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When dinosaurs were first studied in the early nineteenth century, they were compared with modern reptiles such as lizards and crocodiles: and while they do share a common ancestry with these animals, is it an accurate basis for establishing the nature of these creatures?

Anatomists have long recognised that dinosaurs differed from modern reptiles in having a relatively erect posture of the limbs. Whether they were bipedal or quadrupedal, the joint structure shows that their bodies were habitually held off of the ground. In contrast, nearly all modern reptiles have a sprawling posture and are active for only short periods of time.
The nearly vertical posture of the limbs can be explained in many dinosaurs by their great bulk. Neither bone nor muscle is strong enough to support and lift the body if their limbs were sprawled to the side, as in the case of the crocodile. Therefore, if the great weight of the large dinosaurs explains their upright posture, one might expect the smaller dinosaurs to adopt the sprawling stance of crocodiles. However, some of the smallest dinosaurs have a skeletal anatomy that can only be interpreted as indicating an upright posture, and many early genera must have been obligatorily bipedal, resembling large terrestial birds such as the ostrich much more closely than any living lizards.

The skeleton of most dinosaurs suggests a level of locomotion approaching more closely that of large modern animals than that of any modern reptiles. One of the fundamental differences between modern reptiles and mammals is in their capacity for sustained activity. No reptile can run actively for more than a few minutes, after which it must rest for a long period of time. Many mammals, by contrast, can remain active for hours at a time without fatigue.
The differences in their capacity for continual activity lies in the metabolism of the muscles. In reptiles, most of the energy for contraction of the muscles results from the fermentative breakdown of glycogen, giving rise to lactic acid. Lactic acid is a toxic material that must be removed from the muscles. The slow breakdown of lactic acid requires long periods of rest between bouts of activity. Among mammals, most of the energy for muscle contraction is achieved by oxidative metabolism in which the only waste products are water and carbon dioxide, which can be rapidly removed by the blood.

The capacity for continuous muscle activity explains the selective advantage for an upright posture in even the smallest dinosaurs. Most paleontologists now assume that dinosaurs must have relied primarly on oxidative metabolism for muscle contraction, and had achieved a roughly mammalian level of activity.
Other aspects of dinosaur metabolism remain more controversial. Modern mammals have approximately ten times the metabolic rate of reptiles of comparable size. Did dinosaurs have a metabolic rate equal to that of mammals, or was it considerably lower, yet still significantly above that of repitles?
Some paleontologists believe that dinosaurs may have had a truly mammalian metabolic rate, but there is some evidence to the contrary. Maintenance of a high metabolic rate requires a great deal more food than is necessary for modern reptiles, and a more effective feeding strategy, as well as a more effective integration of sensory and motor coordination. In birds and mammals, this is associated with a larger brain structure. While some dinosaurs had brain sizes (measured relative to body weight) approaching that of mammals, most did not. The average brain to body weight ratio of dinosaurs was not very different from that of their modern living relatives, the crocodile.

One more important fact that should be considered is the almost complete absence of small dinosaurs. In small mammals, with weights up to a few kilograms, much of the metabolic energy goes towards maintaining a high, constant body temperature. In this manner, the smallest mammals have an exceedingly high metabolic rate because of the problem of heat loss. In comparison to all other major groups of vertebrates, dinosaurs are exceptional in that the average body size of most species is very high. Only two or three species are known to have an adult size of less than around 10 kilograms. In contrast, the vast majority of birds and mammals are of small size.

As it stands today, the common consensus among paleontologists is that dinosaurs were more comparable to mammals and birds in the metabolism. However, there are still differences signifcant enough to place them into their own category corresponding to neither reptile, mammal nor avian but sharing traits with all three.

This has been a 'node your homework' writeup. The following references may prove useful to anyone who wants more information:

  • Gould, S.J. and Lewontin, R.C., 1979, The spandrels of San Marco and the Panglossian paradigm: a critique of adaptionist programme. p. 581 - 598.
  • Van Valen, L., 1973, A new evolutionary law: Evolutionary theory.
  • Thomas, R.D.K, 1979, Encyclopedia of Paleontology.
  • Vermeij, G.J, 1987, Evolution and Escalations: an Ecological History of Life.

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