such as Saltasarus and one of the largest dinosaurs ever, Argentinosaurus, still roamed.
As plants and herbivores are always locked in an evolutionary struggle, it has been suggested that the increased pressure on plants by the voracious dinosaurian herbivores encouraged a takeover by plants that could grow and reproduce more rapidly. Such plants could colonize a grazed area quickly, producing a new generation before the herbivores returned to crop the area once more. One of the plant types best suited for such an environment was the angiosperms, or flowering plants. Today almost all the species we recognize as plants fall into this category, but in the Late Jurassic they were a struggling minority dominated by the slower gymnosperms. At about the same time that low- feeding dinosaurs were taking over as the major herbivores, angiosperms were flourishing. Worldwide, the great forests changed: The decline of gymnosperms accelerated and flowering plants multiplied. Broad forests of oak, hickory, and magnolia dotted the landscape, while swamp cypresses, giant sequoias, and China firs took over in swampy areas. Are the dinosaurs thus responsible for the flowers we know today, if only indirectly?
All families of ornithischia peaked in numbers and diversity during the Cretaceous, with specialized teeth for chewing the new plants with tougher leaves. One branch of ornithopods gave rise to the hadrosauriformes, such as the Early Cretaceous Iguanodon and mid-Cretaceous Ouranosaurus. By the Late Cretaceous this group evolved the clade hadrosauridae—the “duckbilled” dinosaurs, which were to spread widely and diversify extensively across the northern continents. Some species evolved large, elaborate cranial architecture, an extreme example being the trumpet-headed Parasaurolophus. The hadrosaurs attained large sizes (commonly up to 12 meters long and even longer for some Asian forms like Shantungosaurus) and became one of the dominant herbivore groups as the sauropods declined.
Theropod evolution also peaked in the Cretaceous, which saw a vast array of the largest carnivores ever to walk the earth. Each part of the world evolved its own version of gigantic carnivore: Tyrannosaurus and Giganotosaurus appeared in North and South America respectively, while Spinosaurus and Carcharodontosaurus stalked the wetlands of Africa. Meanwhile, at the other end of the scale, the small theropods exploded in diversity and abundance, from small feathered types to medium-sized “raptor” forms. Deinonychosaurs, dromaeosaurs, and troodontids were representatives of the many derived lineages of coelurosauria that were part of the clade Eumaniraptora. These were small to medium sized bipeDalí carnivores, with specialized arms and some with feathers, including the infamous sickle-clawed Velociraptor. Some taxa such as Microraptor had well-feathered arms and also rear legs, displaying skeletal features indicating at least a gliding ability. Other branches of the eumaniraptorans underwent extensive miniaturization, tail reduction, and evolved shoulder girdles with potential for powered flight; the Avialae, the group that would evolve various groups of true winged dinosaurs and, ultimately, Aves—birds in the truest sense. In the latter part of the Cretaceous the flying reptiles were forced to share the sky with a vast array of essentially flying dinosaurs: primitive birds and their descendants. As the Cretaceous progressed, the marginocepha- lians also evolved extensively from their Late Jurassic ornithischian relatives. Marginocephalia— dinosaurs with extensions and protrusions of bone around the skull—further split into two main lineages: the pachycephalosauria (“thick-headed lizards”) and the ceratopsia (“horned faces”). The ceratopsia lineage started in the mid-Jurassic, marked by basal members such as Yinlong from China, but during the latter part of the Cretaceous they evolved an enormous variety of frill-necked species, including Triceratops and its close relative Torosaurus. The latter has the largest skull of any land animal that ever existed. It seems that in some terrains the marginocephalians came to dominate the hadrosaurs, and at least in one location, the Saskatchewan province of southern Canada, the author has observed the last dinosaur remains occurring before the end of the Cretaceous are the marginocephalian Triceratops.
At the end of the Cretaceous period, approximately 65.5 million years ago, events occurred to cause apocalyptic environmental pressures to bear down on the planet. Despite the vast scope of the dinosaur spectrum, no member of Dinosauria outside the clade Aves proved capable of surviving the subsequent mass extinction.
Birds From Dinosaurs?
The phlyogenetic relationship between dinosaurs and birds has been intensely debated for at least a century, amid growing recognition of the many skeletal features found in both groups. Some theropod dinosaurs have a hand and wrist structure virtually identical to that of birds except in size, following a pattern found only within Dinosauria. They also have almost identical shoulders, hips, thighs, and ankles. Theropod dinosaurs have light, hollow bones as birds do, often with a dense system of blood vessels—a feature never found in reptiles. Birds and dinosaurs have a secondary palate—reptiles do not. Some dinosaurs are known to have uncinate processes, small extensions of bone on the ribs that assist high-flow breathing in nearly all modern birds. This feature suggests at least some dinosaurs had an avian type respiratory system. Likewise, the vertebrae of theropods and sauropods have pleurocoels: scoops and hollows in the bone that accommodate expanding air sacs in modern birds.
Dinosaurs also laid eggs, and although their eggs differ somewhat from those of birds, at least some dinosaurs are known to have nested in colonies and cared for their young in the nest. Incubation behavior has been supported by certain fossil discoveries, such as a large oviraptorid skeleton found in the classical brooding position on top of a nest containing eggs. The number of eggs found and the absence of eggs in the adult’s body support the idea that the oviraptor was truly incubating the eggs, not simply laying them. There are many other interesting specimens, such as the small Mei long found with its tail encircling the body and head tucked around onto the back or wing, perfectly matching a typical sleeping or resting posture of living birds.
Even one of the most evident differences between birds and dinosaurs, the question of feathers versus scales, is no longer much of a difference at all. Many dinosaurs are now known to have evolved feathers, including asymmetric “flight” feathers, “downy” feathers, and other types that develop in different stages of modern bird life.
Several theories have been put forth to explain the similarities between birds and dinosaurs. One is that the birds are simply descendants of dinosaurs, as first suggested by Thomas Henry Huxley in the late 1860s. Another is the theory of parallel evolution—perhaps their similarities arose in response to similar environmental pressures as birds and dinosaurs evolved from an earlier separate ancestor. A third theory, originally proposed by Abel in 1911, is that dinosaurs are actually descended from birds rather than the other way around. The parallel evolution and birds-came- first theories had the majority of scientists convinced until 1973, when John Ostrom from Yale University successfully demonstrated that the skeleton of Archaeopteryx, considered by most researchers to be the earliest bird, was actually that of a coelurosaurian dinosaur with feathers. Ostrom later showed that other dinosaurs such as Velociraptor possessed the most distinctive of all bird-like bones: the furcula (wishbone).
Many of the difficulties in solving the bird versus dinosaur problem stemmed in part from the lack of skeletons of early birds. However, an explosion in the discovery of such skeletons in the 1990s has produced both early birds and “transitional” forms in large numbers. This evidence has greatly strengthened the argument that birds are descended from dinosaurs. The transitional Unenlagia (“half-bird”) from Argentina is especially noteworthy, having arms that fold exactly as bird wings do. The Mongolian nesting oviraptorid dinosaurs demonstrate bird-like brooding behavior, or at least a protectiveness of their eggs not characteristic of reptiles. The dromaeosaurid-like bird Rahonavis from Madagascar also has a peculiar mixture of bird and dinosaur characteristics, with well-developed wings but also the sickle claw typical of the maniraptoran carnivorous dinosaurs.
However, undoubtedly the greatest discovery has been a number of feathered dinosaurs unearthed recently in China. Feathers were long considered a bastion of “birdness,” and although the driving force behind feather evolution is not well understood (they likely did not evolve originally for flight; perhaps for insulation, display, or even as a metabolic clearance system for excess minerals), it had generally been considered that only the discovery of a genuine feathered dinosaur would provide definitive proof that birds had a dinosaurian origin. The announcement by Chinese paleontologists that they had discovered Sinosauropteryx, a basal coelurosaur dinosaur with evidence of an integumentary structure similar to feathers, thus sparked much debate. This revelation was followed by more Chinese finds from the same area, dinosaurs that were definitely feathered. Protarchaeopteryx and Caudipteryx were first announced as basal birds on this shifting disbelief. However, several investigators have since demonstrated that both are theropod dinosaurs—Caudipteryx is an ovirap- torosaur, and Protarchaeopteryx is a manirap- toran dinosaur. Also discovered were a feathered therizinosaurid, Beipiaosaurus; the dromaeosau- rid Sinornith osaurus; and a plethora of superbly preserved similar specimens. Microraptor, another dromaeosaurid, even displayed “flight”- style feathers on its legs. The resulting arrangement of wing surfaces almost resembles a biplane. Most paleontologists now accept that birds— Aves—arose somewhere within the theropod dinosaur lineage; the only remaining question is how far back they were present and so exactly where the split occurs. An emerging consensus seems to indicate that the common ancestor of theropods and birds may date back at least to the Early Jurassic.
As more and more specimens are found, the evidence in favor of birds as dinosaur descendants continues to grow. At the same time, the dividing line between theropod dinosaurs and birds becomes less and less clear-cut. The gap between birds and dinosaurs has essentially been bridged—birds are highly evolved dinosaurs.
It is now known that in the Triassic period, dinosaurs spent many millions of years competing with their archosaur and mammal contemporaries, without achieving dominance. Dinosaurs expanded and evolved at an easy tempo, and although they diversified early on, their adaptations were hardly unique among their contemporaries. Their erect gait, for example, was essentially shared by some other archosaurs (e.g., aetosaurs, ornithosuchids, and some crocodylomorphs) and the dinosauro- morphs who also (presumably) shared similar physiology. But after the end of the Triassic extinction events, dinosaurs had assumed not only the roles of top predator and large herbivore but also most other roles all the way down. Only niches for very small animals remained to be filled by mammals and other reptiles. Why did dinosaurs survive when their closest relatives, which had a similar posture and many of the same anatomical adaptations, did not? And given that mammals have such an enormous advantage today, what are we to make of the whole Mesozoic era when, for 165 million years, the dinosaurs reigned supreme and few mammals could grow larger than a modern domestic cat?
One of the few measurable differences in the comparison of dinosaurs to their archosaur and dinosauromorph cousins was their limb proportions and foot posture. Dinosaurs have a longer fibula and tibia in relation to the femur and had better evolved the digitigrade stance. Could such subtle differences have provided a crucial advantage in the competition for food during periods of rapid environmental change? Possibly. The increased leverage of the legs may have allowed dinosaurs to get food resources faster or to retain energy during low food availability. But such anatomical differences do not explain how dinosaurs also managed to keep mammals subservient for so long, when we know that warm-bloodedness is a potent evolutionary advantage. We must therefore also consider the physiological characteristics of dinosaurs: that they may also have been warmblooded, to some degree, and capable of significantly different respiration than that of their mammal and reptile competitors.
Among today’s fauna, we find no large land predators that are cold-blooded. Crocodiles, the sole exception, occupy only one very specific ecological niche and are basically water dwellers.
During the entire Cenozoic era as well, virtually all large predators were warm-blooded. This is because the position of top predator is very competitive, and the ability to control and maintain body temperature (i.e., being warm-blooded) means that the animal is not as dependent on the environment for its activity rate. It can hunt at any time of the day (or night), in any season; warm-blooded animals can thus operate at maximum efficiency and tend to have a strong advantage on land.
Of course, the terms warm-blooded and coldblooded are something of an oversimplification. Virtually all animals, if examined under the proper conditions, will appear to be “warmblooded”; that is, their internal body temperature will be roughly constant over time. The more important aspect of this trait is the mechanism by which body temperature is maintained. The terms endothermic and ectothermic are more appropriate: An ectothermic animal relies on heat from the outside (i.e., the environment) to maintain body temperature, whereas an endothermic animal relies on the heat generated within its own body by metabolic processes. Endothermic animals will therefore have a higher metabolic rate, because they need to generate more internal heat. There are even animals whose metabolic processes fall in between endothermy and ectothermy. This is far from an either/or proposition, as a large number of physiological mechanisms are potentially involved. The enormous range of dinosaur sizes implies that they did not share a common physiology, nor did they use the same strategies and mechanisms to maintain their metabolic state. Given that all we have left are lifeless bones and footprints, is it even possible to produce evidence supporting the warm-blooded dinosaur hypothesis? Surprisingly the answer is yes, although such evidence must be largely inferential. For example, we can consider lines of evidence in bone structure, histology, and bone isotope composition, growth rates, posture, speed and agility, rate of evolution, similarities with birds, insulation, and the existence of Arctic and Antarctic faunas.
By comparing these aspects of dinosaurs, mammals, and cold-blooded reptiles, we find considerable support for the idea that dinosaurs were warm-blooded. At the very least, they clearly had a much higher activity level and metabolic rate than do modern cold-blooded creatures.
Not surprisingly, some counterarguments and alternatives have been presented as well. The question has yet to be decided, but on balance the likely outcome seems to be heavily weighted in favor of at least partially warm-blooded dinosaurs. The same tests for endothermy have been applied to the fossil remains of mammal ancestors (cynodonts, dicynodonts) and archosaurs. In both cases the results suggest that they were more warm-blooded than cold-blooded. Thus, it seems that endothermy may have evolved in both lines at an early period. In this scenario, dinosaurs were simply one group in a long line of warm-blooded animals.
An additional point of difference that may help explain dinosaur success is their increased activity potential as a result of evolving a unidirectional airflow lung and air sack respiratory system, as seen today in their avian descendents. Compared to mammals, which have a “piston” lung system whereupon the diaphragm does most of the work in sucking air into the lungs, at least some lineages of dinosaurs evolved a more avian-type respiration system that was “high flow” and more efficient at extracting oxygen out of the air. The evidence is in the pneumaticity of bones in sauropod and thero- pod lineages, uncinate processes on the ribs of some maniraptoran dinosaurs, and the astounding growth rates of some clades. During the Late Triassic extinctions, such potential may have been critical when atmospheric oxygen content was less than half that of present day.
Although much work needs to be done to confirm these lines of investigation, the combination of new fossil discoveries and modern-day technology are without doubt overturning the old paradigm of dinosaurs as sluggish, mal- adapted creatures. We now see that they represent an epitome of productive evolution; as a group they were innovative, adaptive, and successful long term. The ability to metabolize oxygen efficiently and maintain constant, high body temperature meant dinosaurs could remain active and successful during times of environmental stress. It required one of the greatest environmental catastrophes the world has ever known to bring them down and clear the way for our own ancestors.
The Great Extinction
Earth’s geological record contains evidence of many different extinction events, some large enough to disrupt ecosystems on a global scale. Three such events occurred during the dinosaurs’ reign. The first occurred in the Late Triassic and has been postulated as one of the main reasons for their rise to dominance. Another, at the end of the Jurassic, is associated with the disappearance of many large sauropods and ornithopods. This extinction may also have been related to the proliferation of flowering plants. Then we have what is probably the most notorious extinction of all time: the Cretaceous-Tertiary (K-T) event that destroyed all nonavialan dinosaurs 65 million years ago. Earth also lost a massive range of other taxa—terrestrial and marine, vertebrates and invertebrates. Floral communities also went through a dramatic turnover in abundance and diversity. The great reign of the dinosaurs was over, and no convincing evidence has ever been found that dinosaurs survived for even a short time after the K-T event.
The carnage has been attributed to several large-scale geological events occurring at or near the K-T boundary. This includes a period of massive volcanism, which poured out cubic kilometers of basalts to form the Deccan traps in India and Pakistan; global sea-level regression, which is notable in the sedimentary record of North America’s western interior; and finally an asteroid impact large enough to form a 170 kilometerdiameter crater on the Yucatan Peninsula of Mexico. Individually or collectively, these events are thought to have invoked habitat loss and fragmentation, global climate change, and food chain disruption. The rapid environmental devastation overcame the abilities of many organisms to adapt and survive, causing one of the five biggest mass extinctions of life on Earth.
Whereas any of these cataclysmic events may account for the loss of life observed in the fossil record, all raise problems that have not yet been resolved. In particular, the apparent selectivity of the extinctions has yet to be explained. Large, nonavialan vertebrates such as dinosaurs were entirely wiped out, but so too were large marine reptiles. Amphibians survived very well, but many lizards and small reptiles did not. The avialae (birds and their closest relatives among the dinosaurs) survived, but flying reptiles such as pterosaurs did not. It can be recognized that life on Earth suffered a relatively rapid disruption that impacted ecosystems all over the world, if to varying degrees. As the selectivity applies to both land and sea, freshwater and saltwater, a successful theory of mass extinction must be able to explain why some survived and others did not.
Unfortunately, few areas of the world record this period of geological time in detail. Probably the best known and most studied are the upper Hell Creek and lower Tullock formations of eastern Montana in the United States. One research site produced more than a hundred samples of vertebrates from 12 major taxa: sharks, bony fish, frogs and salamanders, multituberculate mammals, placental mammals, marsupial mammals, turtles, lizards, champsosaur, crocodilians, orni- thischian dinosaurs, and saurischian dinosaurs (not including birds). At this site it was observed that nearly half of the total species survived across the K-T boundary, but 75% of the extinctions occurred in just four of the groups: sharks, lizards, marsupials, and dinosaurs.
The same pattern of selective termination is seen worldwide in both animal and plant communities: Some life forms were entirely wiped out, whereas others, in seemingly similar conditions, remained untouched. Whereas the patterns of the K-T mass extinctions make the cause very difficult to interpret, the asteroid impact hypothesis has stood the test of time. For one thing, it has made predictions that have been supported by exploration and evidence, such as that an iridium-enriched layer of soil should occur globally and that extinctions should be not only global but relatively abrupt and synchronous. This turns out to be the case. It also has the advantage of being the most parsimonious explanation. While it fails to explain certain details of the selectivity, our understanding of the complex interactions caused by such impacts is still incomplete. Furthermore, the dinosaurs are known to have previously survived sea-level transgression/ regression and volcanism on large scales. While acknowledging that these changes may have played a role, the asteroid impact better explains the scale and tempo of this environmental catastrophe. Although it may be attractive to invoke all the geological events in unison (the “everything that could go wrong did go wrong” theory), this solution is not necessary in light of the simple and effective asteroid impact.
Fortunately for Homo sapiens, the demise of dinosaurs freed up numerous ecological niches into which placental mammals could expand and diversify. The dinosaurs’ death left behind a fertile world in which mammals could finally come out of the shadows and go on to become dominant terrestrial vertebrates.
Today, it is interesting to compare the K-T event to the mass extinction of life currently under way. Human activities are causing an impact of similar strength and rapidity on the habitats of many species, including birds. It would be ironic as well as tragic should our species go on to achieve what the K-T mass extinction could not: the extinction of birds, the last true descendants of dinosaurs.
Sporadic work on fossils started as long ago as the 15th century, but it took hundreds of years for scholars to agree that fossils were even of biological origin. For one thing, recognizing this fact implies that extinction is real: that most of the organisms that have ever existed are now gone, leaving only their descendants. Researchers then (and now) had to deal warily with the extrapolations of their discoveries. Supporting the idea of extinction could be life-threatening in cultures where the dominant religion held that all animals were created at the same time to populate a world that looked much like it does today. It took a great deal of physical evidence collected from sites all over the world to raise the threshold of enlightenment.
During the 18th century, fossil marine reptiles and other “different” creatures such as pterosaurs were being found in quantity all over Europe. This led to swelling public interest, a passion that really ignited when a huge and viciously toothed Mosasaur jaw, the “Monster of Maastricht,” was found in Belgium in 1770. The preponderance of evidence shone light on the debate over extinction and the origin of fossils, and the latter carried the day. Researchers soon moved on to classifying and interpreting the remains, a task made easier by the newly developed technique of comparative anatomy.
By the 1820s fossilized dinosaur bones had been found in English gravel quarries. Their strange appearance caused a plethora of interpretations. Various publications likened them to lizards, birds, mammals, and crocodiles. Scientists, artists, and the public proved insatiable, demanding more knowledge on what these long-lost animals looked like and how they might have behaved. But a combination of factors would collude to imprint one particular interpretation of dinosaurs on the public and scientific psyche.
The first dinosaur described by scientists was dubbed Megalosaurus, which literally means “big lizard.” The next was named Iguanodon, simply because its leaf-shaped teeth are similar to those of the South American iguana. Having been tentatively identified as prehistoric ancestors to modern reptiles because of the shape of their teeth, dinosaurs quickly became associated with reptiles in every way. In 1842 Sir Richard Owen coined the term Dinosauria (“fearfully great lizards”) for this growing group of strange creatures, and his work was published (almost as a footnote) in Report on British Fossil Reptiles. A side effect of this usage was that most subsequent dinosaur names were suffixed with saura, meaning “lizard” or “reptile.”
Laypersons and scientists alike began to assume they were cold-blooded and sluggish, as some reptiles are today. Such an interpretation complemented other attitudes of the time, namely, that human beings were the pinnacle of nature. Any extinct life was necessarily more primitive, slow, and dim-witted than humans. Thus, the first complete dinosaur skeletons went on display as very large lizards with odd spikes and claws. This representation of dinosaurs as splay-legged, heavy-set quadrupeds would hold sway for almost the next 150 years.
Even at this early stage, however, several investigators realized that these animals strongly resembled birds. Thomas Henry Huxley in 1868 went so far as to suggest that birds might be descended from dinosaurs. He and other workers found considerable evidence in contradiction of the sluggish reptile theory, evidence supporting a more active and dynamic model of dinosaur behavior. But the dinosaurs-are-lizards paradigm was already entrenched; it would take time and several bursts of scientific endeavor to break it down.
In 1877 the hub of dinosaur discovery moved from Europe to North America. A vast number of complete pristine skeletons were discovered at Como Bluff in Wyoming and later on in Utah. Some of the discoveries included dinosaur lineages recognized as bipeDalí, a distinctly unreptile-like trait, but this had little effect on the prevailing view. In 1925, however, Earl Douglas used the accumulated evidence in his collection of 350 tons of sau- ropod bones to mount and display the massive specimens in a parasagittal stance, with their legs oriented beneath the body like columns. This started a debate that was to last for decades. The argument was finally settled by the fabulous Paluxy dinosaur trackways, discovered in the bed of a Texas river. The careful measuring of the undoubted sauropod footprint tracks confirmed that sauro- pods indeed had a fully erect stance and not the previously accepted sprawling reptilian posture.
Another part of dinosaurs’ reptilian image had been washed away, at least anatomically, but the entrenched idea of sluggish, cold-blooded behavior persisted. Andrews and Osborn found fossilized dinosaur eggs and nests while exploring the Gobi Desert in Mongolia during the 1920s, indicating dinosaurs’ similar habits to modern birds. Theropod dinosaurs, such as Velociraptor later discovered in the same area, also beautifully displayed hollow bones similar to birds. Despite growing evidence that dinosaurs were more bird-like in their morphology, habits, and physiology, the concept was still not getting widespread traction.
In 1969 John Ostrom published detailed work involving the Early Cretaceous theropod Deinonychus, demonstrating that it was a lightly built and likely agile predator. This convinced many that at least some dinosaurs were very active and were more bird-like than reptilian in their behavior. Ostrom also investigated horned and duckbilled dinosaurs, reinterpreting them as sophisticated feeders rather than plodding primitives. This began a period often called the “dinosaur renaissance”: Research accelerated amid a growing sense that dinosaurs were substantially dynamic animals. Earlier recognized but ignored aspects of the birddinosaur relationship were revisited and examined afresh, fueled by new discoveries. The Trexler family’s 1978 discovery in Montana of hadrosaur hatchlings still within their nests led to the description of Maiasaura (“good mother lizard”) by Jack Horner. This convincing evidence of postbirth parental care increased the growing perception of dinosaurs as dynamic animals with complex behavior. Then in the 1990s, a flood of small feathered dinosaurs with clearly bird-like attributes was discovered in China. These specimens helped confirm the new paradigm, and dinosaurs are now widely regarded as lively rather than sluggish creatures.
In some ways another wave of “dinomania” started around this time, to be inflamed later by blockbuster films such as Jurassic Park and the BBC television series Walking With Dinosaurs. Previously esoteric questions, such as whether dinosaurs were really warm-blooded and how they came to be extinct, became topics of interest to the public at large. People wanted to learn about how they mated, how they cared for their young, and what their migration patterns were. This trend of popularity seems set to continue with every generation.
New discoveries and further research continue to evolve our interpretations of dinosaurs, but old habits die hard and many aspects of these fascinating creatures are counterintuitive. An example is the customary view that sauropods were high feeders, using their extensive necks to reach up and browse on trees similar to the modern-day giraffe. New discoveries and ongoing research show that although there were certainly some very tall sauro- pods that specialized in mid and high browsing, most seemed better adapted for low fodder.
The discovery and study of Nigersaurus, a medium-sized Cretaceous sauropod from West Africa displays extreme adaptations for a lowbrowsing feeding strategy first established among diplodocid sauropods during the Jurassic. The alignment of the cervical column, combined with an endocast of the braincase revealing the ear canals, indicates Nigersaurus‘s head was held habitually pointing at the ground. In this groundlevel posture it could nip off soft vegetation with its linear dental battery running along the front of its muzzle containing many hundreds of small teeth that continually replaced themselves at a monthly rate. Reinterpretation of the Jurassic sau- ropods Apatosaurus and Diplodocus has revealed they too were likely specialized for low browsing, including lacustrine feeding, based on their headdown attitude, neck pose and flexibility, specialized dentition, and a center of mass located back near the hind limbs.
Nigersaurus is an example of new dinosaurs that help broaden our outlook on all dinosaurian biology (beyond the fact that sauropods may not have resembled any living forms in how they carried out their lives). If we let the fossil bones tell their own story, these remarkable creatures help us critically reexamine preset rules of interpretation and thus dinosaurs have finally recovered from several centuries of the misplaced lizard stereotype to play an ongoing role as ambassadors of adaptability, complexity, and long-term survival.
The study of dinosaurs is now progressing at an unprecedented rate. Advances in the sciences of taxonomy, phylogeny, biogeography, paleobiology, and stratigraphic distribution have been providing frequent leaps of insight. Vigorous research programs are under way in many countries around the globe, many involving entirely new methods such as genetic sequencing and the search for biomolecules. Meanwhile, new detection and surveying technology has greatly accelerated the discovery and excavation of new dinosaur remains. There is also a trend toward more attentive excavation, which helps preserve fossil evidence of soft tissue. Finally, many researchers are choosing to focus on specific information that may assist us in dealing with our own biodiversity loss and climate change.
The total number of dinosaur types that have ever lived is estimated at 900 to 1,200 genera. If this is accurate, then the known fossil record of dinosaurs is presently only about 25% complete. This sets a tantalizing and challenging target for dinosaur paleontology. As research continues, there can only be one certainty—this perennially fascinating animal group has many more surprises in store for us.
Mark James Thompson
See also Archaeopteryx; Evolution, Organic; Extinction; Extinction and Evolution; Extinctions, Mass; Fossil Record; Fossils, Interpretations of; K-T Boundary; Paleontology
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