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Journal of Paleontology
but ‘Trionyx’ allani differs substantially by being about twice as large, having a coarsely netted carapacial sculpturing, possessing open suprascapular fontanelles, and exhibiting substantially smaller plastral callosities that lack a surface ornamentation. Gilmoremys gettyspherensis n. sp. resembles ‘Trionyx’
latus (as described by Gardner et al., 1995) in overall size and number of neural elements, but ‘Trionyx’ latus differs by lacking a sutural contact between the nuchal and the remaining carapacial disk, exhibiting well-developed suprascapular fonta- nelles, lacking a preneural, possessing a finely knit carapacial surface sculpturing that is not developed along the margins of the shell, and having significantly smaller costals VII and VIII. Gilmoremys gettyspherensis n. sp. resembles ‘Trionyx’
robustus (as described by Gilmore, 1919) in overall carapace outline, presence of a preneural, and proportions of costals VIII, but ‘Trionyx’ robustus differs by being about twice as large, being thick shelled, having a carapacial surface sculpturing consisting of deep, rounded pits, and having much better developed hyo/hypoplastral callosities that fully cover the medial and lateral processes. The morphology of ‘Trionyx’ austerus (as described by
Figure 4. Time-calibrated strict consensus cladograms retrieved from the phylogenetic analysis performed herein. The full tree is provided in Supplementary data set 2.
Discussion
Alpha taxonomy.—Although a total of 14 fossil trionychid species have been named from Campanian sediments exposed throughout North America, only six are currently recognized as valid (Gardner et al., 1995; Vitek, 2012; Sullivan et al., 2013; Vitek and Joyce, 2015): Aspideretoides foveatus (Leidy, 1857), Axestemys splendidus (Hay, 1908), ‘Trionyx’ allani (Gilmore, 1923), ‘Trionyx’ austerus (Hay, 1908), ‘Trionyx’ latus (Gil- more, 1919), and ‘Trionyx’ robustus (Gilmore 1919). Gilmoremys gettyspherensis n. sp. resembles Aspidere-
toides foveatus (as described by Gardner et al., 1995; Brinkman, 2005) in overall size, outline of the carapace, and presence of a preneural, but Aspideretoides foveatus differs, among other characters, by being thicker shelled and by possessing a carapacial surface texture consisting of coarse pits that fade toward the midline, fewer neurals, smaller costals VIII, a broader entoplastron, only a single lateral hyoplastral process, and only poorly developed xiphiplastral callosities. Gilmoremys gettyspherensis n. sp. resembles Axestemys
splendidus (as described by Gardner et al., 1995) in carapacial outline, presence of a preneural, proportions of costals VIII, and the overall arrangement of the hyo-, hypo, and xiphiplastra. However, Axestemys splendidus differs by being about twice as large, by being thicker shelled, by having a coarsely netted carapacial surface ornamentation, fewer neurals, and less elongated xiphiplastra, and by lacking hyoplastral shoulders. Gilmoremys gettyspherensis n. sp. resembles ‘Trionyx’
allani (as described by Gardner et al., 1995) in the overall arrangement of the carapace, including presence of a preneural,
Hay, 1908) is still poorly characterized, but this taxon can nevertheless be easily distinguished from Gilmoremys getty- spherensis n. sp. by being much larger and extremely thick shelled.
These comparisons highlight that Gilmoremys gettyspher-
ensis n. sp. can be distinguished easily from all previously named Campanian taxa from North America. Although up to eight fossil trionychids may compose the
Maastrichtian trionychid assemblage of North America (Hol- royd et al., 2014), only six are currently recognized by name (Vitek and Joyce, 2015): Axestemys splendida (Hay, 1908), Gilmoremys lancensis (Gilmore, 1916), Helopanoplia distincta Hay, 1908, Hutchemys sterea (Hutchison, 2009), Hutchemys tetanetron (Hutchison, 2009), and ‘Trionyx’ beecheri Hay, 1904. The two named Maastrichtian species of Hutchemys (as described by Hutchison, 2009) are easily distinguished from Gilmoremys gettyspherensis n. sp., among others, by being smaller and thicker shelled, by exhibiting a pitted shell sculpturing, and, most significantly, by possessing well- developed hyoplastral shoulders that suturally articulate with well-developed entoplastral callosities. Helopanoplia distincta (as described by Joyce and Lyson, 2017) most readily differs from Gilmoremys gettyspherensis n. sp. by being larger, by lacking free rib ends for costal ribs I–VI, by exhibiting a carapacial and plastral surface sculpturing that ranges from netting to isolated tubercles, by only possessing only a single lateral hyoplastral process and much more broadly developed hyo-, hypo-, and xiphiplastral callosities. Differences with Axestemys splendida are already listed. ‘Trionyx’ beecheri (as described by Hay, 1908; personal observation of the type specimen) finally differs by possessing a carapacial surface
texture consisting of fine netting, a plastral texture consisting of fine raised tubercles, and large hyo/hypoplastral callosities that cover the lateral processes completely. Gilmoremys gettyspherensis n. sp. broadly overlaps in
morphology with Gilmoremys lancensis (as described by Joyce and Lyson, 2011): both species have a relatively thin shell, carapacial sculpturing consisting of fine pits combined with
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