REFINING THE CHRONOLOGY OF A UNIQUE MASS GRAVE OF GIANT TORTOISES WITHIN THE LATE PLEISTOCENE RAYMONDVILLE FLUVIAL SYSTEM, WILLACY COUNTY, TEXAS

The paleontology of the Rio Grande Valley of South Texas is an underrepresented field in the scientific literature. Only a handful of fossils have been documented in the area, with much of it being fragmentary fossils that were found throughout the Valley and nearby regions, mainly Northeastern Mexico. Based on what has already been found, it is an open question on what the region’s natural environment was like tens of thousands to millions of years before humans settled here. The main goal for this study is to fill in the gaps of knowledge on the evolution of the prehistoric Rio Grande Valley by using the fossils of vertebrates found within the area as paleoenvironmental proxies of this change. Of particular interest is a late Pleistocene (~120,000 to 10,000 years ago) site in Willacy County, Texas. This site contains a mass concentration of relatively complete fossils of several Giant Gopher Tortoises (Gopherus hexagonatus) that was exposed in the late 1980s when several archaeological surveys were conducted during the development of drainage canals in an area near the city of Raymondville. Numerous large adults with more or less complete shells were recovered at this site along with large whole and broken bones. Especially remarkable was the discovery of a female individual that had preserved burrowing behavior and a nest of 5-6 eggs; further indicating a population of interbreeding giant tortoises being preserved in this unique site. As mass fossil graves of this magnitude are uncommon, this site provides an excellent opportunity for paleontological study. Recent fieldwork nearby the original tortoise excavation sites on this site by our group have recovered additional fossils, including a horse tooth, numerous tortoise shell fragments, and plant roots found in situ. Detailed sedimentology and stratigraphy studies done on this system has also revealed evidence that these tortoises were living in an extensive fluvial system much like the Rio Grande River today. Here we present the results of the Optically Stimulated Luminescence chronology of this system using sediment samples collected from the field by our lab.