Skeletochronology: A Method For Determining The Individual Age and Growth of Modern and Fossil Tortoises (Reptilia: Testudines)

Abstract

Skeletochronology is a method used to estimate the individual ages of animals by counting lines of arrested growth (LAGs) within skeletal tissues. This method was applied to evaluate the ages of modern gopher tortoises, Gopherus polyphemus, from north central Florida and fossil tortoise species, Gopherus laticuneus and Stylemys nebrascensis, from the White River Group in northwestern Nebraska. Different skeletal elements were tested for growth lines and the humerus was determined to be the most useful bone for analysis. LAGS in Gopherus polyphemus were correlated with alternative age estimations based on scute annuli counts, carapace lengths, and plastron lengths. While age estimates were similar in younger individuals, the alternative aging techniques did not accurately reflect the ages of older individuals. Scute wear, sexual dimorphism, and decreased growth with age are factors contributing to this discrepancy. Similarly, LAGS were found in the humeri of Gopherus laticuneus and Stylemys nebrascensis. Fossil specimens range in age from 0 years (hatchling) to over 40 years old. Fossil samples provide insight into changing ecological conditions during the Eocene-Oligocene Transition. This critical period appears to show a shift from a Stylemys nebrascensis to Gopherus laticuneus dominant ecosystem.