GASTROPODS FROM THE CRETACEOUS RIPLEY AND PLIOCENE CALOOSAHATCHEE FORMATIONS: SHELL GEOMETRY, ORNAMENTATION, AND PREDATION

Gastropod shells are some of the most isometric structures found in nature. Shells can be modeled using the parameters outlined by Raup (1966, J. Paleo. 40, 1178-1190), which quantify translation rate, whorl expansion rate, and distance from the coiling axis. We measured Raup’s (1966) parameters and used them, together with qualitative coding for ornamentation and predation, to analyze fossil shells from the Coon Creek Tongue of the Ripley Formation (Cretaceous, TN) and the Caloosahatchee Formation (Pliocene, FL), both of which were deposited in the Mid-Gulf Coast Basin. Our findings show similar coiling patterns in both formations. Gastropods use shells as defense against predators such as other gastropods (which leave drill holes) or durophages (which leave repair scars, if the gastropod survives crushing or peeling). Ornamentation—additional material such as spines or ridges on the exterior of the shell—is posited to act as a defense against predators. Specimens from the Caloosahatchee site tend to be more highly ornamented than those from the Coon Creek site, which may be evidence of increased predation pressures. We observed the greatest number of predation traces (either drill holes, repair scars, or both) on moderately ornamented shells, rather than heavily ornamented shells. However, repair frequency can be biased by many factors, including survivorship or preservation. Repair scars occur more frequently than drill holes in both formations, which may indicate that durophages were a more intensive cause of predation than drilling gastropods. Overall, morphological differences between the Pliocene and Cretaceous localities were primarily reflected in degree of ornamentation rather than shell coiling parameters.