Missing molluscs: Field testing taphonomic loss in the Mesozoic through early large-scale aragonite dissolution

As a field test for Cherns and Wright's previously published hypothesis based on Paleozoic faunas—that the paucity of certain molluscan groups in the fossil record may be largely a taphonomic, not ecologic, effect—a typical Mesozoic molluscan-dominated fauna is compared with its early-silicified (lagerstätten) equivalent from a similar depositional setting. A Lower Jurassic offshore carbonate ramp facies in South Wales comprises alternating thin limestones and organic-rich shales; the former are largely diagenetic and carry marine isotopic signatures. Their typical fauna is mainly well preserved, calcitic and bimineralic (calcite + aragonite), epifaunal and semiinfaunal bivalves. Evidence of aragonitic shells is normally restricted to molds, common in inner ramp settings in high-energy skeletal grainstones, but mostly poorly preserved and restricted to deep-burrowing bivalves in offshore deposits. However, locally, early silicification in offshore facies has preserved much more diverse faunas with abundant, formerly aragonitic, molluscs, including many more infaunal burrowers. It appears that major aragonite dissolution normally distorted both apparent diversity (65% decrease in bivalve diversity) and the trophic structure of the offshore facies, providing aragonite that probably sourced the diagenetic carbonates. We suggest that aragonitic shells were selectively dissolved in the upper sediment column in lower-energy settings, where high organic contents favored microbial decay and acidity; such early dissolution was absent from the higher-energy facies that originally had low organic contents. Taphonomic loss through early skeletal aragonite dissolution was an equally important process in Mesozoic offshore shelf environments, and although still leaving depleted molluscan-dominated faunas, resulted in a massive distortion of diversity.