22-2 δ18Op of Tooth Enamel(oid) Reveals Endothermy in Some Late Cretaceous Vertebrates

Session: Working Up an Apatite: Teeth as Paleo -Ecological and -Climatological Archives

Presenting Author:

Authors:

Abstract:

Although assessing thermoregulation in extinct vertebrates is often challenging, stable oxygen isotope analyses of biogenic phosphate (δ¹⁸O_p) in tooth enamel(oid) can be a useful proxy for inferring the thermoregulation strategies and possible magnitude of warming by an organism. Here, we measure the δ¹⁸O_p of various shark (Ptychodus, Cretoxyrhina, Cretalamna, Scapanorhynchus, and Squalicorax), bony fish (Xiphactinus), and mosasaur (Platecarpus and Tylosaurus) taxa from the Late Cretaceous Smoky Hill Chalk Member of the Niobrara Formation, Kansas in the Western Interior Seaway (WIS) to assess thermoregulation with particular focus on suspected marine endotherms. We use the ectothermic bony fish Enchodus as a reference for ambient conditions in the WIS. We then compare WIS δ¹⁸O_p values to those reported for the same taxa from temporally equivalent formations in the Gulf Coastal Plain (GCP) to assess thermoregulation at a higher taxonomic level.

Within the WIS, most sharks measured here (Cretalamna, Scapanorhynchus, Squalicorax) were ectothermic, having δ¹⁸O_p value 95% confidence intervals (CIs) that overlap with co-occurring Enchodus. Both mosasaur taxa, the shark Ptychodus, and the bony fish Xiphactinus have significantly lower δ¹⁸O_p values than co-occurring Enchodus that we interpret as being consistent with endothermy. Although not rising to the level of statistical significance, Cretoxyrhina has δ¹⁸O_p values slightly lower than those of Enchodus. When comparing δ¹⁸O_p values in the WIS to those reported for the GCP, we find that certain taxa (Ptychodus, Platecarpus, Tylosaurus) have similarly low δ¹⁸O_p values in both locations, but the magnitude of Cretoxyrhina and Tylosaurus offsets from co-occurring, local Enchodus in the WIS and GCP are significantly different. This offset difference may be due in part to differences in respective study sample sizes and methodologies, or, in the case of Cretoxyrhina, migration and ontogenetic habitat partitioning. Previous studies have suggested that endothermy in sharks evolved during the Late Cretaceous with Cretalamna or Cretoxyrhina. Our data here, however, is consistent with previous δ¹⁸O_p values that suggest that Cretalamna was ectothermic.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10505

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.