MORPHOLOGICAL RESPONSE TO PREDATION ON FINE TEMPORAL SCALES BY THE DEVONIAN BRACHIOPOD PHOLIDOSTROPHIA

Numerous studies (Signor & Brett, 1984; Alexander, 1990) have documented a pattern at global scales of morphological change in brachiopods during the mid-Paleozoic. Other studies (Leighton, 1999, 2001) have provided evidence that brachiopod ornament could serve an antipredatory function. These results have suggested the hypothesis that morphological change in mid-Paleozoic brachiopods was a response to increased predation pressure. However, no prior study of brachiopods has demonstrated a morphological response to predation at fine spatial and temporal scales. As there is evidence for both an increase in drilling predation intensity and widespread morphological change (increase in ornament) in brachiopods during the Middle Devonian in eastern North America, this time and place is appropriate for a study of the relationship between ornament and predation. The present study from the Silica Shale (Givetian) of Ohio examined predation on the "stropheodontoid" *Pholidostrophia* from multiple shale beds within the formation. The results demonstrate that (1) the genus *Pholidostrophia* had two morphotypes, one lamellose, and one smooth; (2) lamellose individuals had thicker shells than smooth individuals; (3) based on a stereotyped boring site selection for both the pedicle valve and muscle field, *Pholidostrophia* was attacked by a predatory driller; (4) the driller preferred to attack smooth individuals (binomial test, p < 0.03), suggesting that lamellae, or the associated shell thickness, may have inhibited the predator; and (5) the percentage of lamellose individuals varies positively from bed to bed with drilling frequency (r=0.909, p < 0.02). Drilling frequency initially increased and then decreased through time. Maximum drilling frequency against *Pholidostrophia* for any bed was 16.3%. These results corroborate the hypothesis that lamellae in *Pholidostrophia* were a morphological response to changes in predation intensity. Lamellae may have been an adaptation or alternatively may have been an example of a plastic, induced defense.