The Beginning of the Mesozoic: Paleoecological and Paleo- environmental Analysis of the Lower Triassac Virgin Limestones (Nevada and Utah)

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The mass extinction at the end of the Permian has been established as the greatest perturbation which the Earth's biosphere has experienced since the origin of metazoan life. Although relatively little is known about the cause of this mass extinction, even less is known on biotic conditions in its aftermath or the nature of biotic recovery from this event. Fossil faunas and limestones in the Lower Triassic Virgin Member of the Moenkopi Formation (southern Nevada and southwestern Utah), deposited 5 my after the end of the Permian, present an opportunity to learn how benthic faunas in shallow marine carbonate environments were faring in this recovery interval. Preliminary study of the Virgin limestones indicates deposition in a normal marine environment with benthic macroinvertebrate assemblages of relatively low species richness. Of potentially great significance are intervals of apparent normal marine stromatolites, which may represent temporary expansion from marginal marine refugia into normal marine settings due to restriction of grazers, caused by the mass extinction. Evidence for the beginning of recovery is demonstrated through the presence of abundant specimens of the earliest articulate crinoid, which had evolved from some unknown Paleozoic inadunate that survived the crisis. Research will illuminate the way in which biotas recover from mass extinctions.