Environmental determinants of marine benthic biodiversity dynamics through Triassic-Jurassic time

Ecology is thought to be of crucial importance in determining taxonomic turnover at geological time scales, yet general links between ecology and biodiversity dynamics are still poorly explored in deep time. Here we analyze the relationships between the environmental affinities of Triassic-Jurassic marine benthic genera and their biodiversity dynamics, using a large, taxonomically vetted data set of Triassic-Jurassic taxonomic occurrences. On the basis of binomial probabilities of proportional occurrence counts, we identify environmental affinities of genera for (1) carbonate versus siliciclastic substrates, (2) onshore versus offshore depositional environments, (3) reefs versus level-bottom communities, and (4) tropical versus non-tropical latitudinal zones. Genera with affinities for carbonates, onshore environments, and reefs have higher turnover rates than genera with affinities for siliciclastic, offshore, and level-bottom settings. Differences in faunal turnover are largely due to differences in origination rates. Whereas previous studies have highlighted the direct influence of physical and biological factors in exploring environmental controls on evolutionary rates, our analyses show that the patterns can largely be explained by the partitioning of higher taxa with different evolutionary tempos among environments. The relatively slowly evolving bivalves are concentrated in siliciclastic rocks and in level-bottom communities. Furthermore, separate analyses on bivalves did not produce significant differences in turnover rates between environmental settings. The relationship between biodiversity dynamics and environments in our data set is thus governed by the partitioning of higher taxa within environmental categories and not directly due to greater chances of origination in particular settings. As this partitioning probably has ecological reasons rather than being a simple sampling artifact, we propose an indirect environmental control on evolutionary rates. Affinities for latitudinal zones are not linked to systematically different turnover rates, possibly because of paleoclimatic fluctuations and latitudinal migrations of taxa. However, the strong extinction spike of tropical genera in the Rhaetian calls for an important paleoclimatic component in the end-Triassic mass extinction.