PALEOECOLOGY OF AN EARLY TRIASSIC (DIENERIAN(?)-SMITHIAN) MARGINAL MARINE SYSTEM: ANALYSIS OF THE LOWER MEMBER OF THE LOWER TRIASSIC UNION WASH FORMATION

The paleoecology of marginal marine environments during the Early Triassic is not well known, and most studies have focused on normal marine settings, usually documenting the unusual occurrences of low diversity opportunistic generalists and disaster forms dominating the open marine realm. Typically, these types of organisms are restricted to harsh marginal marine waters or make up minor components of normal marine faunas, as they are generally out-competed by more specialized taxa. The Lower Member of the Lower Triassic Union Wash Formation is a mixed carbonate-siliciclastic sequence that was deposited in intertidal and lagoonal environments and contains a low diversity fauna comprised almost entirely of echinoids (usually spines or debris), with bivalves occurring infrequently. Echinoids appear to be endemic to the region, and likely inhabited only the intertidal and shallow lagoonal environments of the member, as the deeper portions of the lagoon appear to have been affected by harsh environmental conditions, likely abnormal salinities, based upon paleoenvironmental interpretations. Echinoids may have capitalized upon a vacancy created following the End-Permian mass extinction and appear to act as an opportunistic taxa in the marginal marine environments of the Lower Member, surviving on available detritus and thriving in the absence of other taxa. There is an unusual absence of disaster forms in the Lower Member despite the fact that the depositional environments found there are typical of the settings that opportunistic taxa reside in during periods of normal marine conditions. With reduced competition in open marine environments due to the effects of the End-Permian mass extinction and likely harsh Early Triassic environmental conditions in offshore environments, an opportunity may have existed for disaster forms to leave marginal marine settings and dominate open marine environments.