Abstract:
The Late Cretaceous Mississippi Embayment includes some of the most complete, well-preserved mosasaur specimens in the world. Here I investigate the paleobiology of these extinct swimming lizards through schlerochronological analysis of fossil teeth with unparalleled temporal resolution. By analyzing the oxygen isotope composition of enamel phosphate in eight consecutive, fully erupted teeth, I reconstruct the ecological niche of a Clidastes propython individual from the Mooreville Chalk in Pickens County, Alabama. The isotopic record is then compared with the previously studied Platecarpus ictericus mosasaurs from the time-equivalent Kansas Niobrara Chalk of the Western Interior Seaway. Phosphate oxygen isotope records from all eight teeth in the C. propython specimen correlate well and are spliced to build a longer record of water composition change. The spliced isotopic record is characterized by two primary features: 1.) well-correlated, semi-regular negative spikes in δ18O that have a 12 to 20 day recurrence and are up to ~4.0‰ amplitude, and 2.) a long-term decrease in δ18O from ~21.5‰ to ~20.0‰ (‰ V-SMOW), followed by a gradual increase to ~22.0‰ and a final decrease to ~20.0‰. The short-term excursions in δ18O indicate rapidly changing water habitats for the mosasaur, possibly resulting from incursion into freshwater sources with low δ18O composition. Semi-regular incursion of the C. propython individual into freshwater suggests that mosasaurs had osmoregulatory function similar to those of their living relatives, sea snakes, who must drink freshwater periodically. The long-term trend in δ18O further suggests that the C. propython individual may have migrated from open ocean water into the shallow, evaporative, 18O-enriched environments of the Mississippi Embayment. The mean δ18O value of the Alabama C. propython is ~3.7‰ higher than that of the Kansas P. ictericus. This difference is likely due to the varying isotopic composition of the open, central Western Interior Seaway vs. the evaporative, lower latitude Mississippi Embayment. An important similarity between the P. ictericus and the C. propython is that they both exibit semi-regular, negative excursions in δ18O, further implicating the freshwater incursions to be the result of a biological requirement spanning multiple, if not all, mosasaur genera.
