THE ROLE OF SUBSTRATE PREFERENCE IN MID-MESOZOIC BRACHIOPOD DECLINE

Brachiopods were a dominant member of the Paleozoic evolutionary fauna between the early Ordovician and late Paleozoic. However, they are today restricted to various marginal habitats, having been almost entirely supplanted by bivalves in modern marine communities. Though brachiopods sharply declined during the Permian-Triassic event, their final decline occurred following a brief recovery in abundance and diversity during the Jurassic. It is hypothesized that a driving cause of this decline was the increasing restriction of brachiopods to carbonate substrates, as opposed to softer, siliciclastic substrates. Important ecological drivers of this environmental restriction include fouling by burrowing organisms as well as the susceptibility of brachiopod larvae to grazing organisms in the absence of cryptic environments, with increases in burrowing and grazing coinciding with brachiopod decline. Indeed, most living brachiopod groups occur on hard, usually carbonate, sediments and in cryptic environments, with the exception of a few minor groups. In the fossil record, brachiopod occurrence in carbonate rocks vs. siliciclastics, compared to bivalves, can be used as a substrate preference proxy. Analysis of occurrence data from the Paleobiology Database shows a brachiopod preference for carbonates when compared to bivalves, particularly during the middle Mesozoic. This preference, coupled with declining global carbonate deposition, likely played a role in brachiopod decline following the middle Mesozoic.