THE DISTRIBUTION OF CARCHAROCLES CHUBUTENSIS AND CARCHAROCLES MEGALODON TEETH WITHIN THE CHESPEAKE GROUP, MARYLAND, U.S.A. AND ITS POSSIBLE EVOLUTIONARY IMPLICATIONS

The Miocene Chesapeake Group preserves teeth attributed to two extinct species of macro-predatory sharks; Carcharocles chubutensis and Carcharocles megalodon. Avocational and professional paleontologists who prospect the beaches below the sea bluffs along Chesapeake Bay and tributaries that flow into the Bay know that the stratigraphic distribution of the teeth attributed to these sharks are not equally distributed throughout the formations of which the cliffs are comprised. In situ cusped teeth attributed to C. chubutensis dominate in the lower Calvert Formation (Burdigalian, Lower Miocene), whereas as one ascends the stratigraphic section, these teeth are replaced in the Choptank and St. Marys formations (Serravalian and Tortonian respectively) by reduced - to the uncusped teeth of C. megalodon. The data set does not allow for a description of the precise nature of this evolutionary transition. However, it does show that this shift in tooth morphotype is not simply the result of environmental selection through time in preference of one species over the other. The waning of dental cusps in the Carcharocles lineage occurred in the north Atlantic during deposition of the Plum Point and Calvert Beach Members of the Calvert Formation (both Langhian, Middle Miocene) throughout which time comparable paleo-environments existed. Perhaps the loss of cusps was driven by a functional demand placed on Carcharocles teeth to become more efficient in cutting vs. puncturing; the loss of lateral cusps allowed the tooth to present a longer uninterrupted serrated blade for cutting into the bodies of the ever-increasing size of prey on which C. megalodon was specializing.