We analyze stable oxygen isotopes (δ18Op) of tooth enamel(oid) from fossil marine predators from the Smoky Hill Chalk Member of the Niobrara Formation (Kansas, USA) to investigate the thermoregulatory modes of shark, bony fish, and mosasaur taxa that lived in the Western Interior Seaway (WIS) of North America during the Late Cretaceous. Through comparing these taxa with the co-occurring ectothermic bony fish †Enchodus, which serves as a proxy for ambient conditions, we assess the relative δ18Op values of 21 individual organisms across nine genera, using data measured from 47 fossil teeth. The mosasaur taxa (†Platecarpus and †Tylosaurus) exhibit significantly lower δ18Op values than †Enchodus that are consistent in the direction and magnitude expected for endothermy in this group. Most shark taxa, by contrast, are likely ectothermic because their δ18Op values overlap with those of †Enchodus. The durophagous shark †Ptychodus and large predatory bony fish †Xiphactinus, however, record significantly lower δ18Op values than Enchodus and other sharks, which we interpret as indicative of elevated body temperatures consistent with endothermy and migration. Comparison with the Gulf Coastal Plain (GCP) of North America reveals that †Ptychodus, †Cretoxyrhina, †Platecarpus, and †Tylosaurus have similarly low δ18Op values across both regions. The consistency of δ18Op value offsets for †Ptychodus across species and regions suggests that endothermy was a trait inherent to this genus. Differences in δ18Op value offsets (from local †Enchodus values) between WIS and GCP for †Tylosaurus and †Cretoxyrhina may reflect differences in respective study sample size, methodology, and ecological factors, namely migration, and regional ecological differences and ontogenetic habitat partitioning within †Cretoxyrhina. Further, these new data support previous findings that †Cretalamna was ectothermic and †Cretoxyrhina was endothermic. The prevalence of endothermy in Late Cretaceous sharks is likely higher than previously thought, challenging hypotheses of endothermy evolution in Late Cretaceous sharks.
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