The ecology of MesozoicGryphaea, Exogyra, andIlymatogyra(Bivalvia: Mollusca) in a modern ocean

Abstract

Physical models of six species of the Mesozoic bivalve generaGryphaea, Exogyra, andIlymatogyraand hollow hemispheres of comparable size were manufactured, the former from molds of fossil specimens. The survival (maintenance of a position within 1 cm of the sediment-water interface) of populations of these models on 3 sediment types in the lower intertidal of the Gulf of Mexico was monitored over an 18 mo period. Bulk sediment density had no detectable influence on the mean mass density (g cm−3) of the population of models which maintained position at the sediment-water interface. Increased intensity of sediment reworking strongly decreased the probability of survival of the models. Model shells took up orientations consistent with previous laboratory studies and observations of fossils preserved in situ. Some of the models were attacked and broken open by blue crabs and stone crabs, and crab predation was an important source of “mortality” of the models. These results are consistent with the hypothesis that increased sediment reworking and the evolution of durophagous predators in the Cretaceous caused the extinction of exogyrid and gryphaeid bivalves and the near-total elimination of the reclining mode of life in post-Cretaceous times.