BIOTURBATORS AND THE MID-MESOZOIC DECLINE OF BRACHIOPODS

Brachiopods suffered greatly during the late Permian mass extinction but recovered and were surprisingly common in many Middle Triassic to Middle Jurassic communities. However, by the mid-Cretaceous, brachiopods had largely disappeared from shallow-marine ecosystems. This gradual decline in abundance coincides with an increase in depth and intensity of sediment mixing, suggesting that substrate disruption by bioturbators may have played a role in the marginalization of brachiopods, which today tend to live cryptically or on firm to hard substrates. We explored the links between brachiopod occurrence and substrate character to evaluate whether brachiopods became increasingly restricted to firmer substrates during their mid-Mesozoic decline. Brachiopods preferentially occurred in carbonate lithologies that were likely firmer on average because of early diagenetic cementation. That preference for carbonates, relative to bivalve occurrences, increased during the Mesozoic. At the local community scale, brachiopods were abundant in communities from both carbonate and clastic substrates in the Paleozoic, but were greatly reduced in siliciclastic communities during the Mesozoic. In addition to an increasing restriction of brachiopods to carbonates during the Jurassic and Cretaceous, brachiopods were more likely to occur in ferruginous carbonates, which may indicate early diagenetic cementation or hardground formation. Although substrate consistency varies within lithological categories, the occurrence and abundance trends are consistent with the hypothesis that increasing biological substrate disruption progressively restricted the habitat distribution of brachiopods, ultimately leading to their near-disappearance from many shallow marine communities.