Abstract
Characterization of morphological variation in the shells of extant Eastern Box Turtles, Terrapene carolina, provides a baseline for comparison to fossil populations. It also provides an example of the difficulties inherent to recognizing intraspecific diversity in the fossil record. The degree to which variation in fossils of T. carolina can be accommodated by extant variation in the species has been disagreed upon for over eighty years. Using morphometric analyses of the carapace, I address the relationship between modern and fossil T. carolina in terms of sexual dimorphism, geographic and subspecific variation, and allometric variation. Modern T. carolina display weak male-biased sexual size dimorphism. Sexual shape dimorphism cannot be reliably detected in the fossil record. Rather than a four-part subspecific division, patterns of geographic variation are more consistent with clinal variation between various regions in the species distribution. Allometric patterns are qualitatively similar to those documented in other emydid turtles and explain a significant amount of shape variation. When allometric patterns are accounted for, Holocene specimens are not significantly different from modern specimens. In contrast, several geologically older specimens have significantly different carapace shape with no modern analogue. Those large, fossilized specimens represent extinct variation occupying novel portions of morphospace. This study highlights the need for additional documentation of modern osteological variation that can be used to test hypotheses of intraspecific evolution in the fossil record.
Highlights
The recognition of extant, intraspecific diversity in the fossil record provides an important body of evidence with which to connect hypotheses of evolution developed from the modern biota to records of evolution on long time scales [1]
The purpose of this study is to address the controversial relationship between modern and fossil T. carolina through three major goals: (a) quantify the contribution of previously proposed sources of variation, allometric, geographic or subspecific, and sexually dimorphic, to standing variation in the carapaces of modern T. carolina, (b) determine which, if any sources of variation are sufficiently diagnostic that they could be identified in the fossil record, and (c) evaluate the degree to which variation in fossils of T. carolina can be accommodated by these sources of variation
Aspects of shell morphology have long been used as proxies for sex and presence or absence of reproductive maturity [29,163], but the features that are most applicable to the fossil record are those that have been least vetted [84,85]
Summary
The recognition of extant, intraspecific diversity in the fossil record provides an important body of evidence with which to connect hypotheses of evolution developed from the modern biota to records of evolution on long time scales [1]. That connection can be difficult to document. In many species various phylogeographic patterns hindcast dynamic intraspecific evolution in the Pleistocene [2,3,4]. Where a model hindcasts a species’ presence or absence, the fossil record is already used as a source of evidence to evaluate that model [4].
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