Changes in terrestrial vertebrate faunas during the Mesozoic

Abstract

Summary The precise time-range of any terrestrial group is difficult to establish, due to the uncertainties of preservation and discovery and due to the difficulty of ascertaining the relative ages of faunas from different areas. Differences between such faunas may be due to ecological and zoogeographical factors as well as to age differences. Even when a relative time-sequence has been established, it is difficult to correlate this with the standard section for the period; this is particularly true for the Triassic faunas. As seen in the South African record, the climate became progressively hotter and drier from the Carboniferous Dwyka glaciation until the deserts of the Upper Triassic. The earliest Triassic fauna is a swamp fauna which still included elements of most of the more conservative, synapsid reptiles. During the remainder of the Triassic, these synapsids were gradually replaced by the evolving diapsids. The more primitive members of the Diapsida were themselves replaced in the Jurassic by more progressive members of the group. The earliest mammals, those of the Triassic and Jurassic, were probably homoiothermal though not viviparous. None larger than a cat is known until immediately after the disappearance of the dinosaurs. The reasons for this apparent inability of somewhat larger (e.g. pig-sized) mammals to co-exist with the dinosaurs are not obvious.