SUBSTRATE PREFERENCE AND MID-MESOZOIC BRACHIOPOD DECLINE

Articulate brachiopods were dominant members of Paleozoic seafloor communities. However, they are today restricted to various marginal habitats, having been almost entirely supplanted by bivalves on the modern seafloor. Though brachiopods experienced a disastrous decline at the Permian-Triassic boundary, they mounted a recovery which culminated in a brief Jurassic peak in abundance and diversity, followed by their final decline. It is hypothesized that a driving cause of this decline was their increasing restriction to firm carbonate substrates. This restriction was driven by increasing soft bottom disturbance due to the burrowing activity of other organisms. We tested the importance of substrate by comparing brachiopod and bivalve occurrence and abundance in carbonate and siliciclastic rocks. Because slow sedimentation rate often leads to firmer substrates, it was also hypothesized that brachiopods would be associated with proxies for low sedimentation rates such as glauconite. This was tested by examining the association between brachiopod occurrence/abundance in the Paleobiology Database and various low sedimentation rate indicators. Analysis of occurrence data from the Paleobiology Database shows a brachiopod preference for carbonates when compared to bivalves, particularly during the middle Mesozoic. This suggests an important role for substrate preference in the final Mesozoic decline of articulate brachiopods.