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.

The origin and early evolution of dinosaurs

“…Focus has been given to two inferred mass extinction events, at the Carnian-Norian and Triassic-Jurassic boundaries, and two alternative scenarios for the rise of the dinosaurs, the so-called ''competitive'' and ''opportunistic'' models. Studies from the mid-late 20th Century postulated that the replacement of various tetrapod groups, notably pseudosuchians and therapsids, by dinosaurs was a long-term affair driven by competition during the Late Triassic (Cox, 1967;Charig, 1980;Bonaparte, 1982). Its outcome would have been the dominance of dinosaurs over terrestrial ecosystems from Norian/Jurassic onwards, thanks to their ''superiority'' relative to the outcompeted contemporary tetrapods, pushed to extinction.…”

Macroevolutionary patterns in the evolutionary radiation of archosaurs (Tetrapoda: Diapsida)

“…Archosaurs, however, are generally considered a 'successful' radiation of vertebrates for many reasons: they have persisted for hundreds of millions of years; have achieved a global distribution; are and have been exceptionally taxonomically diverse (at least by tetrapod standards); have occupied a range of body types and ecological niches; and are and have been locally abundant in individual faunas and ecosystems. The obvious question is why archosaurs (especially dinosaurs) have been able to achieve such success, and there has been no shortage of hypotheses and speculation (e.g., Cox 1967;Crompton 1968;Bakker 1971;Robinson 1971;Sill 1974;Halstead 1975;Charig 1980Charig , 1984Welles 1986). However, many of these hypotheses are extremely difficult, perhaps even impossible, to test.…”

III: Mesozoic Mammals

“…The Red Beds and overlying Cave Sandstone are generally considered to be Norian and/or Rhaetic (Cox 1973). The division between Cave Sandstone and Red Beds does not correspond to the division between the Norian and Rhaetic, and the age of the transition between these beds appears to differ from locality to locality.…”