21 Geochemical Proxies for Marine and Continental Paleoenvironments

Monday, 15 April 2024
3:50 PM - 5:30 PM
Windsor Ballroom Salon A

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

Paper number: 21-2
Program: Theme Sessions
Time: 4:10 PM – 4:30 PM

Authors

Abstract

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 (δ18Op) 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 δ18Op 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 δ18Op values than Enchodus. Direct interpretation of δ18Op values requires an understanding of the δ18O value of seawater (δ18Osw), but despite this limitation, some interpretations can be made for the recorded δ18Op values. The most likely interpretation for taxa that had δ18Op 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 δ18Op values are most simply explained by endothermy (Ptychodus) or by a combination of endothermy and migration to warmer or lower δ18Osw locations. Further studies of these taxa in other locations will be necessary to further refine these paleoecological interpretations.

Geological Society of America Abstracts with Programs. Vol. 56, No. 2

doi: 10.1130/abs/2024SE-398532

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