High-resolution X-ray computed tomography (CT) scans were obtained for the cranium of UMZC T1041, which is the holotype of Mesochelys durlstonensis (Evans & Kemp, 1975 ). UMZC T1041 was found in the Early Cretaceous (Berriasian) Purbeck Limestone Group in Durlston Bay, United Kingdom (Evans & Kemp, 1975 ). Gaffney & Meylan (1988) referred the material to Pleurosternon bullockiiOwen, 1842 , albeit without justification. Milner (2004) compared the shell remains associated with the cranium of Mesochelys durlstonensis with those of Pleurosternon bullockii, and presented anatomical evidence for the synonymy suggested by Gaffney & Meylan (1988) . This has generally been accepted ever since (see review of Joyce & Anquetin (2019) (link)). Scans of UMZC T1041 were obtained by Roger Benson in 2017 at the Cambridge Biotomography Center, using a X-Tek H 225 µCT scanner (Nikon Metrology, Tring, UK). The cranium was scanned using a beam energy of 130 kV, a current of 250 µA, 500 ms exposure time, 1 frame per 1400 projections, and no filter, resulting in a voxel size of 0.03315 mm. The resulting CT-scans were segmented in the software Mimics (v. 16.0–19.0; http://biomedical.materialise.com/mimics ), and 3D models were exported as .ply files. Figures of digital renderings were compiled using the software Blender v. 2.71 (www.blender.org/ ) . CT-slice data as well as the 3D models are deposited at MorphoSource (Evers, 2020 ).
We mostly compare UMZC T1041 to a selection of likely paracryptodires from the Late Jurassic to Cretaceous of North America and Europe based on description published in the literature, in particular Arundelemys dardeni (as described by Lipka et al., 2006 (link)), Compsemys victa (as described by Lyson & Joyce, 2011 ), Dorsetochelys typocardium (as described by Evans & Kemp, 1976 under the name Dorsetochelys delairi), Eubaena cephalica (as described by Rollot, Lyson & Joyce, 2018 (link)), Glyptops ornatus (as described by Gaffney, 1979a under the name Glyptops plicatulus), and Uluops uluops (as described by Carpenter & Bakker, 1990 ). For comparisons with Dorsetochelys typocardium, we used a 3D model of the holotype (DORCM G.00023) that was made available under a CC-BY-NC-SA license by the “GB3D Type Fossils” projects hosted by the British Geological Survey athttp://www.3d-fossils.ac.uk . The comparisons with Compsemys victa (based on UCM 53971) and Uluops uluops (based on UCM 49223) were complemented by CT scans of these specimens. The original descriptions are cited for all previously described features, but the specimen numbers are cited when novel observations regarding these taxa are made based on these scans. These scans will be further discussed and made public in a forthcoming paper.
We mostly compare UMZC T1041 to a selection of likely paracryptodires from the Late Jurassic to Cretaceous of North America and Europe based on description published in the literature, in particular Arundelemys dardeni (as described by Lipka et al., 2006 (link)), Compsemys victa (as described by Lyson & Joyce, 2011 ), Dorsetochelys typocardium (as described by Evans & Kemp, 1976 under the name Dorsetochelys delairi), Eubaena cephalica (as described by Rollot, Lyson & Joyce, 2018 (link)), Glyptops ornatus (as described by Gaffney, 1979a under the name Glyptops plicatulus), and Uluops uluops (as described by Carpenter & Bakker, 1990 ). For comparisons with Dorsetochelys typocardium, we used a 3D model of the holotype (DORCM G.00023) that was made available under a CC-BY-NC-SA license by the “GB3D Type Fossils” projects hosted by the British Geological Survey at
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