Stable Isotope Analysis of a Platecarpus tympaniticus (Squamata, Mosasauridae) with Actinocamax sternbergi (Mollsuca, Belemnoidea) reveals possible endothermic thermoregulation
Authors:   , Lukens Mitchell 
Published: February 2023

Mosasaurs, ancient marine reptiles, dominated the late Cretaceous oceans. However, their ecological success is a contentious topic. Were they ectothermic, like their modern relatives the varanid lizards? Or endothermic like extant marine mammals? Stable isotopes can reveal temperature and physiological variances within skeletons, but do not differentiate between body temperature and ambient environmental temperature. A rare mosasaur specimen from the Smoky Hill Chalk of a partial, articulated Platecarpus tympaniticus with stomach contents of belemnites provides a possible direct temperature contrast between predator and prey. The belemnites, related to modern coleoids, are identified as Actinocamax sternbergi. These animals possessed body temperatures equivalent to the ambient seawater. Stable oxygen isotope ratio values, reported as δ&lt;sup&gt;18&lt;/sup&gt;O values, preserve temperature signature proxies during bioapatite and calcite precipitation. Utilizing previous stable isotopic studies, these values can be interpreted to paleotemperatures through equations describing known phosphate and carbonate ion deposition. Contrasting temperatures between the mosasaur and belemnites would indicate an elevated thermophysiology of the mosasaur compared to its environment.\r Bone, chalk, and belemnite rostrum samples were collected as powdered samples by drilling into visually undamaged zones. Bone samples underwent two different treatment methods to isolate phosphate and structural carbonate ions for analysis. For the phosphates, bone powder was dissolved in nitric acid to isolate and precipitate the phosphate ions as silver phosphate for analysis. Silver phosphate samples underwent pyrolysis and the resulting carbon monoxide gas analyzed in a mass spectrometer for δ&lt;sup&gt;18&lt;/sup&gt;O values.\r For structural carbonate, bone powder was soaked in a triammonium citrate solution to dissolve secondary calcite formed during the fossilization process, leaving the structural carbonate ion within the bone behind. These treated samples, along with untreated samples, chalk, and belemnite rostrum powder samples were reacted in orthophosphoric acid to release carbon dioxide for analysis in a gas-chromatograph mass spectrometer to give δ&lt;sup&gt;13&lt;/sup&gt;C and δ&lt;sup&gt;18&lt;/sup&gt;O values.\r Bone phosphate values (n=26) (δ&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;P&lt;/sub&gt;) ranged from 14.83 to 21.12‰ with a mean value of 18.30 ± 1.53‰ (1σ). Untreated bone carbonate values (n=6) (δ&lt;sup&gt;13&lt;/sup&gt;Ccc, δ&lt;sup&gt;18&lt;/sup&gt;Occ) ranged from -4.9 to -0.8‰ with a mean value of -2.3 ± 1.6‰; and -9.5 to -7.6‰ with a mean value of -8.5 ± 0.8‰ respectively. Treated bone carbonate values (n=6) (δ&lt;sup&gt;13&lt;/sup&gt;Cc, δ&lt;sup&gt;18&lt;/sup&gt;Oc) ranged from -5.7 to -0.7‰ with a mean value of -3.0 ± 1.9‰; and -9.2 to -7.3‰ with a mean value of -8.1 ± 0.8‰ respectively. Chalk sample values (n=4) (δ&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;M&lt;/sub&gt;, δ&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;M&lt;/sub&gt;) ranged from 2.1 to 2.3‰ with a mean value of 2.2 ± 0.08‰; and -6.5 to -6.4‰ with a mean value of -6.4 ± 0.04‰ respectively. Belemnite sample values (n=3) (δ&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;BEL&lt;/sub&gt;, δ&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;BEL&lt;/sub&gt;) ranged from 0.4 to 0.5‰ with a mean of 0.4 ± 0.06‰; and -3.4 to -3.0‰ with a mean value of -3.3 ± 0.25‰.\r Bone powder samples revealed diagenetic alteration of the structural carbonate. Secondary alteration, however, did not impact oxygen isotope values from the bone phosphate, belemnite rostrum carbonate, or the chalk. Scanning electron microscope and energy dispersion system images confirm the isotopic integrity of the phosphates and carbonates for analysis. Intra- and inter-variation of the bone phosphate values within each bone and across the skeleton suggest temperature fluctuations of the entire body or changes in body water oxygen isotopic composition reflecting migration into different bodies of water throughout the animal’s life.\r Direct comparison of the calculated temperatures from the belemnites (15.0℃), chalk (22.1℃), and the mosasaur (36.2 – 37.5℃) provides evidence that this mosasaur possessed an elevated thermophysiology relative to its prey and environment. Variable bone phosphate oxygen isotope values across the appendicular and axial skeleton infer this mosasaur exhibited heterothermic endothermy, a hot body core with varying temperatures in the flippers. The interpreted temperatures of the chalk and belemnites suggest the presence of a thermocline in the Western Interior Seaway. Diving to feed on belemnites is an inferred behavior from the isotope-derived colder temperatures preserved in the rostrums and avascular necrosis associated to decompression-sickness syndrome in other Platecarpus specimens.