CRETACEOUS OYSTERS ON THE HALF-SHELL: A RECORD OF DUROPHAGOUS PREDATION PRESERVED IN PYCNODONTE CONVEXA

One of the important aspects of the ecological and evolutionary history of marine ecosystems is the fossil record of predation. Until recently, most published studies of marine invertebrates have focused on drilling predation. However, durophagous predators have also been important consumers of marine invertebrates through the Mesozoic Marine Revolution to the present. Durophagous predation can be difficult to quantitatively interpret because successful predation often results in the complete destruction of the shell and only unsuccessful predation attempts are recorded in the shell as healed damage scars. Specimens of the gryphaeid oyster Pycnodonte convexa from the Navesink Fm. of New Jersey contain multiple durophagous predation scars acquired during the life of each individual. Analyses of the frequency and timing of predation attempts throughout ontogeny in P. convexa reveal the following: 1) Individual oysters were preyed on repeatedly throughout their entire growth and many individuals survived multiple predation attempts. There does not appear to have been a size refuge from predation for this species. 2) Shell thickness is uncorrelated with predation intensity. Individual oysters did not thicken their shells in response to being preyed on repeatedly or in response to being preyed on early in ontogeny. 3) Predation scars are rare within the first 2 cm of shell length. This could indicate selection on the part of the predator for larger prey, but more likely results from high juvenile mortality due to successful predation on, and destruction of, small individuals. High juvenile mortality (Deevey’s type 3 survivorship curve) has been attributed to durophagous predation in a variety of modern pelecypod species, including oysters. It should be noted that Richard Bambach co-authored one of the pioneering papers exploring the use of survivorship curves for making autecological interpretations of extinct species (Levinton, J.S. and Bambach, R.K., 1970, Some ecological aspects of bivalve mortality patterns. American Journal of Science, 268:97-112.)