Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas, Florida Museum of Natural History and Department of Biology, the University of Florida, Gainesville, Florida, United States of America.
PLoS One. 2018 Mar 7;13(3):e0193437. doi: 10.1371/journal.pone.0193437. eCollection 2018.
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.
对现存的东方箱龟（Terrapene carolina）壳形态变化的特征进行描述，为与化石种群进行比较提供了一个基准。它还为在化石记录中识别种内多样性所固有的困难提供了一个范例。关于在 T. carolina 的化石中发生的变化在多大程度上可以被该物种的现存变化所容纳，这一问题已经争论了 80 多年。通过对龟甲进行形态计量学分析，我从性二态性、地理和亚种变化以及异速生长变化的角度来探讨现代和化石 T. carolina 之间的关系。现代 T. carolina 表现出微弱的雄性偏性体型二态性。在化石记录中无法可靠地检测到性形状二态性。与将 T. carolina 分为四个亚种的观点相反，地理变化模式更符合该物种分布的各个区域之间的渐变模式。异速生长模式与其他淡水龟鳖类中记录的模式相似，解释了大量的形状变化。在考虑了异速生长模式后，全新世的标本与现代标本没有显著差异。相比之下，几个地质年代较老的标本的壳形状有明显的不同，没有现代的类似物。那些大型的、化石化的标本代表了已经灭绝的、占据了形态空间新部分的变异。本研究强调了需要更多地记录现代骨骼变异，以便用于测试化石记录中种内进化的假设。
