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Shell Shock! Can morphometrics distinguish fossil turtle species?

A big component of paleontological work revolves around identifying morphological characters that diagnose distinct species in the fossil record. But therein lies an unavoidable problem: where is the line between variation among separate species and variation within a species? With modern taxa, this problem can be addressed using other lines of evidence: dimorphism can be solved by observing morphological trends between males and females; morphological differences due to age and maturity can be addressed by examining development; or if dealing with potential subspecies or cryptic species, genetics and genomics can help delineate intraspecific variation.

But what about when we are dealing with fossils? Fossil taxa represent just a snapshot of the possible biodiversity that existed in their respective ancient ecosystems, and fossil detectives are left examining what few clues we have in order to deduce the bigger picture. We often can’t tell from the fossil whether the organism represented a male or a female, a juvenile or an adult, etc. So the million dollar question is, when we think we have a new species, do we really have a new species? Or are the differences in size and shape that we have observed due to sexual dimorphism, age and maturity, or just simply variance in a population?

A new paper was published yesterday in PLOS ONE, by Natasha Vitek, a PhD candidate at the University of Florida. Vitek’s research focuses on Eastern Box Turtles, and her aim is to address some of the concerns that I mentioned above, using shell morphology of extant and extinct Eastern Box Turtles, Terrapene carolina, as an example to test variation within species, with the hopes that guidelines for diagnosing new species can become even clearer.

Modern-day representatives of T. carolina display a lot of variation among individuals within the species. These turtles are a terrestrial North American taxon with a historically widespread distribution. In order to effectively demonstrate the intraspecific variation, most biologists that work on them have separated T. carolina into several subspecies, usually parsed out over geographical ranges with some intergrades. (Subspecies come with their own bag of contention that I won’t get into here, but are an especially difficult biological unit to justify in the fossil record).

Within the fossil record, what was once several described species of Terrapene have more or less been synonymized over the years with T. carolina, with most of the morphological distinctions boiling down to subtle differences in size, shell width, and other variation in shape. Most researchers in this group have come to the conclusion that the variation in the fossil record reflected the same patterns of variation that are observed in modern biota. But is that correct? Are we looking at fossil subspecies, or are we looking at dimorphism or changes across different age levels? This is what Vitek wanted to test.

Using geometric morphometric software, Vitek examined and measured 435 extant specimens and 57 fossil specimens of T. carolina. 

Diagram of landmarks and semilandmarks used to study the shape of the carapace of Terrapene carolina. Open circles represent landmarks. Smaller, closed circles represent semilandmarks. (A) Dorsal view. Carapace drawn based on UF 21176. (B) Lateral view. (C) Posterior view. Carapace in B and C drawn based on UF 151664. From Vitek (2018). CC-BY.

When it came to determining maturity, size cannot be a dependable factor. Instead, Vitek determined that features such as growth rings, ossification within the carapace, and skeletal changes are better at determineing the approximate age of a specimen.

As far as sexual dimorphism, what was previously used as a skeletal feature for determining sex of a turtle turned out to be unreliable feature, according to Vitek. With plastron indentation, which has been used as a sexually dimorphic feature in previous studies, many errors were found where males or females were misidentified in museum collections. In living box turtles, this is not a problem because many soft tissue features, such as head coloration, are reliable in distinguishing male and females. Size is also not reliable to distinguish between males and females. Vitek points out, based on her research, that there is little support for determining sexually dimorphic characters in the fossil record at this point.

In terms of subspecies, Vitek concludes, based on her study of extant and extinct Eastern Box Turtles, that the subspecies system is not a reliable way to represent the variation that is seen in carapace shape in T. carolina. Her study shows that there is a lot of overlap in carapace shape, and she is unable to corroborate the hypothesis that there are diagnosable differences in the shapes of carapaces of the four subspecies. However, Vitek does support that geographical variation or size variation more closely ally with subspecies diagnoses that carapace shape.

Geographic representation of the two spatial eigenvectors (SEs) that account for subspecific differences in shape and shape differences predicted by those SEs in carapaces of modern T. carolina. Predicted mean shapes show maximum and minimum values of (A) SE2 and (B) SE3 in (C, F) dorsal, (D, G) lateral, and (E, H) posterior views, respectively. Line drawings indicate hypothetical carapaces consistent with each landmark configuration and are not exact predictions of all sulcus locations. From Vitek (2018), CC-BY.

What about the fossil record for these turtles? Vitek is able to conclude that, overall, the interpretation of intraspecific variation in fossilized T. carolina is reflective of the variation in modern T. carolina. However, some morphological characters, such as size in some of the fossil populations, do not follow the predicted trajectory of the analysis and may represent something more than just extra growth. In addition, some of the fossil specimens that differ greatly in the shell morphology could possibly be misdiagnosed as T. carolina, and could possibly belong to T. putnami, another problematic species that is known from a portion of a plastron that wasn’t examined for this study.

What is important about this study is that it shows that paleontologists should not work in a bubble of extinct taxa only. The variation in fossil taxa could have easily been (and at one point was) misconstrued as several distinct species, when in fact it represented intraspecific variation. But taking into account patterns of variation seen within living representatives of the group helped shed light on a more accurate interpretation of the extinct biota.

Reference:Vitek NS (2018) Delineating modern variation from extinct morphology in the fossil record using shells of the Eastern Box Turtle (Terrapene carolina). PLoS ONE 13(3): e0193437.

Featured Image: Eastern Box Turtle (Image Credit: Andrea Janda/Flickr/CC BY-NC-ND 2.0)

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