Mesozoic Mammalian Evolution

Abstract

The Mesozoic history of the Mammalia is approximately twice as long as the Cenozoic. Because of their small size the remains of early mammals were often overlooked. In the last two decades development of new collecting techniques has greatly augmented their fossil record. Now a large number of fossils document the evolutionary transition from mammallike reptiles to earliest mammals that occurred in the Late Triassic, approximately 190-200 M.Y.B.P. (million years before the present, I-Iowarth 34). During the Jurassic and Cretaceous periods--which had a total duration of over 120 million years--mammals formed a small element of the terrestrial fauna, but major evolutionary changes, including the origin and early differentiation of the Eutheria (Placentalia) and Metatheria (Marsupialia), place. With the extinction of the dinosaurs at the close of the Cretaceous, approximately 64 M.Y.B.P., the mammalian fauna underwent a major radiation and for the first time began to occupy niches available only to large terrestrial vertebrates. THE FOSSIL ~tECOEV ffurassie and Cretaceous mammals were small creatures; the largest yet discovered was no bigger than a terrier. Most were in the size range of modern small rodents and shrews. The minute bones and teeth of these animals frequently have been overlooked by paleontologists in search of skeletons of dinosaurs to dominate museum exhibit halls. Only in the last two decades have collecting techniques adapted for recovery of mammalian and other vertebrate microfossils been developed and consistently employed (McKenna 58). The result has been a multifold increase in the available record, but, as will become apparent, many gaps remain between well-sampled local faunas. The evolutionary history of Triassic terrestrial vertebrate faunas is one of change from dominance of the mammallike reptiles, therapsids, in the Early Triassic to archosaurian dominance at the end of the period. The earliest mammals, descendants of therapsids, have been collected from a series of localities of the Rhaetic (Late Triassic) age. Of these, the largest samples are from deposits formed within fissures and caverns in Carboniferous