USING LATITUDINAL DIFFERENCES IN GROWTH INCREMENT FORMATION IN MERCENARIA MERCENARIA TO LOCATE PALEOBIOGEOGRAPHIC BOUNDARIES IN THE WESTERN NORTH ATLANTIC

Previous studies identified an extinction event in the western Atlantic during the late Pliocene to early Pleistocene. A latitudinal difference in the recovery of molluscan diversity followed this extinction event in Florida and Virginia (Allmon et al., 1996). Today, a latitudinal change in fauna occurs at Cape Hatteras, North Carolina. Determining the past location of this provincial boundary, which we assume was farther north during the Pliocene, is necessary to ensure the validity of latitudinal shifts in biogeographic boundaries based on comparisons of pre- and post-extinction faunas in Virginia and the Carolinas.

A latitudinal difference in the formation of seasonal growth increments in Mercenaria mercenaria shells has been documented in previous studies. Water temperature is likely the primary factor accounting for this pattern. We hypothesized that the latitudinal pattern of summer versus winter growth cessation could be used to reconstruct provincial boundaries in the fossil record. First, however, we tested whether such a method faithfully captures the known biogeographic boundary occurring at Cape Hatteras today. Oxygen isotope time series spanning at least 3 years of growth were used to identify the timing of seasonal growth cessation in M. mercenaria shells from 14 coastal localities from North Carolina (34º13'N) to Rhode Island (41º20'N). We predicted that shells north of Cape Hatteras produced winter cessation increments and those to the south formed summer cessation increments.

Of the 13 specimens for which we have results, 12 of them – spanning the full latitudinal range of our study – have δ¹⁸O values indicating periods of slower growth during the summer season. The specimen collected from Barnegat Bay, New Jersey (39º45'N), was the only shell with δ¹⁸O values indicating periods of slower growth during the winter season. These data do not support the hypothesis that a latitudinal change in Mercenaria mercenaria growth patterns occurs at the modern biogeographic boundary at Cape Hatteras. Our findings suggest that for Mercenaria mercenaria this method is not suitable for identifying the past location of provincial boundaries.