Trachemys scripta (Thunberg In Schoepff, 1792)

Drumheller, Stephanie K., Maddox, Hannah, Stocker, Michelle R. & Noto, Christopher R., 2023, Differentiating convergent pathologies in turtle shells using computed tomographic scanning of modern and fossil bone, Palaeontologia Electronica (a 15) 26 (2), pp. 1-21 : 11-14

SAAF Trachemys scripta

The modern bite marked specimen ( Figure 1 : SAAF Trachemys ) is 24.5 cm long and 19 cm wide. Some of the keratinous scutes are still in place, especially on the carapace, which obscures some of the underlying individual tooth marks in the bone ( Figure 1A ). A line of serial bite marks is associated with a radiating fracture that extends from midline on the posterior margin of the carapace anteriorly roughly two-thirds the length of the shell, angling to the right and terminating roughly halfway between the shell’s midline and its right lateral margin. Smaller, associated fractures radiate from this main break, one propagating posterolaterally towards the right margin of the shell and others forming a concentric, depressed region along the right margin of the longest fracture. A 44.61 mm by 15.19 mm D-shaped fragment of the carapace was displaced and lost some time prior to collection. Four individual tooth marks (widths of 3.59 mm, 4.87 mm, 5.03 mm, 6.11 mm) are visible along the margin of this break, but the fracture, the ventral displacement of the right margin along that break, and the missing fragment of bone make reliable measurements of each tooth mark problematic. A single, bisected puncture (7.41 mm long by 4.70 mm wide) is present roughly 3.5 cm from the shell’s right margin and 4.5 cm posterior to the anteriormost edge. This rounded, slightly fusiform puncture is approximately aligned with the others, 5 cm anterior to the termination of the break.

On the plastron, four additional punctures occur 2 cm medial to the right bridge ( Figure 1B ). The anteriormost and posteriormost of these serial marks are shallow pits, and the two centrally situated marks are deeper bisected punctures, measuring from anterior to posterior 4.33 mmby 3.07 mm and 4.63 mm by 3.16 mm ( Figure 8A ). The spacing between these four marks, again progressing anterior to posterior and measured from the center of each depression, are: 17.53 mm, 19.18 mm, and 18.70 mm. The positions of these serial marks suggest they were formed concurrently with the serial marks on the carapace because they are aligned with one another on opposing sides of the shell. The size and spacing of these serial marks are consistent with the dentition of the crocodylian that had shared the turtle’s enclosure: Mecistops cataphractus .

An additional bisected pit (2.98 mm by 2.48 mm) is present along the plastron’s midline roughly 6 mm to the right of the suture. At least three additional marks, two punctures and one pit, are present near the posterior edge of the plastron. The pit, measuring 3.70 mm by 1.97 mm with no obvious bisection, is located just posterior to the right bridge of the shell and may be associated with the serial bite marks described above. The two punctures are situated on the left side of the plastron. The larger of these is broadly oval shaped with an apparent drag-out mark. Taken together, the total maximum length of that mark is 7.84 mm by 4.32 mm wide. The second puncture is positioned roughly 13 mm lateral to the first and measures 3.90 mm by 3.41 mm with an incomplete bisection in the form of a single notch associated with the impacting tooth’s carina on the anterior margin of the long axis of the trace.

There are regions of this plastron that may also preserve shell disease ( Figure 1B ), one sub-rounded area roughly 29 mm medial to the anteriormost of the serial marks, one 22 mm posterior to the set of two punctures, and a clustering of circular depressions along the right side of the midline, from the center of the plastron towards the posterior margin of the shell. All affected areas are sub-rounded and lack obvious crushing or impact trauma. As such, we tentatively identify these as shell disease, which does commonly affect this taxon ( Garner et al., 1997; Zonneveld and Bartels, 2022).

Internal morphologies show that marks appear V- or U-shaped in cross section, tapering to a rough point. A thick band of compressed cortical bone underlies all punctures ( Figure 1 C-1H). These bands are visible in plain CT data, and variation in bone density is further highlighted in the LUT heatmap, where compressed regions are consistently expressed as a lighter color than the rest of the surrounding bone tissue ( Table 3 ). Breakage and fracture points also show signs of bone compression, but not to the extent found with individual V-shaped marks. An extreme case in SAAF shows a large, U-shaped puncture located on the carapace exhibiting significant bone compression, enough so that the internal arch of the carapace is bowing inwards. This puncture is surrounded by a thick line of white and bright orange when viewed in LUT heatmap settings.

Figures

FIGURE 1. Modern Trachemys scripta shell (SAAF) with bite marks attributed to Mecistops cataphractus. Orthographic models of the shell, based on µCT data shown in dorsal (A) and ventral (B) views. Frames on the models highlight specific bite marks, shown on the right as both direct µCT data (C, E, G) and heatmapped slices illustrating bone density changes (D, F, H). In the heatmapped cross sections, colors range from purple (lowest density), to orange (medium density), to white (highest density). Specific bite marks are indicated with purple arrows. Scale bars in A and B equal 5 cm. Scale bars in C, E, and G equal 5 mm.

FIGURE 8. Characteristic examples of shell disease and bite marks in modern and fossil turtle shells. Modern shell disease on the plastron of Trachemys scripta, specimen UTK 2317 (A). Modern bite marks (bisected punctures) on the plastron of Trachemys scripta, specimen SAAF unnumbered (B). Fossil shell disease on a fragment of turtle shell, specimen DMNH 2013-07-0563 (C). Fossil bite marks (four scores and one pit) on a fragment of turtle shell, specimen DMNH 2013-07-1319 (D). Scale bars equal 10 mm.