Joyce et al.—New species of trionychid from the Late Cretaceous of New Mexico 92(6):1107–1114


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Figure 1. Location of DMNH loc. 5812, type locality of Gilmoremys gettyspherensis n. sp. (1) Overview of surface exposures (green) of the upper Campanian Fruitland and Kirtland formations in the San Juan Basin of northwestern New Mexico; (2) approximate placement of DMNH loc. 5812 (star) in the upper horizons of the Fruitland Formation in the North Escavada Study Area.


USA, Hunter Wash Member, Kirtland Formation, late Campa- nian, Late Cretaceous (Plastomeninae indet. of Sullivan et al., 2013, fig. 20.17); Kane and Garfield County, Utah, USA, Kai- parowits Formation, late Campanian, Late Cretaceous (Trio- nychidae type B of Hutchison et al., 2013, fig. 13.17).


Description.—The carapace of DMNH EPV.125905, the type specimen, includes right costals I–III, V, VI, and VIII, left costals V, VII, and VIII (Fig. 2). All elements with the exception of the nuchal, the preneural, the neurals, and costals IV are therefore present. The available elements indicate that the out- line of the carapace was rounded, with the exception of a broad but shallow pygal notch formed by costals VIII. Although the costal rib is only completely preserved for right costal VI, well- developed breaks along the distal ends of all other preserved costals indicate that all costal ribs would have protruded sig- nificantly beyond the margins of the carapacial disk. We esti- mate the carapacial disk to have been about 22.5 cm along the midline, including the nuchal. The costals vary from about 3.5mmin thickness toward the midline to about 4.5mmtoward their margins, not including the ribs. The margins of the costals are rounded and show no evidence for ‘splitting’ (i.e., separation of surficial and deepest layers of the costal by a trough). The carapace is primarily decorated by a surface pattern consisting of blunt, circular depressions that fade toward the midline. In addition, the carapace is decorated by a series of sinuous ridges that are arranged parallel to the midline and span the length of three to four costals. The ridges are faint and few on costals I, but they become increasingly dense and distinct toward the back of the carapace. The nuchal is not preserved, but its preserved sutural


attachment site with right costal I suggests that the metaplastic portion of the nuchal formed a callosity about half the width of the carapacial disk and that suprascapular fontanelles were absent. The neural series, including the preneural, is not preserved in DMNH EPV.125905, but the medial margins of the available costals provide enough information to allow reconstruction of the most important aspects. The presence of two facets along the medial edge of costal I indicate the presence


of a preneural, which was shorter than neural I. The width of this bone is unknown. Costals II, V, and VI have two facets as well, of which the anterior is shorter than the posterior one. This allows the inference that neurals I–VI were hexagonal and had short posterior sides. The medial margins of costal VII are unfortunately damaged and it is therefore unclear whether neural VII was square (i.e., laterally only contacted costals VII) or hexagonal with short posterior sides. The anteromedial margins on both costals VIII clearly hint at the former presence of a small neural VIII. There is an apparent gap between the anterior three and posterior four available costals, and DMNH EPV.125905 can therefore be reconstructed with confidence to have had eight pairs of costals. Costals I are slightly bowed to the anterior and narrower distally than proximally. Costals II mediate between the anteriorly oriented costals I and the straight costals III and are therefore expanded distally. Costals III, V, VI, and likely costal IV are similar in proportions, expand only slightly distally, and mediate between the anteriorly directly anterior costals and the posteriorly oriented posterior costals. Costals VII are clearly directly posterolaterally, have similar anteroposterior dimensions to the more anterior costals, but are noticeably shorter. Costals VIII are slightly broader than long, have the greatest anteroposterior dimensions of the costal series, contact one another along the midline for much of their anteroposterior length, and frame the shallow pygal notch. Nothoracic vertebrae are preserved inDMNHEPV.125905.


The ribs of all available costals are greatly eroded, with the exception of right costal VI, which still preserves much of the distal end of its rib. It appears all but certain by reference to the available scars that all costal ribs extended significantly beyond the margin of the carapacial disk. Whereas costal rib VII clearly did not underlap costal VI, costal rib VIII may have underlapped costal VII. The plastron of DMNH EPV.125905 includes the left half


of the entoplastron, the right hypo- and xiphiplastron, and the left hyo-, hypo-, and xiphiplastron (Fig. 3). The entoplastron is greatly damaged medially, and it is therefore not possible to assess the original shape of this bone with confidence. We here nevertheless conclude by reference to other trionychids with an


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