A REASSESSMENT OF DUROPHAGOUS PREDATION ON BRACHIOPODS THROUGH THE PALEOZOIC

The frequency of repaired predatory damage is a common means of assessing durophagous (shell-crushing) predation in the fossil record. Meta-analyses have used repair frequency on brachiopods to assess trends in predation through time. However, much of the data for these meta-analyses is from a single source, whose intention was not to evaluate predation through time. We re-evaluate Ordovician through Permian repair frequency using a single protocol and both field and museum data, including more than twenty species-locality-unit combinations. To avoid geographic and taxonomic biases, data collection was constrained to localities from a single, broad paleotropical geographic region (eastern U.S.) and to a single clade, the subphylum Strophomenata. Tropical settings are expected to have the greatest predation rates, and strophomenates are known to have been frequent targets of Paleozoic durophages.

The percent of scarred individuals, and the per capita scar frequency, follow similar U-shaped trends: high values (usually > 40%) before the Devonian, low values (usually < 35%) during the Devonian, and a return to higher values post-Devonian. As repairs indicate failed attacks, changes in repairs through time can be either due to predator success (with evolutionary implications) or to attack rate (can be ecological without invoking evolution). Based on changes in the size of the prey at the time of attack (not to be confused with the size at death, which might have happened much later, subsequent to additional growth), we interpret the decline in repairs during the Devonian to be related to the Paleozoic Revolution, when new and stronger durophages, including vertebrates, diversified. As durophages grew stronger, they were able to take larger prey, success increased, and thus repair frequency declined. The largely post-Devonian adaptive radiation of the order Productida, strophomenates with spines, and the Orthotetidina, cementing strophomenates, may have been evolutionary responses to the rise of stronger durophages. Spines and cementation have been demonstrated to inhibit predation, and may have forced predators to take smaller prey. Overall success may have declined, resulting in greater repair frequencies. The temporal pattern of Paleozoic repairs is consistent with hypothesized evolutionary trends.