The timing and cause of Late Triassic marine invertebrate extinctions: evidence from scallops and crinoids

摘要: According to Sepkoski (1984) and Benton (1986), some 23% of both marine and non-marine animal families became extinct in the late Triassic, making this an interval of mass extinction comparable in importance to the late Cretaceous. Sponges, gastropods, bivalves, cephalopods, brachiopods, insects and a variety of vertebrate groups all suffered heavy losses of families. Among those groups disappearing completely were strophomenid brachiopods, conodonts, conulariids, placodonts, nothosaurs and rhynchosaurs. It has commonly been regarded (see, for example, Hallam, 1981; Raup and Sepkoski, 1984; 1986) that extinctions were concentrated at or near the end of the Triassic, in the Norian stage. However, Hoffman (1985) considered that a significant amount of extinction also occurred in the preceding Carnian stage and Benton (1986) has shown that the single late Triassic extinction peak recognized by Raup and Sepkoski (1984; 1986) is divisible into at least two, with one or more Carnian extinction peaks in addition to a peak in the Norian. The idea of multiphase late Triassic extinctions has been criticized by Newton et al.(1987), principally on the basis of information from a study of bivalves. However, their evidence is not entirely conclusive (see below) and their arguments cannot necessarily be applied to other organisms. It is the main object of the present study to assess the evidence for multiphase extinctions, principally through a detailed analysis of patterns of extinction shown by late Triassic scallops and crinoids. Although these groups represent but a small subset of the total fauna, we believe that any patterns common to the two …