EVIDENCE FOR PREDATORY DRILLING IN LATE PALEOZOIC BRACHIOPODS AND MOLLUSKS FROM WEST TEXAS

Boreholes have been described from every period of the Paleozoic. Although many of these borings are non-predatory in nature, good evidence exists for the parasitic and/or predatory origin of mid-Paleozoic (especially in the Devonian) borings in pelmatozoans and brachiopods. Collections of silicified fossils from Late Paleozoic deposits of West Texas made by G.A. Cooper and housed in the Smithsonian Institution provide an ideal opportunity to look for evidence of predatory drilling during this interval. Examination of these collections revealed a number of boreholes in brachiopods and bivalve mollusks. These drill holes are perpendicular to the shell, smooth sided and, in many cases, beveled in a manner similar to drill holes produced by naticid gastropods today and consistent with predatory/parasitic origin. Of 3140 brachiopod specimens examined, 58 showed unquestionable drill holes (drilling frequency of 1.8%) and another 15 displayed drill holes of questionable origin (drilling frequency of 2.3% if included). Of 654 bivalve mollusk specimens examined, 23 showed unquestionable drill holes (drilling frequency of 3.5%) and another 5 displayed drill holes of questionable origin (drilling frequency of 4.3% if included). These intensities are significantly different (Fisher's exact test p=0.02) and demonstrate that bivalves were more intensely drilled than brachiopods. The samples that yielded drilled specimens range in age from the Late Pennsylvanian through the Late Permian and come from a variety of marine environments. These results demonstrate that drilling predators/parasites were present in various marine habitats throughout the Late Paleozoic. However, drilling intensity in the Late Paleozoic brachiopods and bivalves (less than 5%) appears to have been much lower than in the Late Mesozoic and Cenozoic when it typically exceeds 20% (e.g. Kelley and Hansen, 1993). The results suggest that the intensity of drilling predation/parasitism in marine benthic ecosystems remained low throughout the Paleozoic and supports the idea that drilling intensity did not increase until some time in the Mesozoic.