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1 Department of Earth Sciences, University of Southern California, Los Angeles, CA, 90089-0740, N.Fraser{at}nhm.ac.uk
Following extensive Late Triassic coral-constructed reefs and the aftermath of the Triassic–Jurassic mass extinction, Early Jurassic buildups are rare and constructed primarily by bivalves. The Pliensbachian exhibits a radiation of aberrant pterioid bivalves, the "Lithiotis" bivalves, which include: Lithiotis problematica, Cochlearites loppianus, Gervilleioperna sp., Mytiloperna sp., and Lithioperna scutata. These large bivalves are ubiquitous in shallow, nearshore tropical waters and restricted to the Early Jurassic recovery interval. Field and thin-section observations indicate a strong zonation of "Lithiotis" bivalves in shallow nearshore environments. Gervilleioperna and Mytiloperna are restricted to tidal-flat and inner-platform facies. Lithioperna scutata is found throughout the lagoonal subtidal facies and even in some low-oxygen environments. Lithiotis and Cochlearites construct buildups in subtidal facies, and both taxa exhibit discordant valves—one valve is converted into a pedestal, the other is an opercular valve. The largest buildups attain lengths over 60 m and thicknesses of 3–5 m. It is proposed that the reef-building bivalves Lithiotis problematica and Cochlearites loppianus filled the relatively empty ecological niche of reef-building during the Early Jurassic (Lias), only to be replaced by their predecessors, the scleractinian corals, by the Middle Jurassic. The other "Lithiotis" bivalves (Lithioperna scutata, Mytiloperna, and Gervilleioperna sp.) lived in restricted lagoonal environments with possible fluctuating salinity on Early Jurassic (Liassic) carbonate shelves. Lithiotis and Cochlearites, in contrast to many oysters, do not orient their commissure planes in a single direction. Instead, these two buildup-constructing genera radiate out from a central bouquet, possibly 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. Severe environmental changes and associated unusual seawater chemistry of the protracted Early Jurassic recovery may have led to suppressed coral-reef growth and the rapid radiation of reef-building bivalves.
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