DISSECTING THE "LITHIOTIS" FACIES: IMPLICATIONS FOR THE EARLY JURASSIC "REEF ECLIPSE

Following the large Late Triassic coral-constructed reefs and the aftermath of the Triassic-Jurassic mass extinction, Early Jurassic reefs are exceptional because they are both rare and constructed primarily by bivalves. "Lithiotis" facies bivalves include: Lithiotis problematica, Cochlearites loppianus, Gervilleioperna sp., Mytiloperna sp. and Lithioperna scutata. These large aberrant bivalves are ubiquitous in shallow, nearshore tropical waters and restricted to the Early Jurassic recovery interval. Early Jurassic "Lithiotis" facies ecosystems were traditionally assumed to be estuarine because of morphological similarity of these bivalves to modern oysters and presumed low diversity assemblages. Sites in the western U.S., Italy and Morocco were examined for faunal composition, geometric extent and sedimentologic context.

Field and thin section observations indicate a strong zonation of "Lithiotis" facies bivalves in shallow nearshore environments. Gervilleioperna and Mytiloperna are restricted to intertidal or tidal flat facies. Lithioperna is found throughout the lagoonal subtidal facies and even in some low-oxygen environments. Lithiotis and Cochlearites are found in subtidal facies, constructing bioherms. The largest bioherms attain lengths over 60 m and thicknesses of 3-5 m.

While some of the "Lithiotis" facies bivalves may have been adapted to estuarine environments, Lithiotis and Cochlearites were more likely adapted to well-oxygenated, open marine environments. New work on Lithiotis interbioherm areas in Oregon reveals a diverse assemblage of stenohaline taxa including regular echinoids, spiriferid brachiopods and scleractinian corals. Lithiotis and Cochlearites, in contrast to many oysters, do not orient their commissure planes in a single direction. Instead, these two bioherm-constructing genera radiate out from a central "bouquet", maximizing exposure to light rather than current. Due to their growth habit, extensive calcification, presumed oligotrophic environment and pseudocoloniality, these two genera may have harbored photosymbionts. We propose that the severe environmental changes and associated unusual seawater chemistry of the protracted Early Jurassic recovery led to suppressed coral reef growth and the rapid radiation of reef-building bivalves.