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There are few published data addressing the types, levels, and patterns of intraspecific variation in gross anatomical structures for nonavian reptiles, especially considering the widespread interest in their morphology and evolutionary history. This study examines variation in both discrete and continuous characters of the postnatal skull for a single population of the kinosternid turtle Sternotherus odoratus. The primary purpose of the study was to elucidate those features that exhibit transformations (and by default those that do not) in the latter two-thirds of postnatal ontogeny—that portion heavily relied upon in phylogenetic analyses of macroanatomical features and most often preserved in the vertebrate fossil record. Data are presented for cranial characters historically used to elucidate phylogenetic relationships in turtles and to assess fossil specimens taxonomically.
Results indicate that the number of characters exhibiting postnatal variation, and the levels at which they vary, are surprisingly high considering the conservative nature of both the taxonomic sample and the pool of examined characters. Features associated with the feeding apparatus are among the most variable cranial structures—for both continuous and discrete characters. A relatively large number of discrete transformations occur in structures derived from those cranial elements preformed in cartilage. This concentration demonstrates that developmental trajectories apomorphic for deep nodes in vertebrate phylogenetic history result in relatively late-stage postnatal transformations and high levels of variation in characters informative at much more restricted taxonomic levels. This study provides a baseline of data that future studies examining the ontogenetic and evolutionary history of variation can build upon and it represents a necessary step in understanding the complex system by which variability in developmental modules becomes integrated in the reptile skull.