INDICATIONS OF ENDOTHERMY IN SOME LATE CRETACEOUS SHARKS (PTYCHODUS, CRETOXYRHINA) FROM THE GULF COASTAL PLAIN OF ALABAMA

Some modern shark taxa elevate their body temperature above ambient conditions, this endothermy is related to their large size and/or vascular counter-current exchange systems. The evolution of endothermy in shark lineages is not well known, nor is it known whether this development occurred once, or multiple times. Geochemical analysis of shark tooth enameloid, which is particularly robust to diagenesis, can be used to explore potential endothermy and other paleoecological information about fossil shark taxa. Here, we present oxygen isotope values derived from shark enameloid phosphate (δ¹⁸O_p) from formations of Alabama: the Blufftown Formation and the Mooreville Chalk. These formations are contemporaneous (latest Santonian to early Campanian), but the Blufftown is sandier and thought to represent a more nearshore environment. We analyzed six species of shark across five genera, as well as a co-occurring bony fish taxon (Enchodus), which we used to indicate likely ambient conditions for each formation. At least three individual teeth were analyzed for each taxon from each formation.

Most analyzed shark taxa have δ¹⁸O_p values that are statistically indistinguishable from those of Enchodus, including Cretalamna, Scapanorhynchus, and Squalicorax spp. However, Ptychodus (Mooreville, -2.2‰ VSMOW) and Cretoxyrhina (Blufftown, -3.3‰; Mooreville, -1.1‰) both have significantly lower δ¹⁸O_p values than Enchodus. Direct interpretation of δ¹⁸O_p values requires an understanding of the δ¹⁸O value of seawater (δ¹⁸O_sw), but despite this limitation, some interpretations can be made for the recorded δ¹⁸O_p values. The most likely interpretation for taxa that had δ¹⁸O_p values consistent with Enchodus is that they inhabited the local environment without meaningful migration and had body temperatures consistent with ambient water temperature (poikilothermic). The taxa with anomalously low δ¹⁸O_p values are most simply explained by endothermy (Ptychodus) or by a combination of endothermy and migration to warmer or lower δ¹⁸O_sw locations. Further studies of these taxa in other locations will be necessary to further refine these paleoecological interpretations.