Extinction-Recovery Pattern of Level-Bottom Faunas Across the Triassic-Jurassic Boundary in Tibet: Implications for Potential Killing Mechanisms

Abstract

Scarcity of fossiliferous boundary sections makes the Triassic-Jurassic biotic crisis the most enigmatic of the five major Phanerozoic faunal turnovers. We report a bivalve-dominated level-bottom fauna from the Triassic-Jurassic transition in southern Tibet, which is unique for two reasons: (1) it documents the faunal turnover across the system boundary without major facies changes, and (2) it provides paleobiological data from the immediate postextinction interval. In the extinction pattern, selectivity against burrowing suspension feeders and taxa with completely aragonitic shells emerges, but no major clades or ecological groups disappeared. The earliest postextinction fauna differs in three significant ways from typical survival faunas of other mass extinction events: (1) it was surprisingly diverse, (2) it was heterogeneous (Simpson D = 0.1078) without dominance of disaster taxa or opportunists, and (3) there was a prevalence of highly specialized, morphologically complex forms reaching normal growth sizes. In spite of these unusual features, extinction of typical Triassic taxa occurred comparatively sharply within the underlying beds. These findings support scenarios of relatively short environmental disturbances triggered by the volcanic activity of the Central Atlantic Magmatic Province, which led to rapid extinction of taxa but did not impose ongoing environmental stress on the survival fauna. The nearly instantaneous recovery of level-bottom faunas is in sharp contrast to a prolonged reef eclipse, which probably indicates both higher extinction rates of reef-organisms and intrinsic limitations to the tempo of recovery due to their high level of co-evolution.