SHARK PALEOECOLOGY: APPLICATIONS OF MULTIPLE STABLE ISOTOPE PROXIES

The methodological advancements and sample size reductions of current stable isotope analyses have opened new opportunities to investigate the ecology of extinct shark taxa that have historically been limited to more qualitative paleontological approaches. Most stable isotope analyses target the highly crystallized outer enameloid layer of a shark tooth due to its resistance to diagenetic alteration. Here, we examine the utility of applying multiple stable isotope proxies (stable oxygen isotopes of enameloid phosphate and stable nitrogen isotopes of enameloid-bound organic matter; δ¹⁸O_p and δ¹⁵N_EB, respectively) and evaluate a range of paleoecological inferences from those proxies.

We examine the δ¹⁵N_EB and δ¹⁸O_p of three lamniform sharks (Ptychodus mortoni, Scapanorhynchus texanus, and Squalicorax kaupi) from the Late Cretaceous (Santonian–Campanian) Gulf Coastal Plain to infer paleo-habitats, -trophic positions, and -biology. Sharks are highly migratory, potentially passing through and precipitating teeth in a variety of environmental conditions and trophic structures. When comparing δ¹⁵N_EB of multiple species from a single locality, it is difficult to infer trophic positions without knowing a baseline at the time of tooth precipitation, or whether all included taxa had the same baseline. Measuring δ¹⁸O_p from the same specimens provides environmental clues that aid in making these baseline assumptions. Additionally, δ¹⁸O_p provides clues to environmental conditions (sea temperature and δ¹⁸O of water) as well as shark biology (e.g., thermoregulation). For example, anomalously low P. mortoni δ¹⁸O_p values could be due to incursions into fresher water or higher body temperatures (i.e., endothermy) than surrounding waters. P. mortoni δ¹⁵N_EB values support a marine environment, suggesting that endothermy may be a more likely explanation for δ¹⁸O_p values. S. texanus and S. kaupi have overlapping δ¹⁸O_p­ and δ¹⁵N _EB values suggesting that they feed in similar environments and at similar trophic levels, despite possessing different tooth morphologies. Thus, employing multiple isotopic proxies provides key contextual information that aids in broader paleoecological interpretations.