Morphological variation in the carapace and plastron of terrapene coahuila schmidt and owens 1944

Abstract

Terrapene coahuila is one of four extant species of North American box turtles. It is restricted in distribution to the Cuatro Ciénegas Basin in Coahuila, México. Results of previous examinations of extant T. carolina and T. ornata revealed relatively high levels of morphological variation, but morphological studies of T. coahuila are rare, and data on skeletal morphology are limited. We examined 214 skeletal specimens of T. coahuila and documented variation in 51 mensurative and discrete characters of the carapace and plastron. Overall levels of variation are low, as predicted by previously documented levels of gene flow between the sub-populations of the species. However, significant polymorphism is present in the positions of the anterior and posterior sulci of the fourth vertebral scute and the configuration of neural bones 2, 3, and 7. Additionally, co-ossification of the carapacial bones varies substantially within the sample, but independently of carapace length. Genetic, epigenetic, and environmental controls for those features are not known. In addition to documenting skeletal morphology within T. coahuila, we provide new perspectives on patterns of variation within Terrapene, and contribute data that should help paleontologists to establish more rigorous criteria for the identification of fossil specimens of North American box turtles. Those data will be especially important for critical evaluation of recently discovered early and middle Tertiary fossils that are yielding new insights into the evolution of box turtles and the modernization of the turtle biota.