DRILLHOLES IN EXTANT BRACHIOPODS AND THEIR IMPLICATIONS FOR THE HISTORY OF PREDATION

Distinct drillholes found in invertebrate fossils are among the least ambiguous and most ubiquitous sources of information on biotic interactions available in the fossil record. While much attention has focused on the record of drilling in fossil and Recent mollusks, for other taxa the patterns and processes are poorly known. For example, despite extensive research on brachiopods and widespread interest in drilling predation, only recently have data on the Paleozoic record in that taxon been systematically compiled. Unfortunately, a comparable survey of drillings in Recent brachiopods does not exist; here we report on a preliminary attempt to fill this gap.

In our pilot survey, we examined over 3,400 specimens of Recent brachiopods from the dry collections of the Muséum national d’Histoire naturelle (Paris). These specimens represent a broad range of localities, environments, and brachiopod genera collected, largely by dredging, over the last 150 years. The preliminary data reveal that drilled brachiopods are rare compared to values reported for mollusks. However, they are not as infrequent as might be suggested by the dearth of reports on drilling in brachiopods. Over 3% of all specimens had distinct circular drillholes ranging in diameter from 0.2 mm to 1.3 mm (average 0.6 mm). Even more striking is selectivity or patchiness of drillholes: in a few taxa drilled specimens exceed 10% and even 20% of the sample (recorded only for N>20). Also, whereas the number of specimens collected vary inversely with depth, the drilling frequencies show no such pattern. In fact, although noisy, the data show a reverse pattern: drilling frequency increasing with depth. Finally, over 16% of drilled specimens are multiply drilled, having 2 or more drillholes. Data relevant to identifying the drillers consist of drillhole cross-sections and these vary from straight-sided to strongly tapered, suggesting that a range of taxa may be involved.

A comparison of the Recent to the Paleozoic record of brachiopod drillings reveals a surprising degree of congruence, especially in overall frequency and patchiness, implying a similarity in certain dynamics of these ecosystems separated by over 250 my.