The eastern box turtle (Terrapene carolina) in time and space

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Vitek, Natasha Slonim

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Variation is a key component of the evolutionary process. However, variation often is poorly understood within species. The eastern box turtle (Terrapene carolina) presents an excellent opportunity to study that topic because extant populations have high levels of variation in soft-tissue characters as well as morphological variation in skeletal characters. To explore patterns of spatiotemporal variation, I used geometric morphometrics to quantify shape within three datasets. First, I asked to what extent size explained total shape variation using an ontogenetic series of 101 specimens. Next, I examined to what extent subspecies were morphologically distinct and identifiable in the modern record, and to what degree they explained overall variation using a dataset of 200 modern specimens. Finally, I compared the patterns in the modern biota to those from the fossil record using the previous datasets as well as a fossil dataset of 44 Pleistocene shells of T. carolina. I found that in four views of the turtle shell (dorsal, ventral, posterior, and lateral), size significantly explains 10% - 31% of the variation in shape. Some of the characters correlated with size were historically ascribed to characters of subspecies. Studying the extent to which size explains overall variation in different subsamples of my data allowed me to discover a new way of classifying segments of a population in order to account for size in future studies. Subspecies identification also explained a statistically significant amount of overall shape variation. However, the results of assignments tests and CVAs indicated insignificant or unreliable differences. Results indicate that differences between putative subspecies are more statistically significant than they are biologically significant. They do not support the recognition of subspecies in T. carolina. The inability of statistical analyses to identify individuals of a subspecies based on shell shape means that subspecies cannot be identified in the fossil record. Some of the same relationships between shape and size are present in the fossil record and the modern biota, but other morphological patterns are unique to fossil specimens. Two of the morphotypes co-occur in the same strata, and represent a unique evolutionary pattern not seen in the modern biota.


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