A CENSUS OF MIOCENE SHARK TEETH FROM CALVERT CLIFFS

Shark teeth are the most common vertebrate fossils found along Calvert Cliffs in southern Maryland. 22,560 beach-collected teeth were identified to genus to determine the relative proportions of Miocene (Chesapeake Group) shark teeth present along this section of the western shore of the Chesapeake Bay. A total of 14 genera were represented. The main constituents are: Carcharhinus (45.44%), Galeocerdo (21.05%), Hemipristis (18.21%), Isurus (6.9%), and Carcharias (4.05%). The dominance of Carcharhinus can be partly explained by the presence of ?five species in this genus compared to some other genera, such as Hemipristis, that have only one species present in these deposits. Galeocerdo is represented by two species, Galeocerdo contortus (11.65%) and Galeocerdo aduncus (9.40%). The complete dominance of carcharhiniform sharks over lamniform or other shark groups in the Salisbury Embayment (a vast inland arm of the Miocene Atlantic Ocean), is still evident in modern near-shore marine environments.

The teeth used in this study consisted primarily of private collections given to the Calvert Marine Museum. Certain biases undoubtedly exist, such as over representation of common species and under representation of rare, very small-, and large-toothed species. However, the number of rare, or other teeth removed was probably small in comparison to the total number of teeth donated, thus the relative proportions of common species are likely a good reflection of actual tooth abundance. In as much as different kinds of sharks shed teeth at different rates, these results do not necessarily reflect the relative number of individual sharks present.

The distribution of teeth was not uniform along Calvert Cliffs. There was a significant increase in tooth abundance to the north. Tooth abundance also varied taxonomically with Carcharhinus and Carcharias increasing to the north and Carcharodon decreasing to the north. We propose that these results reflect paleoenvironmental differences in the formations from which the teeth were derived. Multiple transgression-regression cycles characterize the Calvert, Choptank, and St. Mary’s Formations along Calvert Cliffs. This supports the suggestion that a variety of environments sustained different suites of sharks at different times.