Triassic/Jurassic bivalve biodiversity dynamics: biotic versus abiotic factors

Based on the global occurrence dataset, the shift in taxonomic and functional diversity of bivalves at the Triassic/Jurassic transition was examined herein. There is a noticeable decline in diversity at many taxonomic levels (generic, family, and order) along the Triassic/Jurassic boundary. Test changes in the functional diversity (e.g., life habits, mobility levels, and feeding mode) revealed that the percentage of mobile exceeded stationary taxa after the end of the Triassic crisis, while no major changes were observed in the life habit or feeding mode. By the Sinemurian, diversity reached the pre-extinction levels. A significant difference was also found between survivors’ longevity and extinct taxa, where the Early Jurassic (Hettangian) fauna have a longer duration relative to those that became extinct. The Triassic/Jurassic boundary is marked by a marked sea-level fall and a decrease in the mean Sea Surface Temperature (SST), which is associated with increasing siliciclastic and decreasing carbonate rocks. The latter may also point to ocean acidification at the Triassic/Jurassic boundary. The geographic range size of bivalves is slightly changed by the end of the Triassic, where the taxa are slightly characterized by narrower ranges. Hence, the geographic range size, the result of ecophysiology, plays a major role in determining the extinction risk. The difference in the magnitude of the diversity loss (i.e., taxonomically vs. functionally) indicated that the shallower marine habitat destruction resulting from the sea-level fall is the primary cause of the Triassic/Jurassic mass extinction.