Bivalves from the Olenekian (Early Triassic) of south-western Utah: systematics and evolutionary significance

Abstract

The recovery from the end-Permian mass extinction event was a key interval in the history of life, but few modern studies provide systematic data on benthic marine faunas from the epoch immediately following the crisis. Here, the bivalve fauna from the early Spathian (Olenekian, late Early Triassic) Virgin Limestone Member of the Moenkopi Formation is comprehensively documented for the first time. The new genus Sementiconcha (Myophoricardiidae), type species Sementiconcha recuperator sp. nov., and the new species Leptochondria nuetzeli, Eumorphotis ericius, E. virginensis and Pleuromya prima, are described. Leptochondriidae is placed in synonymy with Asoellidae, which is revised. With 27 species belonging to 18 genera, the Virgin Limestone Member records the highest bivalve diversity reported so far from this time interval, questioning previous claims that the recovery from the end-Permian mass extinction was delayed until the Middle Triassic. The two bivalve subclasses (Pteriomorphia and Heteroconchia) that are present in the Virgin Limestone Member clearly differ in their evolutionary contexts. Pteriomorphs of the Virgin Limestone are nearly exclusively composed of genera that survived the end-Permian mass extinction event, whereas heteroconchs are highly dominated by genera that evolved in the Early Triassic. This contrasting evolutionary background probably reflects differential effects of the end-Permian mass extinction event and subsequent crises on these two subclasses, possibly related to differences in filter feeding efficiency and shell mineralogy. The high proportion of infaunal heteroconchs, including deep-infaunal Pholodomyoida, is an additional indicator of a relatively advanced recovery stage, further corroborating that recovery of benthic organisms was well underway during the late Early Triassic. http://zoobank.org/urn:lsid:zoobank.org:pub:EC9802EA-F1D5-4066-A608-2B8DECE7C10D