DINOSAUR PHYSIOLOGY AND THE ORIGIN OF MAMMALS.

Abstract

Today and throughout the Cenozoic mammals have been the dominant class of land vertebrates. Mammals are the most conspicuous elements in most terrestrial vertebrate communities, and the vast majority of large land vertebrates (50 kg and larger) are mammals. The success of mammals probably is due in large measure to their high metabolic rates, great scope for increasing metabolism during activity, and their ability to maintain constant body temperatures by complex mechanisms of heat production, insulation, and heat loss. Mammals evolved from their therapsid reptilian ancestors in the Late Triassic, only a few million years after the first dinosaurs appeared (Crompton, 1968). Although they are common and diverse in most Jurassic and Cretaceous terrestrial microvertebrate faunas, mammals showed little or no promise of developing into the grand variety of large animals seen in modern mammalian communities. From their beginnings in the Late Triassic until the very end of the Cretaceous, a total of over 120 million years, twice the length of the entire Cenozoic, mammals were consistently tiny or at most small vertebrates, with the largest herbivores reaching the size of a large squirrel and the largest carnivores, the size of a cat. The greatest Cretaceous mammals were dwarfed by the largest contemporary lizards-the superficially iguana-like herbivore Polyglyphanodon (Gilmore, 1942) reached a length of a full meter, while some of the Cretaceous flesh-eating monitors such as Sanziwa and Parasaniwa (Gilmore, 1928) were even larger. Dinosaurs dominated the JuraCretaceous, but not in all size categories. Although the two orders of dinosaurs (Saurischia and Ornithischia) included virtually all the large fully terrestrial vertebrates, and many dinosaur genera were elephant-size or larger, the fossil evidence strongly suggests that no small (adult weight 10 kg or less) dinosaurs ever existed in either Jurassic or Cretaceous. The smallest, fairly complete dinosaur skeletons usually indicate a length of a meter from nose to tail tip (Compsognathus, Hypsilophodon, Microvenator, Tenontosaurus). But most of these small specimens probably are juveniles. In some cases, other conspecific specimens show that the adult size was much larger-up to 2.5 meters for Hypsilophodon (Galton, in press), and up to seven meters for Tenontosaurus (Ostrom, 1970a). Skeletal characters, such as the open sutures between centra and arches in the Microvenator vertebrae (Ostrom, 1970a) indicate immaturity in other cases. The most convincing proof of the lack of truly small dinosaur species is their absence in the carefully studied microvertebrate concentrates. In the Late Cretaceous localities at Bug Creek, Montana, and Lance Creek, Wyoming, several dozen species of small vertebrates are found including a wealth of lizards (monitors, skinks, teiids, and alligator lizards) and mammals, and yet nearly all the dinosaur specimens are small fragments of large genera (horned dinosaurs, duck-bills, deinonychids-Estes, 1964; Estes and Berberian, 1970). The author has examined the Yale samples from these localities and has found many