NEW EVIDENCE FOR COMPLEX MOSASAUR PALEOBIOLOGY: OXYGEN ISOTOPES IN ENAMEL REVEAL HABITAT VARIATION OF CLIDASTES FROM THE MOOREVILLE CHALK, ALABAMA

The Late Cretaceous Mississippi Embayment includes some of the most complete, well-preserved mosasaur specimens in the world. Here we 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 8 consecutive, fully erupted teeth, we reconstruct the behavior and ecological niche of an individual of the genus Clidastes. The isotopic record of the Clidastes specimen from the Mooreville Chalk in Pickens County, Alabama will be compared with two Platecarpus mosasaurs from the time-equivalent Kansas Niobrara Chalk of the Western Interior Seaway (WISW).

Isotopic records from all 8 teeth in the Clidastes specimen correlate well and are spliced to build a long record of water composition change. The mean d¹⁸O value of the Alabama Clidastes is ~20‰, which is 3‰ greater than previously studied Kansas Platecarpus mosasaurs. This is likely due to niche partitioning related to the more evaporative, lower latitude habitats of the Mississippi Embayment vs. the central WISW. The spliced isotopic record from the Clidastes shows two primary features: 1) d¹⁸O gradually trends from ~20.5‰ to ~22.5‰, followed by a decrease to ~20‰, with relatively constant low values until a final increase to ~21.5‰, and 2) there are 7 well-correlated positive and negative peaks of up to 5‰ superimposed on the long-term trends. These short-term variations in d¹⁸O suggest rapidly changing water habitats, likely due to paleomigration into freshwater (depleted d¹⁸O) or into warmer, saline environments (enriched d¹⁸O). With ongoing analysis of this highly resolved record, we will continue to reconstruct details of habitat change and compare the paleobiology of Southeastern vs. WISW mosasaurs. Further, correlation of isotopic records of mosasaur teeth will allow us to test anatomical models for tooth replacement patterns. The results will contribute new perspectives on the migratory behavior of large marine reptiles during the greenhouse climates of the Late Cretaceous and may address concerns of how large modern marine organisms and their seasonally-associated behavior, namely migration and reproduction, will be impacted by rising temperatures.