MYSTERIES OF GIANT PLATYCERAMUS OF THE BOQUILLAS FORMATION SEA FLOOR

Sixty-one giant Platyceramus platinus are concentrated in a patch along one bedding plane in the Boquillas Formation east of Lajitas, TX. Only three are preserved in a condition to permit perimeter measurement, and those are one meter in axial length. All of the specimens are thickly encrusted solely with Pseudoperna congesta, a circumstance that is thought to reflect dysoxic conditions. The prevailing sedimentological model is that Platyceramus thrived under weak bottom currents with slow sedimentation rates on a poorly oxygenated bottom. Foraminifers in the calcareous shale directly beneath the inoceramids are nearly 100% planktonic/0% benthic, which may confirm the presumed bottom conditions. Biotic diversity is characterized as extremely low. A Clark-Evans test revealed random distribution of the Platyceramus. Perhaps the occurrence of this cohort indicates colonization during a brief interval of improved benthic oxygenation, a proposition suggested for an inoceramid cohort in Colorado.

Encrustations of the Platyceramus pose a conundrum in that the exterior surfaces of both valves are almost equally and completely encrusted on all of the specimens that have portions of both valves preserved. In some cases, a second generation of oysters may be present, but divining the possible presence of multiple generations is not a clear call on these specimens. In most previous studies, and with good field evidence, Platyceramus is assumed to have lived in a subhorizontal posture even in the uncommon instances wherein oysters encrust both valves. Such encrustations tend to be highly unequal from valve to valve in both coverage and density, conditions that do not mirror the encrustations observed in this population. Platyceramus has never been found in a vertical or even partially buried position, orientations that might explain the Boquillas specimens. Possible solutions previously proposed to account for relatively equal oyster growth over both valves include being flipped over while alive by a passing mosasaur or ptychodid, getting reoriented by storms, or having been bysally attached to a firm-ground. As yet, there is no clear explanation for these Boquillas encrustations.