Joyce et al.—New species of trionychid from the Late Cretaceous of New Mexico 92(6):1107–1114
extended sinusoidal ridges or grooves, costal ribs that protrude beyond the margin of the carapacial disk, a preneural, distally constricted costal I and distally expanded costal II, two lateral hyoplastral processes, only slightly developed hyoplastral shoulders, and full midline contact of the elongate xiphiplastra. It is therefore not surprising that our phylogenetic analysis retrieves them as immediate sisters (see the following). A small number of systematic differences are nevertheless apparent: the Maastrichtian material is consistently larger, costals II are more expanded distally, the xiphiplastra are more elongated, and most significantly, the sinusoidal ornamentation is developed in the form of grooves, not elevated ridges. We find the available character evidence to be sufficient to establish a new species for the Campanian morphotype that we herein name Gilmoremys gettyspherensis. The comparisons made in the preceding highlight that
Gilmoremys gettyspherensis n. sp. is the only known named species from the Late Cretaceous of North America to have surface sculpturing consisting of raised ridges. The only other previously reported material with such ridges has been reported from the Campanian of New Mexico (Plastomeninae indet. of Sullivan et al., 2013) and Utah (Trionychidae indet. type B of Hutchison et al., 2013). The most informative figured specimen from Utah is a left costal VIII (Hutchison et al., 2013, fig. 13.17c) that corresponds fully with that of the holotype of Gilmoremys gettyspherensis n. sp. by being wider than long and exhibiting well-developed raised ridges. The previously described specimen from New Mexico, recovered from the lowermost horizons of the Kirtland Formation (Hunter Wash Local Fauna) is a partial shell. The preserved costals once again exhibit well-developed raised ridges (Sullivan et al., 2013, fig. 20.17a, b). The preserved portion of the hyo/hypoplastra (Sullivan et al., 2013, fig. 20.17c, d) are consistent with the holotype of Gilmoremys gettyspherensis n. sp. in regard to sculpturing, but the hyoplastral shoulder is better developed. As this specimen appears to be slightly larger than the holotype, this difference can be attributed to a greater ontogenetic age. We therefore refer the previously described material from New Mexico and Utah to Gilmoremys gettyspherensis n. sp. with confidence.
Phylogenetic relationships.—Our phylogenetic analysis resul- ted in six equally parsimonious trees. The relationships among plastomenid soft-shelled turtles are fully resolved in the strict consensus tree (Fig. 3), highlighting that the six equally par- simonious trees did not reveal any topological conflict within this clade. The topology generally resembles those of previous analyses (e.g., Joyce and Lyson, 2011, 2017; Joyce et al., 2016) with exception of a more derived placement of Atoposemys superstes (Russell, 1930) as the sister to the Helopanoplia/ Hutchemys clade and placement of Gilmoremys getty- spherensis n. sp. as sister to Gilmoremys lancensis. A literal interpretation of the tree implies that at least three plastomenid taxa existed during the Campanian: Aspideretes foveatus, Gil- moremys gettyspherensis, as the ancestral lineage of the clade formed by Atoposemys superstes, Helopanoplia distincta, Hutchemys spp., and Plastomenus thomasii (Cope, 1872). This number is certainly too conservative as an additional, unnamed plastomenid has been reported from the late Campanian of
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Utah (Helopanoplia sp. of Hutchison et al., 2013) and as some previously described species from the Campanian of North American exhibit plastomenid characteristics, in particular ‘Trionyx’ (originally Plastomenus) robustus (Gilmore, 1919). We therefore infer with confidence that the group originated prior to the late Campanian. The hypothesized sister group relationship between the
Campanian Gilmoremys gettyspherensis n. sp. and the Maas- trichtian Gilmoremys lancensis is not surprising, as the morphology of these two species corresponds broadly. As the Campanian morphotype does not exhibit any characters that must be inferred to be apomorphic, it is possible to conclude that it may represent the ancestral morphology of the Maastrichtian morphotype and that the two form a single, persistent anagenetic lineage. This assertion can be tested in the future by finding both morphotypes in contemporaneous sediments. It is relatively easy to establish the presence of Gilmoremys
gettyspherensis n. sp. in a fauna, as the shells of adults are decorated by sinusoidal ridges. Although the study of turtle remains from the Campanian of New Mexico and Utah is still in its infancy, we here document multiple specimens referable to Gilmoremys gettyspherensis n. sp. Although the fossil turtle faunas of more northern basins of the Western Interior, in particularly those in Alberta and Montana, have been sampled heavily and are much better understood (see Brinkman, 2005 for a summary), we are unaware of any fragments of Gilmoremys gettyspherensis n. sp. reported from those localities. We therefore conclude that this taxon was restricted to the southern basins during the late Campanian. The majority of plastomenid remains are recovered from
siltstone and mudstones and the group is therefore often believed to have preferred ponded environments (e.g., Joyce and Lyson, 2017). The discovery of the type specimen of Gilmoremys gettyspherensis n. sp. in a mudstone once again confirms this conclusion.
Acknowledgments
We thank the late M. Getty (DMNH) for finding and collecting the holotype specimen. Special thanks are due to the late
D. McCuan,DMNHvolunteer, for preparation of the fossil. Our fieldwork in the San Juan Basin has been generously supported by several anonymous donors to the DMNH Laramidia Project. Immense gratitude is extended to the large volunteer corps at
DMNH, who contributed thousands of hours of work in the field, laboratories, and collections resulting, among others, in this publication. Thanks also to BLM Regional Paleontologist P. Gensler and BLM Field Office Paleontologist S. Landon for their logistical support. The holotype specimen described herein was collected under BLM permit NM14-04S. P. Meylan is thanked for providing thoughtful comments that helped improve the quality of the manuscript.
Accessibility of supplemental data
Data available from the Dryad Digital Repository: https://doi. org/10.5061/dryad.7h9p04r.
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