DRILL HOLES IN CENOZOIC BRACHIOPODS FROM AUSTRALIA AND A TEST OF THE “LOW NUTRITIONAL VALUE” HYPOTHESIS

Drill holes are important indicators of biotic interactions and can be used to address ecological and evolutionary questions. Drill holes in bivalves are often attributed to predation; their frequencies have been high since the Mesozoic Marine Revolution. However, reports of drillings in brachiopods are much less common and are primarily documented from the Paleozoic, indicating patchy spatial and temporal distributions. Until recently, few examples of drilling in Cenozoic brachiopods had been reported. However, several recent studies have reported localized high frequencies of drilled Cenozoic brachiopods in Europe, Africa, and the southwestern Pacific. Here we present data from Australia suggesting that on this southern continent Cenozoic brachiopods were also targets of predators.

We examined a number of different Cenozoic brachiopod genera (Tegulorhynchia, Aliquantula, Victorithyris, Austrothyris, Liothyrella, Epacrothyris, Diedrothyris, Stethothyris, Terebratulina, Giraliathyris) from the Carnarvon, Perth, Euclaand Murray Basins. There is an average drilling frequency of 16.1% which is much higher than the average Paleozoic frequency of <1%. The drill holes are cylindrical, and range from 0.1 to 2.0 mm (mean = 0.56 mm) in diameter. We attribute the drill holes to muricid gastropods because of their morphology. However generic identity of the predators is unknown because of the lack of aragonite preservation. Such high drilling frequency of brachiopods seems surprising given that brachiopods are commonly hypothesized to be inadequate prey items for gastropod predators. According to the “low nutritional value” hypothesis, brachiopods do not provide enough energy for their predators in the caloric content of their soft tissue. To evaluate this hypothesis, we used a cost-benefit approach quantifying the trophic interaction between a drilling predatory gastropod and its brachiopod prey by comparing the amount of energy needed to capture and consume the prey relative to the energy provided by the tissue of the eaten brachiopod. Even using conservative assumptions, out results indicate that from a calorific standpoint, brachiopods do provide enough energy for predatory gastropods, as indicated by the extent of drilling. We thus reject the “low nutritional value” hypothesis.