EVOLUTION IN THE EARLY TRIASSIC

The Early Triassic is well-known as a time of recovery from the end-Permian mass extinction. The analysis of recovery dynamics in marine environments has focused on a variety of extrinsic and intrinsic influences. Ultimately these factors inhibited or enhanced evolutionary processes that shaped the direction of the recovery. As demonstrated by an absent to very sparse primary fossil record, the overall environmental conditions of the Early Triassic strongly inhibited the evolution of bryozoans and corals during this time. Other major phyla such as sponges, molluscs, echinoderms, brachiopods, chordates and arthropods were inhibited but to a lesser extent. It appears that some of these animals were actually uniquely adapted to combinations of Early Triassic environmental stresses, while others found refuge in regions with oceanographically ameliorated geochemistry. In the Smithian, during the hottest times in the Phanerozoic, gastropods thrived as microgastropods in shallow marine environments, with evolution of a number of new genera and species in the absence of other taxa that could not survive these hot temperatures. Diversity trends of conodonts and ammonoids are similar during the Early Triassic, and their diversifications may have benefited from the development of a mid-water “refuge zone” between deep anoxic waters and lethally hot surface waters. In particular, the occurrence in ammonoids of significantly more species of oxycones in low latitude Smithian tropical seas may be because these cephalopods, which were likely adapted for survival in low oxygen environments, took advantage of an evolutionary opportunity in the form of persistent oxygen minimum zones in the tropics. A nearshore “habitable zone” which had ready access to atmospheric oxygen through wave mixing has been postulated as another environmental refuge during the Early Triassic, and indeed the distribution of echinoid fossil remains indicates these environments are where they preferentially subsisted. Furthermore, although there was no loss of global ecological functional diversity in marine environments after the end-Permian mass extinction, regional and temporal functional diversity was quite variable, also contributing to the complex backdrop upon which evolution played out during this time.