PALEOECOLOGICAL STUDY OF SCLEROBIONT BORINGS AND ENCRUSTATION OF FOSSIL HYOTISSA HYOTIS OYSTER SHELLS FROM THE CALIFORNIA MIOCENE

Trace fossils represent a powerful tool for understanding ecological relationships and, in the case of soft-bodied organisms, provide a unique opportunity to study tracemakers that may not otherwise be preserved in the fossil record. In this study, we report evidence of sclerobionts on Hyotissa hyotis oysters from the Latrania Formation in the Southeast Coyote Mountains near Ocotillo, California. The upper Miocene Latrania Formation represents an ancient shoreline of the Proto-Gulf of California. We examined 82 target-collected specimens of the large, free-living oyster H. hyotis from this locality. The specimens included in this study were disarticulated oyster valves with the hinges intact. The abundance of bioerosion and trace fossils on the H. hyotis suggests that these oysters provided a hard, biogenic substrate for a diverse community of encrusting and boring sclerobionts. Our goal was to gain insight into the paleoecology of this community.

We analyzed occurrences of borings from clionid sponges, spionid polychaete worms, and pholadid bivalves. None of the borings completely penetrated the shells; thus, bioerosion of the internal surface of the shell is an indicator of postmortem encrustation. Of the collected oysters, 100% showed clionid sponge bioerosion (Entobia isp .) on the external surface of the shell and 95% of these oysters showed sponge borings on the internal surface. Spionid burrows were found on 68% of the oysters, occurring on external and internal surfaces. Although it is possible that some sponge and worm borings may have occurred during the life of the oyster, these borings also occurred postmortem on shells lying on the seafloor. Rare incidences of serpulid polychaete worm tubes (2.4%) and spirorbid worm tubes (1.2%) were also identified. These worm tubes were found on the exterior and interior surfaces of the shells.

Pholadid borings (Gastrochaenolites isp.) occurred on 73% of shells; however, these borings only originated on the external surface of the oysters. Additionally, clionid sponge borings inside of the pholadid borings were rare. A likely explanation is that pholadids only inhabited living oysters. Another possible, although less likely, explanation is that pholadids selectively inhabited oysters arranged on the seafloor in a convex upward position.