The Non-Actualistic Early Triassic Gastropod Fauna:A Case Study of the Lower Triassic SinbadLimestone Member

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Abstract
Through a detailed regional study of the Lower Triassic (Nammalian) Sinbad Limestone Member of the Moenkopi Formation in southeastern Utah, previously qualitative paleoecological observations on Early Triassic gastropods have been quantified. Paleocommunities of the Lower Triassic Sinbad Limestone Member are dominated by gastropod juveniles and small-size adult gastropods (99% of the gastropods are smaller than 1 cm) that form numerous microgastropod packstones and grainstones that were deposited subtidally. The results from this case study of the Sinbad Limestone Member reflect a global Early Triassic phenomenon of microgastropod-dominated open-shelf environments. These characteristics of the paleoecology of Early Triassic gastropod faunas are non-actualistic since, during Ecological Evolutionary Units (EEUs), larger gastropods and normal-marine communities are invariably common somewhere contemporaneous with the occurrence of microgastropod-dominated faunas. Early Triassic microgastropods behaved as repopulation-interval opportunists during the aftermath of the end-Permian mass extinction, proliferating into vacated ecospace after most groups became extinct or dropped to very low numbers as a result of the end- Permian mass extinction. Small-sized adults and numerous juveniles are characteristics of opportunists and of groups inhabiting physiologically and/or chemically harsh environments. The characteristics of the Early Triassic microgastropod fauna indicate prolonged environmental stresses related to the causes of the end-Paleozoic biotic crisis.