Paleobiology of the oldest known articulate crinoid

The evolutionary and ecological recovery of benthic marine invertebrate faunas from the devastating Permian-Triassic mass extinction is poorly understood. In particular. Paleozoic crinoids were decimated, creating an evolutionary bottleneck, so that Early Triassic (Scythian) and Anisian representatives of the genus Holocrinus may be considered the stem group for the monophyletic crinoid subclass Articulata, which includes all post-Paleozoic crinoids. Crinoid ossicles in limestones of the Scythian (Spathian) Virgin Limestone Member (Moenkopi Formation) of southern Nevada and southwestern Utah, identified as Holocrinus? smithi, respresent the oldest known Holocrinus. Limestone units of the virgin were deposited in nearshore and inner shelf environments of an arid epeiric seaway. Although these crinoids are generally completely disarticulated and occur commonly in storm-generated deposits. they are interpreted to have been deposited near their living sites in ‘disturbed neighborhood’ assemblages. Counts of ossicles in bulk samples indicate a relatively low number of individuals, suggesting that populations of H.? smithi occurred in scattered clumps or as isolated individuals. attached to hardgrounds or shell beds. Bivalves are the most common associates, and are commonly more numerous than the estimated number of crinoid individuals, although bivalve species richness is never greater than three. Virgin Limestone fossil assemblages probably represent the first redevelopment of relatively complex tiered epifiaunal communities in the Mesozoic. *Echinodermata, mass extinction, Early Triassic, paleoecology, Crinoida, Articulata.