Differentiating convergent pathologies in turtle shells using computed tomographic scanning of modern and fossil bone Author Drumheller, Stephanie K. Author Maddox, Hannah Author Stocker, Michelle R. Author Noto, Christopher R. text Palaeontologia Electronica 2023 a 15 26 2 1 21 http://dx.doi.org/10.26879/1230 journal article 10.26879/1230 1094-8074 11063361 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. TABLE 1. Previous field numbers, current specimen identification numbers, element and taxonomic identification of each element. * = taxonomic revisions based on Adrian et al., 2019 .

Field Number ( Noto et al., 2012 )	DMNH Specimen Number	Element	Taxonomic Notes
UTA-AASO-125	2013-07-0358	Femur	Protohadros byrdi
UTA-AASO-201	2013-07-0521	Femur	Naomichelys sp. *; previously identified as ornithopod
UTA-AASTL-001	2013-07-0783	Right bridge	“ Trinitichelys” maini *
UTA-AASTL-002	2013-07-0704	Right bridge	“ Trinitichelys” maini *
UTA-AASTL-003	2013-07-0696	Right bridge	“ Trinitichelys” maini *
UTA-AASTL-008	2013-07-0560	Anterior peripherals	“ Trinitichelys” maini *
UTA-AASTL-012	2013-07-0710	Bridge	“ Trinitichelys” maini *
UTA-AASTL-013	2013-07-0690	Xiphiplastron	“ Trinitichelys” maini *
UTA-AASTL-014	2013-07-0675	Carapace	“ Trinitichelys” maini *
UTA-AASTL-015	2013-07-0687	Carapace	cf. “ Trinitichelys ”*
UTA-AASTL-016	2013-07-0685	Shell fragment	cf. “ Trinitichelys ”*
UTA-AASTL-020	2013-07-0686	Costal	Naomichelys sp. *
UTA-AASTL-025	2013-07-0568	Carapace	“ Trinitichelys” maini *
no number (associated with 012)	2013-07-0710	Shell fragment	“ Trinitichelys” maini *
UTA-AASTL-007	2013-07-0711	Hyoplastron	Naomichelys sp. *
UTA-AASTL-009	2013-07-0561	Carapace
UTA-AASTL-005	2013-07-0558	Posterior peripheral	Naomichelys sp. *
UTA-AASTL-006	2013-07-0559	Left costal 7	Naomichelys sp. *
UTA-AASTL-024	2013-07-0567	Shell fragment	Naomichelys sp. *

TABLE 2. µCT scanning parameters for specimens included in this study.

Specimen	kV	µA	Filter	Exposure Time	Voxel size
DMNH 2013-07-0563	205	322	0.75 copper	267 ms	0.06622 mm
DMNH 2013-07-0567	210	288	0.75 copper	267 ms	0.06126 mm
DMNH 2013-07-1319	210	278	0.75 copper	267 ms	0.05895 mm
UTK 2317	150	567	0.25 copper	267 ms	0.09242 mm
UTK 1844	145	684	0.25 copper	267 ms	0.09980 mm
SAAF unnumbered	150	688	0.25 copper	267 ms	0.10399 mm

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 . TABLE 3. Measurements of unmodified and modified shell regions. Cortical bone thickness = average of unmodified region of bone immediately to the left and right of the modified region, taken from µCT slices representing the region of maximum mark penetration into the cortical bone. Pathology thickness = thickness of the observed cortical bone at the point of maximum mark penetration. Note, measurements of 0 are recorded when trabecular bone is exposed in the bottom of the mark, measurements of 0* represent specimens in which the affected bone has fractured, leaving a gap in the bottom of the mark. Maximum depth = measured relative to the projected curvature of the surface of the shell, as suggested by the unmodified bone to the immediate to the left and right of the modified region, taken from CT slices representing the region of maximum mark penetration into the cortical bone. Maximum width = the widest diameter of the modified shell region.

Specimen Number	Cortical Bone Thickness (μm)	Pathology Thickness (μm)	Maximum Depth (μm)	Maximum Width (μm)
SAAF	45288.06	56951.04	172647.04	243635.08
29130.635	62399.93	111264.2	382252.95
32896.835	54984.73	268506.27	391349.41
51405.835	0*	0*	178124.16
UTK 1844	24245.115	0	30261.9	109062.13
25331.58	0	43635.5	157751.88
25386.99	0	52452.4	184845.27
UTK 2317	45288.21	0	104220.47	431636.55
41977.59	0	116430.76	339495.79
42773.075	0	120013.43	283238.1
42092.27	0	158822.71	377231.37
DMNH 0563	97228.65	0	18505.9	64807.59
121232.02	0	32351.63	76673.77
112778.42	0	30624.77	125514.26
DMNH 0567	81907.24	84583.98	84583.98	257895.4
83234.265	84774.67	36879.82	84774.67
88745.795	83148.73	28367.22	86037.54
DMNH 1319	139768.74	77083.42	80875.33	201561.55
107567.655	37105.23	123479.95	463813.03
120727.92	45855.25	77271.17	468758.1

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 ). 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. 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.