The postnatal skull of the extant North American turtle Pseudemys texana (Cryptodira: Emydidae), with comments on the study of discrete intraspecific variation.

This study describes the postnatal morphology of the skull in the extant testudinoid turtle Pseudemys texana. The material basis of this study is drawn from a single population and therefore provides the opportunity to document cranial variation expressed at multiple levels of inference (e.g., ontogeny, sexual dimorphism, individual) within a taxonomically restricted sample. Such descriptions are exceedingly rare for reptiles but are important for evaluating the inherent biological information that skeletal morphology provides to more inclusive biological questions. Results indicate that postnatal variation in Pseudemys texana for discrete cranial characters is considerable, especially when recognizing the taxonomically conservative nature of the sample and the fact that most of the examined characters were drawn from published phylogenetic analyses. Results demonstrate a complex relationship between size, skeletal maturity, and the expression of discrete skeletal characters in this sexually dimorphic reptile. Such complexity potentially could affect our interpretation of evolutionary history based on discrete characters. This study emphasizes the need for increasing the numbers and taxonomic breadth of morphological descriptions based on expanded samples so that a better understanding of the phylogenetic distribution and modular expression of anatomical variability can be achieved. The available empirical data lag seriously behind recent theoretical advances that broaden the application of intraspecific variation data for discrete characters to phylogenetic and evolutionary questions.