A COMPARISON OF BRACHIOPOD ATTACHMENT STRATEGIES THROUGH THE PALEOZOIC

Understanding the relative influence of ecological factors provides insight into changes in diversity through time. Variation in brachiopod diversity and life modes may provide information about large-scale environmental and ecological shifts during the Paleozoic. Relationship to substrate (attachment strategy) is of fundamental importance to sessile organisms, and brachiopods are no exception. Studying the mechanisms that underlie trends in brachiopod attachment strategy is important for understanding brachiopod evolutionary selection, and ultimately, the controls on marine biodiversity. Here, we explore the trends in brachiopod diversity through the Paleozoic within four categories of substrate attachment strategy: pedicle-attached, cemented-attached, spine-bearing, and free-lying. We then compare diversity trends of each category to trends in ecological and environmental variables.

The global generic richness of each of the four brachiopod categories for each geological stage in the Paleozoic was calculated from the Treatise on Invertebrate Paleontology. Major patterns include increases in pedicle and free-lying forms at the end-Cambrian and through the Ordovician, and a sharper decrease in free-lying than pedicle brachiopods at the beginning of the Silurian. In the Early Devonian, pedicle brachiopods reach the peak richness of any category during the Paleozoic. In the Mississippian (Visean), the pedicle brachiopods decrease, while all three other groups increase in richness. Although the richness of the free-lying forms remains low, the trends of all four groups track one another through the Pennsylvanian and Permian.

We suggest that a Late Paleozoic increase in predation intensity, and competition for space with burrowers, were causes for the post-Devonian change from free-lying and pedicle-attached genera to a mix of pedicle-attached, cemented, and spinose genera. Attached brachiopods may have resisted predation if a predator was unable to pull them from the substrate. Spines could provide stability in soft and increasingly-bioturbated substrates, and may have also protected brachiopods by deterring predators or by increasing the likelihood of survival from an attack.