Data from: Oxygen isotope composition of teeth suggests endothermy and possible migration in some Late Cretaceous shark taxa from the Gulf Coastal Plain, USA

We analyzed the oxygen isotope composition of biogenic apatite phosphate
(δ18Op) 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 δ18Op with the δ18Op of a co-occurring
poikilothermic bony fish Enchodus petrosus as a reference for ambient
conditions. E. petrosus tooth enamel δ18Op 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 δ18Op 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 δ18Op values than co-occurring E. petrosus (P. mortoni
δ18Op is 19.1‰ VSMOW in the Mooreville Chalk; C mantelli δ18Op 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 δ18Op values. The low P. mortoni δ18Op 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 δ18Op 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 δ18Op values are also driven by active thermoregulation, this
suggests that mesothermy evolved independently multiple in families of
Late Cretaceous sharks.