A PRELIMINARY TEST OF THE MESOZOIC MARINE REVOLUTION HYPOTHESIS USING REPAIR SCARS ON CRETACEOUS OYSTERS, SOUTHERN ATLANTIC GULF AND COASTAL PLAIN

The Mesozoic Marine Revolution (MMR) hypothesis attempts to tie the evolution of shell crushing and peeling predators (crabs) to an increase in anti-predatory morphology in marine mollusks. Previous work compared Late Cretaceous (~75 mya) and Early Paleogene (~50 mya) oysters from the northern Atlantic and Gulf Coastal Plain (AGCP: Delaware, New Jersey) and showed that shell repair frequencies (repaired scars from failed crab predation) increased through time, consistent with the MMR hypothesis. However, oysters from the southern part of the AGCP have not been similarly tested, and it is quite possible that these warmer water oysters may also experience escalation. We examined three Mesozoic oysters: Texigryphaea (n=35) from the Early Cretaceous Buda Limestone Fm, Austin, Texas, as well as Exogyra (n=21) and Pycnodonte (n=6) from the Late Cretaceous Ripley Fm near Moscow Landing, Alabama. The thick-shelled Pycnodonte and the smaller, thin-shelled Texigryphaea both lack ornamentation, but the larger and thicker Exogyra has costae. We hypothesize that there will be no difference among the three oysters in amount of shell repair. Shell height was measured, and shell repair was recorded as at least one major repair scar per oyster. Results indicate that Texigryphaea was significantly smaller in size than both Exogyra and Pycnodonte. Texigryphaea had 20% major repair compared to 66% in Pycnodonte and 80% in Exogyra. These shell repair results suggest for Texigryphaea that either there were few durophagous predators at that time in Texas, or that predation was nearly 100% effective on their small, relatively thin shells. By the Late Cretaceous, the more robust oysters (Pycnodonte, Exogyra) sustained more shell repair suggesting an escalation in shell body armour.