Dinosaurs - 3

Dinosaurs - 3

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 �brontosaur� 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 Triassic�Early 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.




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