Clues to dinosaurian metabolism
The physiology of dinosaurs is unknown for the simple reason that their temperatures cannot be measured, nor can their food consumption or carbon dioxide and solid waste output be determined (the usual methods of measuring an animal's metabolic rate). Indirect evidence is all that is available. The question whether any dinosaur species was a true endotherm cannot be answered, but some interesting anatomic facts suggest that possibility.
First of all, two dinosaurian clans, the hadrosaurs and the ceratopsians, featured highly specialized dentitions that obviously were effective food processors. Both groups were herbivorous, but unlike living reptiles they chewed their foliage thoroughly. Such highly efficient dental equipment implies that the hadrosaurs and ceratopsians were tachymetabolic. With the exception of the carnivores and possibly some ornithopod predecessors of the duckbills, like Heterodontosaurus and Iguanodon, other dinosaurs generally possessed very weakly developed dentitions.
On another tangent, certain of the predaceous dinosaurs had anatomic features that reflect a high capacity for activity. The 'ostrich dinosaurs' like Struthiomimus , Gallimimus, and Dromiceiomimus, for example, all were obligatory bipeds (two-footed animals) that, on the basis of their long hind legs, must have been very fleet. Further, the dromaeosaurs like Deinonychus , Velociraptor , and Dromaeosaurus , although they also were obligatory bipeds, killed their prey with the talons on their feet. It must have taken a high level of metabolism to generate the degree of activity and agility required by such a skill. The implication is compelling, but conclusive proof of endothermy is lacking.
Dinosaurian posture is also suggestive. Many (but not all) dinosaurs stood upright with the legs positioned directly beneath the hip sockets and, in some, the shoulder sockets. Such an erect posture is present in all nonaquatic endotherms (mammals and birds), but a sprawling or semierect posture is typical of all ectotherms (reptiles and amphibians). Bipedal stance and gait are not possible in any living ectotherm. Why is that? And what is the implication for all of the theropod dinosaurs?
Related to the upright posture of many dinosaurs is the inescapable fact that the head was usually positioned at a high level, often well above the level of the heart. In some extreme cases (Apatosaurus, Diplodocus, Brachiosaurus , and Barosaurus, for instance), the brain must have been several metres above the heart. The importance of this is that a four-chambered heart would need to have been present to pump freshly oxygenated blood to the brain. Brain death follows very quickly when nerve cells are deprived of oxygen, and to prevent it most dinosaurs must have required two ventricle pumps.
In a four-chambered heart, one ventricle pumps oxygen-poor venous blood at low pressure to the lungs to absorb fresh oxygen (low pressure so as not to rupture the pulmonary capillaries). A powerful second ventricle pump circulates the freshly oxygenated blood from the lungs to all other parts of the body at high pressure; the high systemic pressure is needed to overcome the weight of the column of blood that must be pumped from the heart to the elevated brain. In short, like birds and mammals, many dinosaurs apparently had the required double-pump heart that is necessary for an animal with a high metabolism.
The significance of thermoregulation can be seen by comparing modern reptiles with mammals. The rate of metabolism is usually measured in terms of oxygen consumed per unit of body weight per unit of time. The resting metabolic rate for most mammals is on the order of 10 times that of modern reptiles, and the range of metabolic rates of living mammals is about double that of reptiles. These differences mean that endothermic mammals have much more endurance than their cold-blooded counterparts. Some dinosaurs may have been so endowed. They seem to have possessed the cardiovascular system necessary for endothermy, but that capacity does not prove that they were endothermic. The probabilities are that dinosaurs were neither complete ectotherms nor complete endotherms but were somewhere in between.
Form and function
Differentiation of the dinosaurian orders
The two traditional orders of dinosaurs established by Seeley, Saurischia and Ornithischia, long believed to be closely related, are now widely believed to have evolved from a common ancestor‹an as-yet unrecognized (or undiscovered) primitive archosaurian reptile. The chief difference between the two orders was in the configuration of the pelvis. It was primarily on this distinction that Seeley established the orders and named them Saurischia ('Lizard Hips') and Ornithischia ('Bird Hips'), a differentiation still maintained today.
As in all four-legged animals, the dinosaurian pelvis was a paired structure consisting of three separate bones on each side that attached to the sacrum of the backbone. The ilium, above (attached to the spine), and the pubis and ischium, below, formed a robust bony plate at the centre of which was a deep cup‹the hip socket, or acetabulum. The hip socket faced laterally and was pierced or open at its centre for the articulation of the medially projecting proximal head of the thighbone. The combined saurischian pelvic bones presented a triangular outline as seen from the side, the pubis extending down and forward and the ischium projecting down and backward from the hip socket.
The massive ilium formed a deep vertical plate of bone to which the muscles of the pelvis, hind leg, and tail were attached. The pubis had a stout shaft, commonly terminating in a pronounced expansion or bootlike structure (presumably for muscle attachment), that joined its opposite mate in a solid symphysis. The ischium was slightly less robust than the pubis, but it too joined its mate in a midline symphysis. There were minor variations in this structure between different saurischian genera and families.
The ornithischian pelvis was constructed of the same three bones on each side of the sacral vertebrae, to which they attached by coossification. The lateral profile of the pelvis was quite different from that of the saurischians, with a long but low iliac blade above the hip socket and a modified ischium-pubis structure below. Here, the long, thin ischium extended backward and slightly downward from the hip socket. The pubis had a short to moderately long anterior blade, but posteriorly it stretched out into a long, thin postpubic process lying beneath and closely parallel to the ischium. The resulting configuration resembled that of birds, whose pubis is a thin process extending backward beneath the larger ischium.
These anatomic dissimilarities are believed to reflect important differences in muscle arrangements in the hips and hind legs of these two orders. Other marked dissimilarities between saurischians and ornithischians are found in their jaws and teeth, their limbs, and especially their skulls. Details on these differences are given in the following discussions of the major dinosaur groups. The table shows how the two orders are subdivided. It is important to note that the classification of dinosaurs involves a high degree of uncertainty, which may result in variations in the way dinosaurs are grouped depending on the authority.
Saurischia
The order Saurischia is known from specimens ranging from the Middle Triassic to the latest part of the Cretaceous in geologic time and recovered from every continent on the Earth. Two distinctly different suborders are traditionally included in the order‹the Sauropodomorpha (herbivorous sauropods and prosauropods) and the Theropoda (carnivorous dinosaurs). These groups are placed together only because both have the saurischian type of pelvis along with a few other primitive archosaurian features in common. No common ancestor has been widely recognized, and they could just as well be placed in separate orders. A little-known group, the Staurikosauria, is also classified in the order.
Included in this group as infraorders are the well-known sauropods, or ³brontosaur² types, and their probable ancestral group, the prosauropods. All were plant eaters.
Prosauropoda
Most primitive of the Sauropodomorpha were the early (Triassic) saurischians known as prosauropods or plateosaurs. Found in Late Triassic and Early Jurassic rocks (230 to 187 million years old), their remains are probably the most ubiquitous of all Triassic dinosaurs. They have been found in Europe (Germany), North America (New England, Arizona, New Mexico), South America (Argentina), Africa (South Africa, Lesotho, Zimbabwe), China (Yunnan), and Antarctica.
The best-known examples are Plateosaurus of Germany and Massospondylus of South Africa. Prosauropods were not large, as dinosaurs go, ranging from less than 2 metres (7 feet) in length up to about 7 metres (23 feet) and about 1 ton in maximum weight. Because their forelimbs were conspicuously shorter than their hind limbs, these animals (known from very complete skeletons) usually have been reconstructed poised on their two hind legs in a bipedal stance. Their anatomy, however, clearly indicates that some of them could assume a quadrupedal (four-footed) position. Footprints generally attributed to prosauropods appear to substantiate a quadrupedal form of locomotion.
Prosauropods have long been seen as including the first direct ancestors of the giant sauropods, probably among the family of melanorosaurids. That view still prevails, largely because of their distinctly primitive sauropod-like appearance and also because of their Late TriassicEarly Jurassic occurrence. No better candidate has been discovered so far. In general body form they were rather stocky, with a long, moderately flexible neck (containing surprisingly long and flexible cervical ribs) and a head that was small in comparison with the body. The jaw was long and contained rows of thin, leaflike teeth suited for chopping up (but not grinding or crushing) plant tissues, although there is an indication of direct tooth-on-tooth occlusion.
Prosauropod forelimbs were stout and heavily built, with five complete digits. The hind limbs were about 50 percent longer than the forelimbs and even more heavily built. The foot was of primitive design, and its five-toed configuration could be interpreted as a forerunner of the sauropod foot. Walking apparently was semidigitigrade (partly on the toes), with the metatarsus held well off the ground. The vertebral column was unspecialized and bore little indication of the cavernous excavations that were to come in later sauropod vertebrae or of the processes and projections that were to buttress the sauropod vertebral column. The long tail probably served as a counterweight or stabilizer whenever the animal assumed a bipedal position.
A large number of skeletons of Plateosaurus were recovered in the 1920s from a site in Germany. The occurrence there of multiple remains, together with the nature of the enclosing sediments, led to the thought that this assemblage had been overcome by drought and sandstorm while migrating to more suitable environs. Whether migrating or not, the abundance of Plateosaurus individuals in one location lends support to the herding instinct that has been attributed to several kinds of dinosaurs.
The more widely known sauropods - the huge 'brontosaurs' - varied in length from 6 or 7 metres (about 20 feet) in the primitive ancestral sauropods Riojasaurus of South America and Vulcanodon of Africa up to 28 to 30 metres (90 to 100 feet) or more in advanced Jurassic North American forms like Apatosaurus (formerly known as Brontosaurus), Diplodocus, and Seismosaurus. Weights ranged from about 20 tons or less in the smaller kinds like Barapasaurus of India to 80 tons or more for the gigantic Brachiosaurus of Africa and North America. Sauropods were worldwide in distribution but have not as yet been found in Antarctica. In geologic time they ranged from the Late Triassic Riojasaurus to the Late Cretaceous Alamosaurus of North America and Laplatasaurus of South America. Their greatest diversity and abundance took place during the Late Jurassic (163 to 144 million years ago).
Sauropods are notable for their body form as well as their enormous size. Their large bodies were almost barrel-shaped, with long (sometimes very long) necks and tails. They had columnar legs, like those of elephants, with little freedom to bend at the knee and elbow. The legs were maintained in a nearly vertical position beneath the shoulder and hip sockets. Because of their great bulk, sauropods unquestionably were obligatory quadrupeds, and the largest forms could not have assumed a bipedal stance even momentarily.
The sauropod limb bones were heavy and solid. The feet were broad, close to plantigrade (adapted for walking on the soles), and graviportal (adapted for bearing great weight). The five toes were generally short, blunt, and broad, but some kinds featured a large straight claw on the first digit of the forefoot and the first and second toes of the hind foot. These claws probably improved traction. Movement for these animals must have been relatively slow, with short steps necessary because of the comparative inflexibility of the limbs. Running must have been stiff-legged and no better than an elephantine pace of 16 kilometres (10 miles) per hour, if that. Their tremendous bulk placed them out of the reach of predators and eliminated any need for speed.
The vertebrae of the backbone were highly modified, with numerous excavations and struts to reduce bone weight. Complex spines and projections for muscle and ligament attachment compensated for any loss of skeletal strength that resulted from reductions in bone density and mass. The long and sometimes massive tails, characteristic of so many sauropods, would appear to have been carried well off the ground. Tail drag marks associated with sauropod trackways are not known, and damaged (stepped-on) tails are also not known, even though these animals apparently traveled in herds (albeit of undetermined density).
The sauropod tail may have served as a modest whiplike weapon, but there is no evidence to that effect. Another possible use of the tail may have been thermal regulation‹improved heat loss through its large surface area. A more likely explanation of its function is that the massive muscular base of the tail was the critical anchor site of the large, powerful hind leg muscles that produced most of the walking force required to move the many tons of sauropod weight. The muscle arrangement of the tail was precisely that of modern alligators and lizards.
The most important part of any skeleton is the skull, because it provides the most information about an animal, its mode of life, and its general biology. Sauropod skulls were of several main types; the high, boxy Camarasaurus type (often incorrectly associated with Apatosaurus) and the low, narrow, streamlined Diplodocus type. The former had broad, spatulate teeth, while the latter had narrow, pencil-shaped teeth. Both kinds of teeth seem weak and totally ill-designed to crop or chew the volumes of plant food necessary to sustain such large animals. Correspondingly, the jaws were relatively weakly developed, and there is no special evidence indicating powerful jaw muscles to activate the feeding system.
Until recently, sauropods were visualized as swamp or lake dwellers because their legs were thought to be incapable of supporting their great weights or because such huge creatures would naturally prefer the buoyancy of watery surroundings. Not only is that thinking incompatible with their food requirements (food would seem to have been most plentiful on land), but research has refuted it. Experiments with fresh bone samples have shown that bone of the type which composed the sauropods' limb bones could easily have supported their estimated weights. Moreover, there is no feature in their skeletons that suggests an aquatic, or even amphibious, existence. In addition, numerous trackway sites clearly prove that sauropods could navigate on land, or at least where the water was too shallow to buoy up their weight. Accordingly, newer interpretations see these animals as forest inhabitants.
Still another blow has been dealt to the old swamp image by the physical laws of hydrostatic pressure, which prohibit the explanation that the long neck enabled a submerged animal to raise its head to the surface for a breath of fresh air. The depth at which the lungs were submerged would not allow them to be expanded by normal atmospheric pressure, the only force that fills the lungs. Consequently, the long necks of sauropods must be explained in terms of terrestrial functions such as elevating the feeding apparatus or the eyes. On all counts, sauropods are best seen as successful giraffelike browsers and only occasional waders.
Staurikosauria
Very little is known about the animals grouped in this suborder because so few specimens have been found, and all of those are fragmentary. The remains of Staurikosaurus, from the Middle Triassic of Brazil, the specimen for which the suborder was established, consist only of the vertebral column and pelvis, the hind legs lacking the feet, and the lower jaw. The skull and forelimbs, like the feet, are not known. Those parts that are known, however, are similar to those of the later theropods, and a flesh-eating habit is suggested by the piercing teeth of the lower jaw. The skeleton indicates a moderate-size animal of about two metres (seven feet) in length, possibly having a bipedal posture and gait. Appearing as it does in Middle Triassic rocks, it may be the oldest kind of dinosaur known‹if in fact it proves to be a dinosaur.
Several other fragmentary remains from South America may be of related types. Specimens of Herrerasaurus and Ischisaurus from Middle to Late Triassic rocks of Argentina are those of carnivores about the same size as Staurikosaurus or slightly heavier. But again, the material is so incomplete that relationships are still uncertain. What is preserved suggests a theropod identity.
Theropoda
This group includes all the other known carnivorous dinosaurs. No herbivores are recognized in the group. Theropods ranged in size from the smallest known adult dinosaur, Compsognathus , the size of an ordinary chicken and probably weighing 1 or 2 kilograms (2 to 4 pounds), up to the great Tyrannosaurus , which was 15 or more metres (50 feet) long and over 5 metres (16 to 18 feet) tall and which weighed 6 tons or more. In all theropods the hind leg bones were hollow to varying degrees‹extremely hollow and lightly built in small to medium-size animals like Compsognathus, Coelurus, and Ornitholestes and more solid in the larger forms like Allosaurus, Daspletosaurus, and Tarbosaurus. Theropods have been recovered from rocks of the Late Triassic through the latest part of the Cretaceous and from all continents except Antarctica.
In stance and gait, theropods were obligatory bipeds. Their bodies conformed to a common shape in which the hind legs were dominant and designed for support and locomotion. The forelimbs, on the other hand, had been modified from the primitive design and entirely divested of the functions of locomotion and body support. Hind limbs were either very robust and of graviportal (weight-bearing) proportions, as in Allosaurus, Megalosaurus, and the tyrannosaurids, or very slender, elongated and of cursorial (adapted for running) proportions, as in Coelurus, Coelophysis, Ornitholestes, and the ornithomimids.
Theropod feet, despite the group's name, which means ³beast (i.e., mammal) foot,² usually were designed like those of birds. Three main toes were directed forward and splayed in a V-arrangement; an additional inside toe was directed medially or backward; and the whole foot was functionally digitigrade, with the ³heel² elevated well above the ground. Toes usually bore sharp, somewhat curved claws.
The forelimbs varied widely from the slender, elongated ones of Struthiomimus, for example, to shorter, more massively constructed grasping appendages like those of Allosaurus, to the greatly abbreviated arms and hands of Tyrannosaurus. The hands typically featured long, flexible fingers with pronounced, often strongly curved claws, which must have borne sharp piercing talons. Primitive theropods like Coelophysis had four fingers, but the majority were three-fingered. Tyrannosaurids (Albertosaurus, Daspletosaurus, Tarbosaurus, and Tyrannosaurus) and apparently the diminutive Compsognathus were notable for their two-fingered hands on unusually short arms. This separation of function between fore and hind limbs set theropods apart from all other dinosaurs.
As obligatory bipeds, the theropod body plan was reorganized so that the animal, large or small, balanced at the pelvic pedestal by using its heavy tail behind and thrusting the thighs and knees forward to positions directly below the centre of gravity. To shift the centre of gravity back toward the supporting hind legs, the head and neck were arched upward, especially so in those kinds with very large heads. In addition, the bipedal stance required reinforcing the backbone by enlargement of the interspinous ligaments (between the back vertebrae) and the ligaments of the neck. The tail probably served not only as a counterweight to the large body and head but also as a dynamic inertial stabilizer, moving up and down and from side to side to counteract changes in the animal's movement and direction such as lunging and turning during an attack.
Bipedality has sometimes been explained as an adaptation for fast running or for energy conservation. The latter seems unlikely in view of several experiments showing that it requires no less energy to run or walk on two legs than it does on four. Speed does not seem to have been the primary factor either, although some investigators have claimed that a seven-ton Tyrannosaurus could achieve an unlikely velocity of 70 kilometres (45 miles) per hour - faster than a greyhound or a racehorse. Rather, because of their great weight, tyrannosaurids probably could have barely kept ahead of a charging elephant (20 kilometres per hour), whereas the more cursorial-limbed ornithomimids might have been able to keep up with a modern ostrich (70 kilometres per hour). Rather than as a specialization for running, bipedality may have come into being and perfection more as an enhancement of viewing range. Theropods all had large eyes and a wide field of vision.
Theropods that featured large heads, like Allosaurus and Tyrannosaurus, had long, strong lower jaws that undoubtedly were powered by massive jaw muscles. The skull was a highly fenestrated strut work, both for lightness and for strength, providing ample attachment areas for muscles. The jaws are noted for their complement of sharp, bladelike teeth. In nearly all theropods these laterally compressed blades had steak-knife-like serrations along the rear edge and often along the front edge as well. Among the predatory adaptations displayed by most kinds of theropods, the characteristic teeth were the most conspicuous. The diversity of the suborder Theropoda, with its various modes of predation and carnivory, is suggested by the following summary of the group's two infraorders.
Ceratosauria
These are the basal or primitive theropods of medium size like Ceratosaurus, Dilophosaurus, and perhaps Coelophysis from the Late Triassic and Late Jurassic. They may include ancestral stock of most later theropods.
Theropoda
Tetanurae
These comprise all the nonceratosaurian theropods. The tetanuran theropods are subdivided into five distinct categories: the coelurosaurs, the ornithomimosaurs, the maniraptors, the segnosaurs, and the carnosaurs.
The coelurosaurs were small to medium-size carnivores, the smallest known being Compsognathus. Coelurosaurs had very long legs of cursorial proportions and had forelimbs and hands ranging from short (Compsognathus) to long and grasping (Ornitholestes).
Ornithomimosaurs were medium-size to large theropods. They were toothless and apparently beaked, with very long legs and arms. A well-known example is Struthiomimus. Most were ostrich-size and were designed for fast running. The largest kind was Deinocheirus from Asia, known only from one specimen consisting of complete arms and hands almost three metres (nine feet) long‹nearly four times longer than those of Struthiomimus. These animals' cursorial design, toothlessness, and hands unsuited for seizing prey leave their lifestyle and feeding habits unclear.
The maniraptors are also known as deinonychosaurs and include the oviraptors and troodontids. These medium-size predators had long, grasping arms and hands, moderately long legs, and a specialized tail that could be held high for active balance control. Their feet bore the primary killing device, large slicing talons on the inside toes. The best-known examples are Deinonychus of North America and Velociraptor of Asia.
Segnosaurs were medium-size Asian theropods known only from a few examples. The mouth had bladelike teeth at the back but apparently no teeth at the front. The pelvis differed markedly from the normal saurischian design. They are very inadequately understood but seem to have been unlike all other theropods.
The carnosaurs were large to very large (up to 6- or 7-ton) carnivores with blade-toothed jaws, twice or more as long as the arms and hands, powerful hind legs, and taloned feet. It is not certain whether they were predators or carrion feeders. Tyrannosaurus is the most commonly cited example.
Ornithischia
The order Ornithischia, unlike the Saurischia, appears to be a natural group of closely related animals. All were plant eaters, and all are thought to have descended from a common ancestor. The rationale for postulating such an ancestor is based on the common existence of a uniquely ornithischian feature‹a median predentary bone that joined the two lower jaws at the symphysis. Further, a distinctive tooth form, crenulated along the upper edges, occurred in some members of all suborders. Collectively, these features point to a close common ancestry. The order has traditionally been divided into four suborders. However, recent studies have regrouped the members of this order into two major categories, the suborders Cerapoda and Thyreophora.
Ornithischia
Cerapoda
The suborder Cerapoda is divided into three infraorders: Ornithopoda, Pachycephalosauria, and Ceratopsia.
Ornithopoda
The ornithopods ranged in size from the small fabrosaurids, 1 to 2 metres long, to the huge duck-billed types that reached lengths of 10 to 12 metres or more. Ornithopods appear to have flourished the longest of the dinosaur varieties, thriving from the Late Triassic to the latest part of the Cretaceous. They inhabited all land areas except Antarctica.
Ornithopod families included the early and somewhat primitive fabrosaurids, mostly from Eurasia and southern Africa, and heterodontosaurs, also largely from southern Africa. In the latter group was the oldest of the ornithischians, Pisanosaurus, a single very fragmentary specimen from the Late Triassic of Argentina. Better known are the slightly larger hypsilophodonts and much larger iguanodonts, mostly from North America and Europe. Representative of these groups are Hypsilophodon, about three metres in length, and the famed Iguanodon, about nine metres long. Another group, the hadrosaurs, sometimes called trachodonts, were the large duck-billed ornithopods of the Late Cretaceous, especially of North America and Eurasia. An abundant and diverse family, they included more than two dozen known genera. (The Late Triassic or Early Jurassic Scutellosaurus had some similarities to the ornithopods, but its affinities are still uncertain.)
The postcranial anatomy of the ornithopods was fairly uniform. All members had hind legs that were much longer and sturdier than their forelegs. The thighbone (femur) was always shorter than the shinbones (tibia and fibula) and usually bore a prominent process, called the fourth trochanter, just above mid-length for the attachment of the retractor, or walking, muscles. The pelvis was expanded, usually with an elongated and broad prepubic process for the attachment of the protractor, or recovery, leg muscles. The tail was long and sometimes quite deep and flat-sided (commonly equated with tails that afford a sculling action in swimming animals). The vertebral spines of the tail and thoracic region were reinforced by a rhomboidal latticework of ossified tendons. These features, taken together, have led to the conclusion that all ornithopods were at least facultatively bipedal but that most kinds could assume a quadrupedal stance and gait. Some may have been obligatory bipeds‹for example, Fabrosaurus, whose forelimbs were only one-third the length of the hind limbs and whose hands seem better constructed for grasping than for walking.
Ornithopod feet were modified from the primitive five-toed pattern. The three middle toes served as the functional foot; the inside toe was shortened and often held off the ground; and the outside toe was greatly reduced or absent altogether. The toes terminated in broad, almost hooflike bones, especially in the duckbills. The hand reflected the primitive five-part design, although the first, or inside, finger may have been missing in some. The fingers usually ended in broad, blunt bones rather than in claws. In the duckbills the fingers apparently were encased in a mittenlike device sometimes thought to have been a paddle to aid in swimming; more probably it simply broadened the hand to better support the animal's weight on soft ground.
Members of the infraorder differ from one another in the structure of their skulls. In particular, their jaws and teeth show considerable variation among the different families. In the fabrosaurids the teeth were simple, leaf-shaped, laterally compressed elements arranged in a single front-to-back row in each jaw. They were not set in from the outer cheek surface as in most ornithopods. Small incisor-like teeth existed on the premaxillary bones above, but no teeth were present on the predentary below. The lower jaw had no coronoid process for large muscle attachment, and the upper temporal fenestra (the jaw muscle site) was relatively small. Upper and lower teeth alternated in position when the jaw was closed; they did not occlude directly.
In heterodontosaurs the cheek teeth were crowded together into long rows and set in slightly from the outer cheek surface. They occluded directly to form distinct chisellike cutting edges with a self-sharpening mechanism maintained by hard enamel on the outer side of the upper teeth and the inner side of the lower. There were prominent upper and lower tusklike teeth at the front of the mouth, the upper tusks in the premaxillaries, the lower tusks on the dentary bones of the jaw and not on the predentary. The upper temporal fenestra, larger than that of the fabrosaurids, and a prominent coronoid process beneath it indicate the existence of much larger jaw muscles than in the fabrosaurids.
The hypsilophodonts had cheek teeth arranged in tightly packed rows set well in from the outer cheek surfaces. The teeth occluded directly, and the opposing rows formed a long shearing edge similar to that of the heterodontosaurs. There was, however, no canine tusk either above or below. The premaxillaries had small, simple, incisor-like teeth above the beak-covered, toothless predentary. Strong coronoid processes extended up from the lower jaws toward the moderate-size upper temporal fenestrae.
In the iguanodonts the coronoid processes and temporal openings provided for still larger jaw muscles, but the cheek teeth were less regular and compacted than in the primitive ornithopods and consequently did not occlude as uniformly. Both the premaxillaries and the predentary were toothless but probably were sheathed in horny beaks.
Specialization of the teeth and jaws reached a pinnacle in the hadrosaurs, or duck-billed ornithopods. Here a very prominent, robust coronoid process jutted out and up at the back of the stout lower jaw. A large adductor muscle chamber was present above this process and beneath the lateral and upper temporal fenestrae‹clear evidence of powerful jaw muscles. The dentition consisted of numerous tightly compacted teeth crowded into large grinding batteries. The battery in each jaw was composed of as many as 200 functional and replacement teeth with distinct, well-defined wear or grinding surfaces that resulted from very exact occlusion. Both the predentary and premaxillaries were toothless but were enclosed in broad horny beaks, or bills. These bills apparently had edges sharp enough to close, clamlike, for shredding and stripping leaves or needles from low shrubs and branches. Pine needles have been identified in duck-billed dinosaur remains and presumably represent stomach contents.
Some varieties of hadrosaurs are also noted for the peculiar crests and processes on the top of the head. These structures were expansions of the skull composed almost entirely of the nasal bones. In genera like Corythosaurus , Lambeosaurus , Parasaurolophus, and a few others, the crests were hollow, containing a series of median and lateral chambers that formed a convoluted passage from the nostrils to the trachea.
Aside from passing air along to the lungs, the function of these narial crests is not widely agreed upon. Sound production (honking), an improved sense of smell, or a visually conspicuous ornament for species recognition are some of the variously accepted suggestions. Since these animals are no longer considered to have been amphibious, ideas like snorkeling and extra air storage space have generally been discarded.
Pachycephalosauria
In important respects the pachycephalosaurs conformed to the basic ornithopod body plan (some experts include them with the Ornithopoda rather than as a separate infraorder). All appear to have been bipedal, possessed the typical ornithopod ossified tendons along the back, and had simple leaf-shaped teeth, although the teeth were enameled on both sides. The ornithischian type of pelvis was present, but the obturator process of the ischium was not. The pachycephalosaurs are known as domeheads, because of their most distinctive feature‹a marked thickening of the frontoparietal (forehead) bones of the skull (see illustration). The thickness of bone was much greater than might be expected in animals of their size. The suggestion has been made that this forehead swelling served as protection against the impact of head-butting activities such as those seen today in animals like bighorn sheep.
Stegoceras and Pachycephalosaurus of the North American Cretaceous were the smallest and largest members of the group, the former attaining a length of about 2.5 metres and the latter twice that. Pachycephalosaurs existed almost entirely in the Late Cretaceous (although Yaverlandia is from the Early Cretaceous) and have been found mostly in North America and Asia. The origin of the group is not known, but most likely it derived from some undiscovered Late Triassic or Early Jurassic hypsilophodont.
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