REAPPRISIAL OF BIVALVES AND THE END-TRIASSIC MASS EXTINCTION

The end-Triassic extinction (ETE) is linked to the environmental consequences of global warming and possibly ocean acidification, resulting from degassed CO₂ associated with volcanic activity of the Central Atlantic Magmatic Province (CAMP). The ETE is strongly correlated with facies changes combining both sea-level effects and CAMP-related short-term environmental perturbations. Marine bivalves have been used to argue for and against a sudden high-magnitude extinction event. At long-term (stage and sub-stage) scales, regional and global compilations of bivalve species show significant extinction (> 95%) differing from genus-level compilations (40-45%). Data show selectivity against infaunal, aragonitic and low-latitude shallow-water bivalve species. These compiled data, however, are subjected to equivocal and imprecise age assignments inherent in PBDB and other compilations (the compiled correlation effect) and uncertain taxonomic affiliation (especially of poorly-preserved or rare species). This pattern is especially true amongst Triassic megalodontoid hypercacifying clams which exhibit severely depleted species richness prior to the ETE concomitant to the reduction of Rhaetian carbonate platform systems in Alpine Europe yet demonstrate some increase in maximum geometric size leading up to their ultimate demise at the ETE (post-Triassic descendants of the megalodontidae are questionable). At short-term (<100 ky) scales, bivalve occurrences within the extinction interval, beginning at the initial C isotope excursion (CIE) through the main CIE from localities in Austria and Italy (and to a lesser extent in UK and Nevada where species richness was already low) exhibit abrupt decline in species richness and ecological evenness within high-abundance episodic shell beds coincident with the onset of the initial CIE. Skeleton mineralogical data suggest limited to no selectivity against aragonitic bivalves at risk by ocean acidification. Evidence for corrosion/dissolution of bivalve skeletons is not observed. Small sample sizes and episodic occurrences within this interval make it difficult to parse out other deleterious effects (e.g., temperature, salinity and habitat loss/modification due to sea-level changes).