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November 2022 Bulletin

Luminescence chronology of a Mass Grave of Giant Gopher Tortoises (Gopherus hexagonatus), Willacy County, TX

By: Randolph G. De La Garza, Juan L. González, and Zhixiong Shen 


A rare fossiliferous site in Willacy County, Texas, contains a mass concentration of fossil Giant Gopher Tortoises (Gopherus hexagonatus). The site southeast of Raymondville was exposed in the late 1980s during archaeological surveys for drainage canal improvements. Numerous large adults with complete shells were recovered along with partial limb, cranial, and vertebral elements. Especially remarkable was the discovery of a female preserved in burrowing behavior, and clusters of nests containing whole and fragmented eggs, indicating a breeding population. This site provides an excellent opportunity to study the paleoenvironmental context of a prehistoric ecosystem situated within the Rio Grande Valley. Sedimentologic and stratigraphic evidence indicates these tortoises were living in an extensive fluvial system much like the present Rio Grande River. The Raymondville Fluvial System (RFS) deposit is mapped as the Beaumont Formation, but an absolute chronology of the deposit is lacking. We present the first optically stimulated luminescence (OSL) dates of the deposits within the RFS. The average of two dates yields an age of 50.2±2.5 ka, placing the RFS and its unique fossils in Marine Isotope Stage 3c, a time of cooler climate.

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