Sharks today live in a variety of habitats, but the range of environments occupied by extinct sharks is not well known. The water temperature and chemistry of marine environments is incorporated in the composition of fish teeth, as well as the way organisms regulate their body temperature. To better understand the ecology of ancient sharks, we analyzed the chemical composition (stable oxygen isotope values) of six Late Cretaceous–age (86–79 million years ago) shark species from the Gulf Coastal Plain of Alabama, USA. We also analyzed teeth from the fish Enchodus petrosus, which should record ambient water temperature and chemistry, and compared the shark data with that of E. petrosus. Two of the shark taxa (Ptychodus mortoni and Cretoxyrhina mantelli) have much lower values compared with other fossil sharks and the fish E. petrosus. The low P. mortoni values are best explained by P. mortoni having a higher body temperature than the surrounding water, either through active or passive body heating. Similarly, the low C. mantelli values are best explained by both migration and higher body temperatures. It was proposed from other evidence that C. mantelli had higher body temperatures, but this study marks the first quantitative evidence of higher body temperatures in P. mortoni. If P. mortoni had an elevated body temperature, then it is likely that body temperature regulation evolved many times in sharks in the geologic past, as this species is not closely related to other species in which this phenomenon is documented.

We analyzed the oxygen isotope composition of biogenic apatite phosphate (δ¹⁸O_p) in fossil tooth enameloid to investigate the paleoecology of Late Cretaceous sharks in the Gulf Coastal Plain of Alabama, USA. We analyzed six different shark taxa from both the Mooreville Chalk and the Blufftown Formation. We compared shark δ¹⁸O_p with the δ¹⁸O_p of a co-occurring poikilothermic bony fish Enchodus petrosus as a reference for ambient conditions. Enchodus petrosus tooth enamel δ¹⁸O_p values are similar between formations (21.3‰ and 21.4‰ Vienna Standard Mean Ocean Water [VSMOW], respectively), suggesting minimal differences in water δ¹⁸O between formations. Most shark taxa in this study are characterized by δ¹⁸O_p values that overlap with E. petrosus values, indicating they likely lived in similar habitats and were also poikilothermic. Ptychodus mortoni and Cretoxyrhina mantelli exhibit significantly lower δ¹⁸O_p values than co-occurring E. petrosus (P. mortoni δ¹⁸O_p is 19.1‰ VSMOW in the Mooreville Chalk; C mantelli δ¹⁸O_p is 20.2‰ VSMOW in the Mooreville Chalk and 18.1‰ VSMOW in the Blufftown Formation). Excursions into brackish or freshwater habitats and thermal water-depth gradients are unlikely explanations for the lower P. mortoni and C. mantelli δ¹⁸O_p values. The low P. mortoni δ¹⁸O_p value is best explained by higher body temperature relative to surrounding temperatures due to active heating (e.g., mesothermy) or passive heating due to its large body size (e.g., gigantothermy). The low C. mantelli δ¹⁸O_p values are best explained by a combination of mesothermy (e.g., active heating) and migration (e.g., from the Western Interior Seaway, low-latitude warmer waters, or the paleo–Gulf Stream), supporting the hypothesis that mesothermy evolved in lamniform shark taxa during the Late Cretaceous. If the anomalous P. mortoni δ¹⁸O_p values are also driven by active thermoregulation, this suggests that mesothermy evolved independently in multiple families of Late Cretaceous sharks.