The Coelurosauria represent a widely diverse group of theropods ranging in size from the tiny Microraptor and Compsognathus to the enormous Tyrannosaurus. The name Coelurosauria means "hollow-t ail lizards." When the name was proposed in 1920, the group was intended to include mostly small predatory dinosaurs that had lightweight skeletal features. The category became a dumping ground for many difficult-to-classify theropod taxa that were not part of a naturally related group.
The Coelurosauria was recently redefined by French paleontologist Jacques Gauthier and others as a key subgroup within the Avetheropoda. The group Coelurosauria is divided into four main groups and then several subgroups—the Compsognathidae, Tyran-nosauroidea, Ornithomimosauria, Therizinosauridae, and Mani-raptora. The subgroup Maniraptora, in turn, is further divided into several lines of small- to medium-sized non-avian theropods, plus the basal birds. The origin of birds will be discussed in more detail in Chapter 4.
The coelurosaurs represented the final great radiation and evolution of non-avian theropods before their demise at the end of the Late Cretaceous Epoch. As such, coelurosaurs are the most derived (i.e. birdlike) versions of several lines of theropods and represent the extreme trends that were taking place just before the end-Cretaceous mass extinction.
The compsognathids ("delicate jaws") were small basal coelurosaurs with birdlike bodies. Only a few taxa currently make up this group; they lived during the Late Jurassic of Germany and France and the Early Cretaceous of England, China, and Brazil. Compsognathus (Late Jurassic, Germany and France) was discovered in 1861 in the same quarries as Archaeopteryx and was in some ways similar to
this early bird, with a long neck, a tail, and long legs, but not wings or feathers. Compsognathus is known only from two specimens and for many years languished as a mysterious taxon without any obvious closely related kin. A discovery in China in 1995 was to change the status of Compsognathus considerably, however.
In the 1990s, a stream of fossils dating from the Early Cretaceous of northeastern China began to emerge from the province of Liaoning. Among the first to appear was a slab of mudstone with the flattened remains of a small predatory dinosaur. The skeleton of this creature was remarkably well preserved in much the same manner as that of the famous Archaeopteryx. The tiniest of details were visible. The dinosaur measured only 3.3 feet (1 m) long and was complete except for the tip of its tail. With its meat-eating teeth, short arms, and long legs, it looked much like Compsognathus. Most exciting, however, was that the skeleton was outlined with the texture of filaments of fuzz, or "protofeathers," which could be likened to the downy coating seen on modern birds before they reach maturity and grow true feathers. The new dinosaur was named Sinosau-ropteryx ("Chinese lizard wing") by Chinese scientists and soon joined Compsognathus as one of the primitive members of a clade that preceded an explosion of feathered dinosaur and bird evolution in the Early and Late Cretaceous Epochs.
Tyrannosaurs were once categorized as members of the Carnosauria because of their large size and superficial similarity to carnosaurs such as Allosaurus; however, closer examination by American paleontologist Thomas Holtz in 1994 revealed some derived traits of the foot that tyrannosaurs shared with coelurosaurs, including the ornithomimosaurs and troodontids. Resurrecting an old hypothesis first postulated by the legendary German paleontologist Friedrich von Huene (1875-1969), Holtz's well-reasoned and exacting analysis resulted in a major redefinition of tyrannosaurs and their place in the evolutionary tree of dinosaurs. Rather than being closely related to such early large predators as Allosaurus, T. rex and its kin were actually an extraordinary branch of the same dinosaur lineage that gave us birds. The subsequent discovery of earlier and smaller members of the tyrannosaur line has firmly rooted Holtz's view of tyran-nosaur evolution and helped better define coelurosaurs as a natural group of related organisms.
Tyrannosaurs are known for several features unique to their clade. Their broad, massive skulls were supported by short, powerful necks. Unlike the bladelike teeth of most theropods, the teeth at the tip of the tyrannosaurs' upper jaw were shaped like a "D" in cross-section, and in derived tyrannosaurids, the teeth were thick and banana-shaped, with sharp serrations and deep roots. This gave these predators the ability to bite and crush bone with their powerful jaws. Tyrannosaur heads were massive and seemingly oversized in proportion to their stout bodies. The tyrannosaur forelimbs were short and strong and had only two digits—a derived trait not seen in other theropod clades.
The roots of the Tyrannosauroidea tree began with much smaller coelurosaurs that arose in the Late Jurassic Epoch. The traits that link them to T. rex are limited at first to certain features of the skull. The earliest known relative of the tyrannosaurs was Guan-long, from the Late Jurassic of China. It lived 160 million years ago, which was before Archaeopteryx and other known feathered dinosaurs and early birds, but could be related to the line of theropods that led to Tyrannosaurus some 90 million years later. Guanlong was not the top predator of its day, measuring only about 10 feet (3 m) long, a far cry from the 43-foot (13 m), 7-ton (6.3 metric ton) behemoth that was Tyrannosaurus. Guanlong, whose name means "crowned dragon," was described in 2006 by a team led by Xing Xu of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. The name is a tip of the hat to Tyrannosaurus rex, the "tyrant lizard king," but also refers to an unusual and prominent oval-shaped nasal crest that sits atop the centerline of the skull. The relationship of Guanlong to tyrannosaurs is not obvious to the casual observer. Among the major differences were its small size, head crest, long arms, and three claws on each hand—not two claws as known for later tyrannosaurs. Examination of the skull openings and teeth, however, reveals close affinities with tyrannosaurids, so it would appear that the evolution of coelurosaurs in the direction of Tyrannosaurus began in the skull.
The next important link in the evolution of tyrannosaurs was another Asian theropod named Dilong (Early Cretaceous, China), or "emperor dragon," described in 2004 by Xing Xu and colleagues. Dating from 128 million years ago, Dilong was discovered in the fossil-rich deposits of Liaoning in northeastern China. At only about 5 feet (1.5 m) long, this lightly built theropod was also far smaller than T. rex, but it shared traits of the skull and teeth, thus linking it to tyrannosaurids.
Specimens of Dilong also show that it was cloaked in Sinosauropteryx-like "protofeathers," a trait that insulated its body and that was unrelated to flight (see Chapter 4). Dilong also had three digits on its hands, not two as in the later, l arge-bodied tyrannosaurs. The presence of protofeathers in Dilong suggests that all tyrannosaurs—even the mighty T. rex—may have been feathered at some stage of their lives. Because feathers in non-avian dinosaurs would have been used primarily for insulation, display, or gliding, larger tyrannosaurs were probably only feathered as hatchlings or juveniles. Upon reaching maturity, their body size probably would have been sufficient to retain enough heat to maintain their thermo-regulatory requirements.
The Asian localities of Guanlong and Dilong suggest that early tyrannosaurs may have originated there, but several other specimens of proposed early tyrannosaurs, most of which are fragmentary or of questionable affiliation, hail from the Late Jurassic of England (Iliosuchus); Portugal (Aviatyrannis); and Utah (Stokesosaurus); and from the Early Cretaceous of England (Eotyrannus). Even when Guanlong and Dilong are taken into account, there remains a considerable gap in the evolutionary record of tyrannosaurs leading to the Late Cretaceous appearance of Tyrannosaurus (Late Cretaceous, western North America) and its close relatives.
Tyrannosaurus was the largest predatory dinosaur in the Northern Hemisphere during the Late Cretaceous. Its highly derived adaptations veered somewhat away from the body plan found in most other theropods. It had extremely short but muscular arms with only two claws per hand. Its teeth were thick and chunky and its head wide, bulky, and heavily muscled in a way that optimized its jaws for biting down firmly with great force. Unlike the car-charodontosaurid giants of the Southern Hemisphere, which used their bladelike teeth to slice away at the flesh of prey, tyrannosaurs were built for brute force. Tyrannosaurs were likely pursue-and-bite predators, taking advantage of their long legs, huge raptorial feet, muscular necks, and jaws lined with sturdy, bone-crunching teeth. After chasing down their prey, tyrannosaurs probably completed the kill using a combination of biting to the neck or clamping down on the muzzle of the prey, each of which could result in mortal injury or suffocation. Their foot claws were used to hold down the prey rather than to slash them. Tyrannosaurs probably fed by clamping
down on the body with their jaws, then pulling and twisting with their enormous strength to rip chunks of meat from their prey.
Tyrannosaurus is the largest and best-known tyrannosaur. It had smaller close relatives in the Late Cretaceous of Asia and western and eastern North America, including Albertosaurus (Alberta and
Montana); Daspletosaurus (Alberta and Montana); Gorgosaurus (Alberta); Appalachiosaurus (Alabama); Alectrosaurus (Mongolia and China); Alioramus (Mongolia); Tarbosaurus (Mongolia and China); and possibly Dryptosaurus (New Jersey).
Ornithomimosaurs ("bird mimic lizards") bore a striking, although superficial, resemblance to modern-day ostriches and other large, flightless ground birds. Although they are not closely related to ostriches, ornithomimosaurs had a similar body plan, with long legs for running and a long, S-curved neck with a small head, the latter typically equipped with a toothless beak. Ornithomimosaurs were among the fastest of dinosaurs. Estimated top speeds for ornithomimosaurs range from 30 to 50 mph (50 to 80 km/h). They had long, slender arms and hands equipped with three sicklelike claws for defending from predators or possibly grasping prey. The most basal members of this clade had small teeth, a feature missing from more derived taxa in favor of toothless beaks. The group Ornithomimosauria is divided into three subgroups: the Harpymimidae, the Deinocheiridae, and the Ornithomimidae.
Pelecanimimus ("pelican mimic"), from the Early Cretaceous of Spain, is the oldest and most basal ornithomimosaur currently known. It is represented by an excellent specimen consisting of the front half of the skeleton, but no legs. Pelecanimimus was small, measuring only about 6 to 8 feet (1.8 to 2.5 m) long. Its skull was long and low, and its jaws contained an enormous number of teeth— especially considering that most other members of this line of the-ropods were toothless. Surprisingly, Pelecanimimus had about 220 teeth, more than half of which were found closely packed in the den-tary (the forward-most bone of the lower jaw). The premaxilla and maxilla (upper jaw) had teeth only in the forward part of the mouth, followed by bony ridges instead of teeth to complete the upper biting surfaces. This is remarkable, considering that the next-oldest basal ornithomimosaurs, Harpymimus (Early Cretaceous, Mongolia) and Shenzhousaurus (Early Cretaceous, China), had only 10 or 11 and
7 or 8 teeth, respectively, and these were found only in the dentary. In most other respects, Pelecanimimus, Harpymimus, and Shen-zhousaurus resemble smaller versions of later ornithomimosaurs.
Having evolved from earlier lines of tooth-bearing predatory dinosaurs, it was assumed that ornithomimosaurs acquired their toothless condition through the gradual reduction of the number of teeth over time. Harpymimus, discovered in 1984, did much to support this idea. The discovery of Pelecanimimus, a geologically earlier taxon than Harpymimus, suggested otherwise. When Pelecanimimus was first described in 1994 by Spanish paleontologist B.P. Pérez-Moreno and colleagues, they needed to reverse some established logic about the progression from teeth to beaks in ornithomimosaurs. Pérez-Moreno reasoned that closely packed teeth could have become increasingly fused over many generations, effectively creating a cutting and ripping surface analogous to that of a toothless beak. Harpymimus and Shenzhousaurus might represent theropods in the middle of such an evolutionary transition, having already adapted to a closely fused cutting edge.
The subgroup Deinocheiridae is represented by only one highly enigmatic taxon, Deinocheirus ("terrible hand"), from the Late Cretaceous of Mongolia. The only known specimen of this thero-pod consists of a pair of long, robust forelimbs that measure about
8 feet (2.5 m) long and are adorned with massive, 10-inch (25 cm) claws, plus some fragments of ribs and vertebrae. The arms most closely resemble those of other ornithomimosaurs—hence the current association of Deinocheirus with this clade. If this dinosaur was indeed an ornithomimosaur, the length of the forelimbs suggests that the entire animal could have been between 25 and 35 feet (8 and 11 m) long.
The subgroup Ornithomimidae includes the best-known and most derived ornithomimosaurs. All of these taxa are found in the Northern Hemisphere, as are all known basal ornithomimosaurs, and hail from either Asia or western North America. Two of the most studied taxa are Gallimimus (Late Cretaceous, Mongolia) and Struthiomimus (Late Cretaceous, Alberta and Montana).
Gallimimus ("chicken mimic") was described by Mongolian paleontologist Rinchen Barsbold and Polish paleontologists Halszka Osmolska and Ewa Roniewicz in 1972. It is known from several specimens ranging in size from juvenile to adult. The largest known specimen of Gallimimus reached 13 to 20 feet (4 to 6 m) in length and may have stood about 6 feet (2 m) tall at the hip. Gallimimus had legs especially well adapted for running and a long tail that served as a counterbalance while it ran. Its skull was long, with large eyes and a toothless, almost ducklike beak.
Struthiomimus was discovered in 1914 and first described by Henry Fairfield Osborn (1857-1935) of the American Museum of Natural History. Struthiomimus represents the first well-understood ornithomimosaur. Measuring between 10 and 13 feet (3 to 4 m) long, this ornithomimosaur had the characteric slender forelimbs, grasping claws, and small, ostrichlike head associated with other members of this clade.
The toothless beak of ornithomimosaurs has been a source of much speculation regarding the diet of these theropods. As early as 1917, Osborn suggested that these dinosaurs subsisted on an herbivorous diet of soft shoots and buds from plants. Evidence of gastroliths found with several ornithimimosaur specimens supports this hypothesis because stomach stones are used by some herbivorous dinosaurs to help pulverize tough plant material. This would not have precluded ornithomimosaurs from also eating small vertebrates such as lizards or small birds as well as insects. As such, ornithomimosaurs represent a branch of the theropod evolutionary tree that may have been omnivorous. The recent discovery of evidence of soft-tissue structures in the beaks of two ornithomimosaur specimens also suggests that some specimens may have used their beaks to strain food particles from water in streams and lakes.
Maniraptora is a natural group of related theropods that includes the evolutionary stock from which birds arose. This is not to say that all maniraptorans were small or birdlike. Members of this clade vary considerably in their outward appearance and size, ranging from small to medium-sized theropods. Anatomical traits that unite maniraptorans include modifications to the wrist and fore-limbs that made grasping and twisting of the hand possible and led to the flight stroke in birds; a clavicle, or "wishbone," in the collar area; a downward- or backward-pointing pubis bone in the pelvis (most saurischians had a forward-pointing pubis); a shortened tail that was stiff at the distal (outer) end; long arms; and a three-clawed hand that was larger than the foot. There is growing evidence that all subgroups within Maniraptora had feathers. Maniraptorans are divided into five subgroups: the Oviraptorosauria, Troodontidae, Therizinosauroidea, Alvarezsauridae, and Dromaeosauridae.
The Oviraptorosauria included theropods that measured from a modest 6.6 feet (2 m) to 27 feet (8 m) long and that currently are known with certainty only from the Northern Hemisphere. Known from 20 or more taxa, the oviraptorosaur body is similar to that of other small theropods and has the long, slender arms and grasping claws of other maniraptorans. Some members of the group—such as Oviraptor, Rinchenia, and Citipati, all from the Late Cretaceous of Mongolia—had large crests at the front of the skull, extending from the nasal bones. The Oviraptor skull is unique among dinosaurs and quite alien in appearance, even among theropods. The skull was tall with a short snout and large, toothless beak. The purpose of the large nasal crest is not entirely understood, but in addition to providing a visual display, it may have been adapted for the purpose of making sound and possibly to enable the dinosaur to absorb more moisture from the warm air of its arid environment.
The toothless beak of most oviraptorosaurs was part of a strong, muscular jaw. Without teeth, one might assume that these oviraptorosaurs were either insectivores or herbivores, and it would appear that they were well adapted for a variety of foods. Their jaws could have been used to crack open clams and mollusks that lived in freshwater streams and lakes in their habitat. The powerful grasping claws of oviraptorosaurs strongly suggest that they also may have fed on small vertebrates—such as frogs, lizards, snakes, birds, and mammals—which they could snatch up and dispatch with a crushing bite.
The first described member of the Oviraptorosauria was Oviraptor, discovered in Mongolia by famed fossil hunter Roy Chapman Andrews (1884-1960) and his team, working for the American Museum of Natural History. Andrews had found the remains of the little theropod in the midst of a nest of dinosaur eggs. Because abundant adult fossils of the horned dinosaur Protoceratops had also been found in the vicinity, it was assumed by paleontologist Henry Fairfield Osborn, who first provided a scientific description of Oviraptor, that the predator was plundering a nest of Protoceratops at the time of its untimely demise. Hence the name Oviraptor philoceratops—"egg stealer fond of ceratopsians."
Years later, in 1994, Mark Norell (b. 1957) and colleagues, also of the American Museum of Natural History, announced the discovery of an Oviraptor embryo within one of the very kinds of eggs that were once thought to have been those of the horned dinosaur Protoceratops. Soon after correcting this case of mistaken identity, at least two additional oviraptorosaur fossil specimens of Citipati were discovered in which an adult individual was sitting atop a nest of its own eggs, apparently brooding in the manner of modern birds. This is one of the strongest cases from the fossil record of dinosaurs that at least some taxa engaged in some degree of parental care over their unhatched eggs.
One of the most primitive or basal oviraptorosaurs was Cau-dipteryx (Early Cretaceous, China), a remarkable creature known from at least nine nearly complete specimens. Caudipteryx comes from the same fossil deposits in the Liaoning region of northeastern China that is now famous for a variety of vertebrate, invertebrate, and plant fossils, including many birds and feathered dinosaurs.
Described in 1998 by paleontologists Qiang Ji, Phil Currie, Mark Norell, and others, Caudipteryx provided the first definitive evidence of modern birdlike feathers on a non-avian theropod. Close study showed that its jaw was nearly toothless except for four premaxillary teeth; this suggested that it and other basal oviraptorosaurs, including Avimimus and Incisivosaurus, were evolving in the direction of a toothless beak, as seen in later members of this group. Caudip-teryx was between 3 and 4 feet (1 m) long. Its arms were short, its neck was long, and its hind legs were long and suited for running. Impressions of feathers found with the skeletal elements show that Caudipteryx had short, downy feathers covering its body, plus a tuft of longer feathers on its forelimbs and a fanlike spread of feathers on its short tail. The feathers were short and similar to those seen in modern flightless birds. Several specimens of Caudipteryx have been found with a collection of gastroliths in the abdominal region, suggesting that this animal may have been an herbivore with a gizzard similar to modern birds.
Another important oviraptorosaur specimen is that of Nomingia (Late Cretaceous, Mongolia). Although consisting of only a partial postcranial skeleton, this specimen is significant because the blunt tail of this oviraptorosaur ended with several short, fused vertebrae, a feature interpreted by some paleontologists as that of a pygostyle. In birds, the pygostyle is a place of attachment for the tail feathers. Moreover, an as yet-undescribed caenagnathid at the Carnegie Museum in Pittsburgh includes a tail tipped by several very short, interlocking vertebrae that, when articulated, resemble a pygostyle, although because they're not fused together they cannot really be considered one.
The maniraptorans known as the Troodontidae make up a small clan known from 10 taxa found in western North America, Uzbekistan, Mongolia, and China. Dating from the Early Cretaceous Epoch, troodontids were lightly built theropods that measured no more than 6.5 feet (2 m) long. Their anatomy speaks to an active predatory lifestyle: long arms with raptorial hand claws, long legs for running, and a mouth full of small, recurved, serrated teeth seen in some species. Of all the dinosaurs, troodontids possessed some of the largest brains relative to body size, which extended their sensorial acuteness beyond that seen in most other theropods. Troodontids are known for having large eyes and probably also had excellent hearing. These sensory advantages, combined with their lightly built anatomy, suggest that troodontids were active and agile predators.
The fossil record of troodontids is spotty. The best-known taxon, Troodon ("wounding tooth"), is known from at least 20 partial specimens found in such localities as Montana, Alberta, Wyoming, and Mexico. The animal was originally named by American paleontologist Joseph Leidy (1823-1891) in 1856, based only on a tooth. The best-understood skull of a troodontid is that of Saurornithoides (Late Cretaceous, Mongolia), for which both an adult and juvenile specimen are known. Byronosaurus, Mei, Jinfengopteryx, and Sinor-nithoides (Early to Late Cretaceous, Mongolia and China) are also known from complete or nearly complete skulls.
The troodontids possessed another anatomical feature seen in a related group of maniraptorans: a deadly, sicklelike claw on the second toe of each foot that could be raised off the ground while the animal walked or ran. This claw was probably used for attacking prey and became a larger and more lethal weapon in the Dromaeosauridae.
The therizinosaurs represent another branch of theropods that adapted to eating plants. The bizarre morphology of this group is accented by the enormous, scythelike claws on their forelimbs, a distinctive feature that inspired the name therizinosaur, which means "reaping lizard." Therizinosaurs have been found in Northern Hemisphere outcrops dating from the Early to Late Cretaceous in North America and Asia. As a subgroup within the Coelurosau-ria, therizinoaurs are defined as all taxa representing the last common ancestor of Therizinosaurus (the most derived member) and Beipiaosaurus (the least derived member) and all of its descendants. There are currently 12 recognized taxa of therizinosaurs.
The therizinosaur body plan was that of a rotund body with wide hips, a long neck, a small head, and long arms with sicklelike claws. The animals were small to large-sized theropods that evolved from small, goose-sized taxa to monstrous, overgrown turkeylike creatures that may have measured up to 33 feet (10 m) long or more.
The long necks, leaf-shaped teeth, bipedal posture, and four-toed hind limbs of therizinosaurs invite comparisons with prosauropods. Like prosauropods and ornithomimosaurs, the most derived therizinosaurs were probably vegetarian and used their long necks to reach high into trees to grasp leaves. They possibly used their clawed hands to shovel branches into their mouths, where their teeth could strip off the leaves and buds. For many years, the remains of known therizinosaurs were scant and partial, leaving much guesswork as to their true evolutionary affinity within the dinosaurs. With nothing more to go by, the anatomical traits of therizinosaurs appeared to mix features of prosauropods, ornithischians, and theropods, creating a confusing picture.
The description of Beipiaosaurus (Early Cretaceous, China) in 1999 added much to the knowledge of therizinosaurs. This early, small therizinosaur measured about 7 feet (2.2 m) long and was the most complete therizinosaur found to date. This dinosaur had a bony beak and cheek teeth, an early adaptation toward more efficient plant eating. It also had three functional, weight-supporting toes—not four like the later therizinosaurs. This suggests that the four-loed foot was derived by therizinosaurs on their own and not inherited from a prosauropod ancestor. As if to dramatize the theropod link even more, Beipiaosaurus was covered with a bushy coat of feathers, further linking it to Maniraptorans and other non-avian dinosaurian relatives of birds. The anatomical affinities of therizinosaurs are so close to those of maniraptorans that many cladistic studies now place the clade Therizinosauroidea within the Maniraptora.
An important basal therizinosaur was Falcarius (Early Cretaceous, Utah). Measuring up to 13 feet (4 m) long, this medium-sized theropod is known from an abundance of fossil specimens recently discovered in Utah and described by paleontologist James Kirkland (b. 1954) and his colleagues. Kirkland says that literally thousands of disarticulated specimens of Falcarius have been found in two adjacent Utah sites, apparently drowned by a flood as the dinosaurs gathered or migrated as a large group. It will take many years for paleontologists to fully comprehend the amount of information available from a site of these extraordinary proporations. Even during the early stages of analysis, however, it appears that Falcarius may fill an important evolutionary gap between earlier carnivorous coelurosaurs and the later herbivorous therizinosaurs. Several adaptations seen in Falcarius make this evident, including the development of leaf-shaped teeth similar to those that evolved separately in sauropods, a bony beak at the front of the mouth, and expansion of the pelvic region to accommodate greater intestinal volume.
Therizinosaurus (Late Cretaceous, Mongolia) was one of the last and most derived members of the clade. Discovered in fragmentary pieces over the course of several fossil-hunting expeditions by a joint team of Soviet and Mongolian paleontologists, it was officially described by Russian E.A. Maleev in 1954. Still, the Therizinosaurus specimen consisted only of elements of the forelimb and possibly the hind limb, so the precise nature of the animal remained largely unknown until fossils of other therizinosaurs came to light. Descriptions of Erlikosaurus and Segnosaurus, both from the Late Cretaceous of Mongolia, emerged between 1979 and 1981, revealing detail about the skulls, teeth, and postcranial skeletons of therizinosaurs.
In addition to the discoveries of the early therizinosaurs Beipiao-saurus and Falcarius, knowledge about the later therizinosaurs has grown considerably because of several discoveries beyond the provenance of Mongolia. Nothronychus (Late Cretaceous, New Mexico) included a fragmentary but informative partial skeleton and skull. Erlianosaurus and Neimongosaurus, both from the Late Cretaceous of China, were important to understanding the spinal, limb, and pelvic elements of this clade.
The Alvarezsauridae include five known taxa of enigmatic theropods from the Early and Late Cretaceous of Argentina and Mongolia that may be more closely related to non-avian theropods than to birds. As these creatures have come to light only since the 1991 description of Alvarezsaurus (Late Cretaceous, Argentina), not enough is yet known about them to fully understand their evolutionary relationship to true birds. Alvarezsaurids were small;
they were no more than 6.6 feet (2 m) long. They had tiny teeth, long and narrow snouts, and extremely short forelimbs with a single, pronounced claw each. Alvarezsaurids were flightless.
Shuvuuia (Late Cretaceous, Argentina) and Mononykus (Late Cretaceous, Mongolia) are the two best-known specimens. The only good specimen of an alvarezsaurid skull is that of Shuvuuia; it was long and slender and had an upper jaw that was hinged at the point where it connected to the skull, thus providing great flex. This was a feature found in later birds but not found in other non-avian dinosaurs. The specimen of Shuvuuia also showed evidence of a downy covering of protofeathers. The relationship of alvarezsaurids to other non-avian theropods is clouded by the fact that the alva-rezsaurids' single-clawed, stubby forelimb adaptation is unlike that of either maniraptoran dinosaurs or birds. These theropods could not fly, but there is evidence yet to be clarified that might reveal whether they evolved from earlier flying ancestors or from non-avian theropods.
In a category of its own is another early bird named Rahona-vis, from the Late Cretaceous of Madagascar. The evolutionary affinities of this theropod are not entirely certain. When it was first described, by paleontologist Catherine Forster and her colleagues, Rahonavis ("menace from the clouds bird") was believed to be a primitive bird with close ties to the dromaeosaurid dinosaurs; like them, Rahonavis included a large, curved claw on the second toe of the foot. About the size of Archaeopteryx, Rahonavis was certainly a predatory creature with dinosaur roots, but the viability of its wings was questioned. Along with Shuvuuia and Mononykus, which also had close ties to birds but could not fly, Rahonavis suggests that some theropods may have become birds and then secondarily lost the ability to fly in later generations.
All of these specimens date from the end of the Cretaceous, some 75 million years or more after the first bird, Archaeopteryx. Such enigmas illustrate that fossils provide an incomplete picture of the true diversity of past life, and that evolution is capable of going in many directions at the same time, as dictated by the habitat and succession of traits that inhabit a given taxa. By the end of the Cretaceous, Madagascar was separated from the landmass that now forms India. The isolation of Rahonavis and other dinosaurs on this island during the Late Cretaceous may have resulted in some highly specialized adaptations not seen in other parts of the world. The Dromaeosauridae, known to many members of the public as "raptors," were small to medium-sized predators that are the closest known relatives of birds, making their anatomy of special significance to paleontologists interested in understanding the evolutionary relationship between dinosaurs and birds. Dromaeosaurs ("swift lizards") were bipedal with three weight-supporting toes on the feet that included a large, recurved, and retractable killing claw on the inner toe. Their forelimbs were long with flexible wrists and grasping hands that had three fingers with pronounced, sicklelike claws. The dromaeosaur tail was long and greatly stiffened, probably for improved balance and maneuverability. Their skulls were generally slim and light. Their teeth were fairly uniform in size, bladelike, and serrated, and included four somewhat less curved teeth in the front of the upper jaw. The smallest known dromaeosaur is also one of the smallest known dinosaurs: the birdlike Microraptor (Early Cretaceous, China) was a mere 2.5 feet (76 cm) long. The largest known raptors are known only from partial specimens. Achillobator (Late Cretaceous, Mongolia) was up to 20 feet (6 m) long and Utahrap-tor (Early Cretaceous, Utah) was about 23 feet (7 m) long and had an enormous foot claw measuring 9 inches (23 cm). Eleven taxa of dromaeosaurs are currently known from decent fossil evidence.
The first dromaeosaur described was Dromaeosaurus (Late Cretaceous, Alberta), but was known only from an incomplete skull and a few other bones. The importance of dromaeosaurs in understanding the evolution and biology of dinosaurs took on much greater significance with the discovery of Deinonychus in Montana in the late 1960s. Described by paleontologist John Ostrom in 1969, Deinonychus became one of the best-known dromaeosaur taxa and ignited modern debates about the lifestyle and metabolism of the-ropods as well as the evolutionary link between dromaeosaurs and birds. Deinonychus was a slender but powerful theropod measuring about 10 feet (3 m) long.
Deinonychus provided the first fairly complete picture of a drom-aeosaur, and Ostrom quickly recognized its significance for the evolutionary history of theropods. He was struck by the similarities between Deinonychus and Archaeopteryx, the first bird, and detailed many anatomical similarities between the two. It was Ostrom's conclusion that dromaeosaurs and Archaeopteryx both evolved from a common ancestor—probably a small, bipedal, predatory dinosaur from the Early or Middle Jurassic. By the time of Archaeopteryx in the Late Jurassic Epoch, early birds had begun to evolve separately from their non-flying theropod relatives, the dromaeosaurs. Whether Archaeopteryx was an important link in this chain or merely an evolutionary dead end has yet to be proven. Likewise, the search continues for a common ancestor of both birds and dromaeo-saurs from fossil deposits of Middle Jurassic age.
In addition to its anatomical links to birds, Ostrom also revived the debate over the lifestyle of theropods. What he saw in the anatomy of Deinonychus was an agile, alert, and quick-moving predator. The quarry in which the original specimens of Deinonychus were found contained several individuals of this theropod as well as a skeleton of the larger herbivorous dinosaur Tenontosaurus. Ostrom interpreted this as suggesting that Deinonychus hunted in packs, an activity requiring i ntra-species coordination and communication. In his view, it was incorrect to picture all dinosaurs as being passive, stupid, and slow-moving. It seemed that at least some species, such as Deinonychus, were active and possibly endothermic, two ideas that have since become accepted once again by most specialists in the field.
The retractable claw on the foot of dromaeosaurs was a remarkable adaptation. It was probably used to slash the prey—either to disembowel them outright or to weaken them through blood loss—or to pin down small prey. When not in use, the claw could be neatly raised out of the way so that the predator could walk or run without being inhibited. When brought into play during an attack, the claw was leveraged perfectly with the force of the leg to deliver maximum power to the point of contact, digging deep into the prey if the dromaeosaur was able to put its weight behind it.
At least two genera of early dromaeosaurs, Microraptor and Sinornithosaurus, show clearly that at least some members of the clan were feathered. It would be safe to say that later dromaeosaurs also possessed feathers, even if only at the early stages of their growth. Microraptor had long feathers on its forelimbs and hind limbs, effectively giving it four wings. It also had a phalanx of feathers lining the end of its tail. Microraptor was not a bird, though, and is considered a flightless feathered dinosaur that could probably glide. The evolution of feathers, which is described more fully in the next chapter, may have first come about to provide insulation for these animals. According to this hypothesis, the role of feathers in flight was a secondary application.
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