The end-Triassic bivalve extinction event

Abstract

One of the most important episodes of mass extinction of animal groups in the whole of the Phanerozoic took place in the latter part of the Triassic, but the time of major faunal turnover of the vertebrates preceded that of marine invertebrates by several million years. As shown by ammonites, brachiopods and conodonts, decline from a Carnian—early Norian diversity peak was followed by a significant extinction episode at the end of the period. This pattern is confirmed at generic and subgeneric level by the bivalve molluscs, probably the most abundant and diverse macroinvertebrate group. This paper analyses the bivalves in detail and attempts to seek a relationship of the extinction event to environmental changes as recorded by late Triassic facies. Stratigraphy of the youngest Triassic is briefly discussed. Following a recent recommendation, the Rhaetian stage is dropped and the Upper Norian (Sevatian) subdivided into three new ammonite zones. It is suggested that the classic northwest European “Rhaetic” correlates with the Kössen and Zlambach Beds of the Alps and corresponds at least approximately with the upper two of these zones. It is estimated that nearly half the existing bivalve genera and nearly all the species failed to survive the end-Triassic extinction event. The trigonioid, unionoid and hippuritoid orders were especially strongly affected but most of the pholadomyoid genera survived. Three major ecological categories are distinguished: a shallow neritic group, to which most taxa belong, a euryhaline group that could tolerate marginal marine and lagoonal conditions, and a deep neritic group, notably Halobia and Monotis. All groups were strongly affected by the extinction but species of the deep neritic group were apparently the most susceptible. Seven biogeographic regions are distinguished for the purpose of analysis and it is shown that provinciality is slight, but some important taxa are endemic to the Tethyan and Pacific regions. Endemic genera prove to have been much more subject to extinction than cosmopolitans. There are very few places in the world with a more or less continuous marine sequence of deposits across the Triassic-Jurassic boundary. Details are given of sections in the Alps, northwest Europe and Nevada and briefer mention made of others. With regard to late Triassic palaeogeographic changes, those involving climate were neglibible, but there was a more or less progressive regression of epicontinental seas following a Ladinian—early Carnian transgressive maximum, with a corresponding shallowing in the more continuously marine Tethyan region. That this first-order effect was due to a eustatic lowering of sea level is fairly certain, but the situation is less clear for second-order transgressive-regressive cycles. The late late Norian (“Rhaetian”) cycle is probably, however, the results of a minor eustatic event. By the end of the Triassic the sea had regressed substantially and the mass extinction event is though to be bound up partly with this phenomenon and partly with the phase of widespread anoxicity that occurred during the succeeding Hettangian transgression.