DID MOSASAURS NEED TO DRINK FRESHWATER? OXYGEN ISOTOPE EVIDENCE OF ESTUARINE INCURSION BY THE MARINE REPTILE CLIDASTES PROPYTHON, MOOREVILLE CHALK, ALABAMA

Age-old questions about extinct organism behavior and biology can be answered through new geochemical applications to fossil material. Here we address the question of whether the extinct marine lizards, mosasaurs, migrated to freshwater environments through schlerochronological analysis of their teeth. From oxygen isotope composition of enamel phosphate in eight consecutive, fully erupted teeth, we reconstruct the ecological niche of a Clidastes propython individual from the Late Cretaceous Mississippi Embayment (Mooreville Chalk, AL). The daily-resolution isotopic record of the C. propython is then compared with an identical study of Platecarpus ictericus mosasaurs from the Kansas Niobrara Chalk of the Western Interior Seaway (WISW).

Oxygen isotope records from all 8 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 of 12 to 20 day recurrence and 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‰. The short-term excursions in δ¹⁸O likely indicate rapidly changing water habitats for the mosasaur, possibly resulting from incursion into freshwater sources that are ¹⁸O-depleted. Semi-regular incursion of the C. propython individual into freshwater suggests that mosasaurs may have had osmoregulatory function similar to those of their living relatives, sea snakes, who must drink freshwater periodically. The long-term trend in δ¹⁸O also suggests that the C. propython individual may have migrated from open ocean water into the shallow, evaporative, ¹⁸O-enriched environments.

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 niche partitioning related to the open ocean central WISW vs. the evaporative, lower latitude Mississippi Embayment. An important similarity between the P. ictericus and the C. propython is that they both exhibit 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.