Morphological response of prey to drilling predation in the Middle Devonian

Abstract

Numerous studies have documented a pattern at global scales of an increase in potentially anti-predatory ornament in brachiopods during the mid-Paleozoic. Other studies have provided evidence that brachiopod ornament could serve an anti-predatory 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 the species level over short temporal scales. The present study from the Silica Shale (Givetian) of Ohio, USA demonstrates that (1) the genus Pholidostrophia had two morphotypes, one lamellose, and one smooth; (2) lamellose individuals had thicker shells than smooth individuals; (3) Pholidostrophia was attacked by a predatory driller; (4) the driller preferred to attack smooth individuals; and (5) the percentage of lamellose individuals varies positively from bed to bed with drilling frequency ( r=0.909). 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.