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, U.S.A. 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. E. petrosus tooth enamel δ¹⁸O_p values are similar between formations (21.3‰ and 21.4‰ 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 multiple in families of Late Cretaceous sharks.