MORPHOLOGICAL RESPONSE TO PREDATION IN AN UPPER-PENNSYLVANIAN BELLEROPHONTID GASTROPOD

Organisms risk predation on a daily basis; thus predation can be one of the most important agents of natural selection. During the Mid-Paleozoic, several clades experienced morphological change, possibly in response to a major adaptive radiation in durophagous (crushing) predators occurring at that time (Signor and Brett, 1984). One clade thought to have responded morphologically to this adaptive radiation is bellerophontid gastropods (Signor and Brett, 1984), but thus far no fine-scaled studies have directly tested this hypothesis. Here we examine possible evidences of predation on Euphemites, a relatively un-ornamented bellerophontid gastropod, from three Upper-Pennsylvanian shale units in North-Central Texas: East Mountain (oldest), Colony Creek, and Wayland (youngest).

Shell size and thickness were measured, and repair frequency (RF) was calculated from 739 specimens from the bellerophontid genus Euphemites. Shell size increases (one-sample Kolmogorov-Smirnov, p<0.001) from the Colony Creek shale to the Wayland Shale. Increased shell size in prey organisms may lead to fewer successful attacks because predators are functionally constrained by the size of their mouths or attacking appendages. Euphemites found in the Wayland Shale have thicker shells than those found in the East Mountain and Colony Creek Shales (ANOVA, p<0.025). Increased shell thickness in prey organisms may lead to fewer successful attacks by predators because breaking thicker shells requires more force.

Repair frequency increases from 2.4% in the East Mountain Shale to 4.7% in the Colony Creek Shale to 6.4% in the Wayland Shale. Changes in RF may be the best proxy for intensity of durophagous predation but, because RF only considers failed attacks, this proxy is problematic. An increase in RF can be caused by: 1) taphonomic biases, 2) an increase in the number of predators or a predator's change in prey preference (more attacks) or 3) an improvement in prey defenses (more survivors). We interpret this increase in RF to be caused by an increase in the number of survivors of predatory attacks. This interpretation is supported by an increase in shell size and thickness, both of which would tend to inhibit successful predation.