ELEVATED DRILLING INTENSITY IN EARLY PENNSYLVANIAN BRACHIOPODS FROM THE BIRD SPRING FORMATION OF SOUTHERN NEVADA

Drill holes on shelled invertebrates are important indicators of predator-prey interaction in the fossil record and offer a small glimpse into overall community dynamics. Drill holes are commonly present on post-Paleozoic mollusks where research shows drilling frequencies regularly exceed 20% of sampled specimens during the Cenozoic. However, growing data from the literature of predatory/parasitism traces on Paleozoic brachiopods indicate drilling frequencies are far lower than that seen in Mesozoic and Cenozoic mollusks, with less than 1% of specimens typically drilled. Although brachiopod drill hole data are well documented for the Paleozoic, it is far from complete. Here we report drilling frequencies of Early Pennsylvanian brachiopods from the Bird Spring Formation at Arrow Canyon, Nevada. A total of 1,117 specimens were examined from 5 horizons beginning approximately 6m above the Mississippian-Pennsylvanian global boundary stratotype section and point (GSSP). Drilled specimens were observed in all but the uppermost sampled horizon where the sample size is significantly smaller (n=56). Of the 1,117 total specimens, 43 contain drill holes equating to a drilling frequency of 3.8%. However, within individual horizons drilling frequencies are much higher, with a peak drilling frequency of 11% from the second sampled horizon, ~15m above the Mississippian-Pennsylvanian boundary. Drill holes are circular, perpendicular to the shell, relatively small (0.38-1.24mm), occur almost exclusively on ventral valves (~98% of drilled specimens), are found on shells with a length range of 4.08-24.6mm and occur on at least four different genera. The athyrid, Composita, comprises 81% (n=35) of all drilled specimens and exhibits a peak drilling frequency of 18% within the second horizon above the Mississippian-Pennsylvanian boundary. The more frequent predation of the smooth-shelled, Composita is consistent with previous observations that taxa with weak or no ornamentation are at a higher risk of being drilled, particularly in the tropics. Furthermore, most drill holes occur on the two most abundant taxa sampled, suggesting predation may have played a role in the increase in evenness observed during the Early Pennsylvanian at Arrow Canyon.