SUBSTRATE AVAILABILITY IN THE UPPER CRETACEOUS OYSTER EXOGYRA SAY (OSTREIDA: GRYPHAEIDAE): A CASE STUDY IN THE MAASTRICHTIAN OF MISSISSIPPI AND ALABAMA

Oysters typically encrust solid, often hard surfaces on the seafloor, although substrates may vary in their construction and composition. Auto-attachment (oyster-oyster) prevails in intertidal or subtidal oyster reefs near shore, as in estuaries. However, non-reefing oysters of the deep inner and middle shelfs attach to a greater variety of objects. In places on the seafloor, and in fossiliferous sedimentary facies resulting therefrom, accumulations of refractory organic debris suitable to settlement by oyster larvae may contain a diversity of skeletal taxa. Attachment scars in oyster shells resulting from overgrowth of objects in this debris can be potentially revealing about the nature of the substrate materials as well as the organisms that produced it.

Fossil occurrences of marine macroinvertebrate species are typically based on skeletal remains and/or moldic representations thereof. As most mollusk species possess aragonitic shells that do not regularly preserve in toto, particularly in carbonates, they may go undocumented, or, if internal molds are produced before shell decomposition, identified at less useful higher taxonomic levels. Impressions of skeletal taxa in attachment scars, or bioimmurations, may thus reveal otherwise hidden biodiversity, particularly among the benthos.

Five localities in upper Maastrichtian sediments of Mississippi and Alabama were selected for their abundance of reasonably well-preserved Exogyra shells. The sites are somewhat evenly spaced north-south within the outcrop belt of the Owl Creek and Prairie Bluff formations on the eastern side of the Mississippi Embayment. Taxonomic composition was determined among bioimmured organisms identifiable within the attachment scars of left valves from each location. The identified bioimmured taxa were classified into groups with varying degrees of specificity, most ranging from class- to even genus-level identifications, including several non-taxonomic groups, like ‘amorphous pebble’ or ‘unidentifiable.’ Preliminary results suggest that auto-attachment in Exogyra decreases proportionally with distance from the upper part of the embayment and increasing depth. Diversity of bioimmured objects increased proportionally but substrate taxa became increasingly unidentifiable along the same transect.