A PRELIMINARY TEST OF THE MESOZOIC MARINE REVOLUTION HYPOTHESIS USING REPAIR SCARS ON CRETACEOUS OYSTERS, GULF AND ATLANTIC 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 Coastal Plain (Delaware, New Jersey) and showed that shell repair frequencies (repaired scars from failed crab predation) increased through time, consistent with the MMR hypothesis. However, Mesozoic oysters from the southern part of the United States have not been similarly tested. Two Mesozoic oyster species were examined for predatory shell repair: Early Cretaceous Texigryphaea mucronata from the Buda Limestone (“18-Wheeler site” near Harker Heights, Texas) and Late Cretaceous Exogyra cancellata from the Ripley Formation near Moscow Landing, Alabama. T. mucronata lacks ornamentation, while E. cancellata has thick costae which should deter predatory attacks. Fifty specimens of both species were measured for their shell height, length, and thickness. Major and minor repair scars as well as growth disruptions were recorded. Results indicate that T. mucronata was significantly thinner than E. cancellata (6.73 mm vs. 14.90 mm, respectively) and smaller (40.31 mm vs. 83.21 mm) based on mean measurements. Frequency of major shell repair on E. cancellata (0.80) was almost twice that of T. mucronata (0.48) and was significantly different (X² = 11, d.f. = 1, p<0.05). No differences were found for minor repair or growth disruptions between the species. Both oysters show high frequency of major repairs on their shells, comparable to their Late Cretaceous northern counterparts, suggesting predator activity was high at both time intervals at these sites. What is unusual, is that smooth, thin-shelled T. mucronata had nearly 50% of the shells with multiple major repairs: does this mean that the predators were not well-weaponised at that time? By the Late Cretaceous, E. cancellata had a better record of shell repair likely related to its robust shell with thick costae.