Faunal diversity, heterogeneity and body size in the Early Triassic: testing post-extinction paradigms in the Virgin Limestone of Utah, USA*

Abstract

The recovery intervals after mass extinction events are portrayed as times of ecological stress, with effects including reduced body size (Lilliput Effect), reduced taxonomic diversity and lowered ecological complexity. As the end-Permian mass extinction is the largest of the ‘Big Five’ events, with a prolonged recovery interval postulated during the Early Triassic, we expect these effects to be most pronounced in Early Triassic faunas. We have used new data and analytical techniques on the fauna of the Spathian Virgin Limestone to test whether the Lilliput Effect can be observed, whether the fauna is vertically and laterally homogeneous, and whether taxonomic diversity is significantly lower than that of comparable nearshore marine deposits transgressive over redbed sequences from other deposits not regarded as falling into post-extinction recovery intervals. The Lilliput Effect can be observed in some cases, but no uniform trend in body size reduction emerged. When the distributions of the body size across a range of marine invertebrate taxa was compared between the Virgin Limestone and the latest Triassic Penarth Group of Britain there was no support for a significant reduction in body size among the Virgin Limestone fauna. Evidence for faunal homogeneity was also limited. The alpha diversity of individual sites in the Middle Triassic Muschelkalk of central Europe was generally higher than Virgin Limestone sites, after accounting for differences in sample size using rarefaction, but there was some overlap. Although the beta diversity of the Muschelkalk is also apparently higher, a number of biases may be elevating Muschelkalk diversity. Future comparisons between recovery and ‘normal’ faunas need to ensure that as many confounding factors as possible are controlled for and that multiple samples are tested.