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Dinosaurs Metabolism. From the time of the earliest discoveries in the 19th century, experts like
Owen, Leidy, Marsh, and Cope classified all then-known Dinosaur remains as reptilian because they exhibited a
set of anatomic features that were typical of living reptiles like turtles, crocodiles, and lizards. Dinosaur all had
lower jaws constructed of several bones, featured a reptilian jaw joint, and possessed a number of other
nonmammalian characteristics.
Clues to dinosaurian metabolism. The physiology of Dinosaur 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 Dinosaur generally possessed very weakly developed dentitions. On another tangent, certain of the predaceous Dinosaur 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) Dinosaur 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 Dinosaur 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 Dinosaur 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 Dinosaur 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 Dinosaur 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 Dinosaur 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 Dinosaur 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.
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