RECONSTRUCTING THE PALEOECOLOGY OF THE ALABAMA MOSASAUR CLIDASTES FROM PHOSPHATE OXYGEN ISOTOPES IN FOSSIL TOOTH ENAMEL

Here we present a fresh look at the paleobiology of marine reptiles from the Late Cretaceous Mississippi Embayment of the Gulf Coast through schlerochronological analysis of growth lines in fossil teeth with unparalleled resolution. By analyzing the phosphate oxygen isotope composition from eight consecutive, fully erupted teeth, we reconstruct the ecology and migration behavior of an individual of the mosasaur genus Clidastes from the Mooreville Chalk, Pickens County, Alabama, which facilitates comparison with mosasaurs of the ancient Western Interior Seaway (WISW) of the United States.

Preliminary isotopic records from 3 of the 8 consecutive teeth in the Alabama Clidastes mosasaur correlate well and are spliced to build a longer record. The mean d¹⁸O value of the Alabama Clidastes is ~20‰, which is 3‰ greater than previously studied Kansas Platecarpus mosasaurs of the same age. This is likely due to paleogeography and niche partitioning related to the more evaporative, lower latitude habitats of the Mississippi Embayment vs. the open central WISW. Preliminary isotopic analysis of growth increments shows two primary trends: 1) d¹⁸O gradually decreases from 21.5‰ at the base of the tooth to 19‰ near the middle, then increases to 21‰ at the crown tip, perhaps as a result of seasonal temperature variation; and 2) every 3 to 7 growth lines are superimposed positive and negative peaks in d¹⁸O, with a difference of up to ~4‰, and are well correlated in all three consecutive teeth. These short-term variations in d¹⁸O suggest changing water habitats, likely due to paleomigration into freshwater (depleted d¹⁸O) or into warmer, more saline environments (enriched d¹⁸O). With ongoing analysis of highly resolved samples from the Clidastes jaw, we will be able to reconstruct details of habitat change in a time series, and compare the paleobiology of Southeastern mosasaurs with those from the central WISW to interpret the paleoecology of mosasaurs. In addition, there are concerns of how large modern marine organisms and their seasonally-associated behavior, namely migration and reproduction, will be impacted by rising temperatures. Our results should help inform these discussions by contributing perspectives on the migratory behavior of large marine reptiles during the warm greenhouse climates of the Late Cretaceous.