EFFECT OF SELECTIVE PREDATION AND SIZE REFUGIA ON STROPHOMENATES IN THE FOSSIL RECORD

Modern populations typically have a great abundance of juveniles relative to adults and one would expect the fossil record to present a similar pattern. Instead, the fossil record of strophomenate (concavo-convex) brachiopods has numerous large (i.e. adult) individuals, and few juveniles. Three hypotheses have been proposed to explain this phenomenon: 1) differential preservation, 2) rapid growth early in life decreases their probability of preservation while small or 3) juveniles have a higher rate of destructive mortality (e.g. predation) and thus are less likely to be preserved. This is intriguing because strophomenates have a higher repair frequency than most Paleozoic clades.

We tested this third hypothesis by examining over 200 well preserved, articulated specimens of multiple strophomenate genera from the Ordovician (Indiana), Devonian (Ohio) and Pennsylvanian (Texas). Each individual's anterior-posterior length was measured. For scarred individuals, the length from scar to hinge line (Scar-Hinge length) was recorded. This Scar-Hinge length is a proxy for the minimum possible size of the prey at the time of attack. For each genus, the specimen size range was significantly different from the range of size at the time of attack (t test, p<0.05). Little to no overlap exists between these two ranges, e.g. *Strophomena* lengths range from 15.5-28.6 mm, while their Scar-Hinge lengths range from 4.1-12.0 mm. Although the ranges of body size and Scar-Hinge lengths vary between genera, this pattern also applies to the other taxa as well.

This pattern suggests that Paleozoic predators, similar to modern predators, may have been size-selective in their choice of prey, preferring small individuals. If so, a size refuge might have existed. Once an individual reached a certain size, its likelihood of being attacked decreased significantly. Conversely, smaller/younger individuals are more likely to be attacked and destroyed and therefore less likely to be preserved in the fossil record. Two important implications can be drawn from these results: 1) The lack of smaller individuals in the strophomenate populations is, at least in part, due to greater predation pressure on this segment of the population, and 2) strophomenate brachiopods in the fossil record may represent only specimens that have grown into a size refuge.