1108
Journal of Paleontology
formations for more than a century. These remains have tradi- tionally been grouped into the ‘Hunter Wash Local Fauna,’ defined as the fossiliferous horizons in the upper 12.2 meters of the Fruitland Formation and the lower 16.8 m of the Kirtland Formation in theHunterWash area (Clemens, 1973; Sullivan and Lucas, 2003, 2006). This definition, however, is dependent on the recognition of a clear interformational contact. Considerable disagreement persists over the nature and stratigraphic location of the contact between the Fruitland andKirtland formations. As first defined, the Fruitland/Kirtland contact was recognized as grada- tional, though the Fruitland Formation was observed to be typi- cally sandier than the overlying Kirtland Formation (Bauer, 1916). Later work by Fassett and Hinds (1971) examined the contact in outcrop and subsurface logs across the San Juan Basin and defined the contact as the top of the highest coal or carbonaceous shale. This definition was largely followed by later work (Fassett, 2000, 2010), though the gradational nature of the contact between the lower shale member of the Kirtland formation (‘Hunter Wash Member’ of Hunt and Lucas, 1992) and the upper Fruitland For- mation make the definition geologically useless, with the only easily definable contact at the base of the overlying Farmington Sandstone Member (‘Farmington Member’ of Hunt and Lucas, 1992). Other attempts to define the contact (e.g., Hunt and Lucas, 1992; Lucas et al., 2006) have placed the top of the Fruitland at the base of the ‘BistiMember’ or ‘BistiBed,’ an indurated but laterally discontinuous sandstone horizon. Regardless of the definition used, the Gilmoremys gettyspherensis type locality (DMNH loc. 5812) is stratigraphically below the Bisti Bed by at least 10 m and falls comfortably within the Fruitland Formation, within the recognized horizons of the Hunter Wash Local Fauna. The locality is likely bracketed by dateable ash horizons,
one in the lower horizons of the Fruitland Formation just above its contact with the underlying Pictured Cliffs sandstone (‘Dog Eye Pond’ [DEP] ash of Fassett and Steiner, 1997) and the other in the lower Kirtland Formation (Ash 2 of Fassett and Steiner, 1997). The originally published dates for both of these ashes, 75.56 ± 0.41 Ma and 74.56 ± 0.13 Ma, respectively (Fassett and Steiner, 1997), have more recently been recalculated using updated standards. Interestingly, these recalculations are in marked disagreement: DEP = 75.16 ± 0.41 Ma and Ash 2 = 74.17 ± 0.13 Ma (Roberts et al., 2013); DEP = 76.029 ± 0.41 Ma and Ash 2 = 75.023 ± 0.13 Ma (Fowler, 2017). Refining the geochronology of the Fruitland Formation through identifi- cation and dating of newash beds, and redating known ash beds, is imperative for the comparison of the Hunter Wash Local Fauna with other Western Interior Basin faunas. Regardless of the precise date, all published and recalculated ages firmly place the type locality in the late Campanian. The holotype specimen was recovered from a blocky, car-
bonaceous mudstone horizon likely representing a ponded, overbank depositional environment located in San Juan County, New Mexico, USA (Fig. 1). Disarticulation of the carapace and plastron, accompanied by the vertical displacement and irre- gular orientation of recovered elements at the site, suggests soft sediment bioturbation typical of shallow pond or slow water environments on a fluvial floodplain. It is likely the postmortem remains settled to the bottom of a low-energy, shallow-water environment and experienced in situ disarticulation with negli- gible transport.
Materials and methods
All materials used herein were collected over the course of the past decades by independent teams from several institutions from Upper Cretaceous (Campanian) sediments exposed in New Mexico and Utah. Previously published materials from New Mexico (Sullivan et al., 2013) and Utah (Hutchison et al., 2013) are housed at the State Museum of Pennsylvania and Utah Museum of Natural History, respectively. New material pre- sented herein is housed at the Denver Museum of Nature and Science.
Repositories and institutional abbreviations.—Types, figured, and other specimens examined in this study are deposited in the following institutions: Denver Museum of Nature and Science (DMNH), Denver, Colorado, USA; State Museum of Pennsyl- vania (SMP), Harrisburg, Pennsylvania, USA; Utah Museumof Natural History (UMNH), Salt Lake City, Utah, USA.
Systematic paleontology
Testudinata Batsch, 1788, sensu Joyce et al., 2004 Trionychidae Gray, 1825, sensu Joyce et al., 2004 Gilmoremys Joyce and Lyson, 2011
Type species.—Aspideretes lancensis Gilmore, 1916.
Gilmoremys gettyspherensis new species Figures 2, 3
Holotype.—DMNH EPV.125905, a partial, disarticulated shell consisting of right costals I–III, V, VI, and VIII, left costals V, VII, and VIII, the left half of the entoplastron, the right hypo- and xiphiplastron, and the left hyo-, hypo-, and xiphiplastron (Figs. 2, 3); DMNH loc. 5812, North Escavada Study Area, San Juan County, New Mexico, USA (Fig. 1); Fossil Forest Mem- ber, Fruitland Formation, late Campanian, Late Cretaceous.
Diagnosis.—Gilmoremys gettyspherensis n. sp. can be diag- nosed as a representative of Pan-Trionychidae, among other characters, by the absence of peripherals, pygals, and scutes, and as a representative of Gilmoremys by a relatively thin shell; carapacial sculpturing consisting of fine pits combined with extended sinusoidal ridges or grooves that cross the length of several carapacial elements; costal ribs that protrude beyond the margin of the carapacial disk; the inferred presence of a pre- neural; a distally constricted costal I and distally expanded costal II; two lateral hyoplastral processes; low hyoplastral shoulders (i.e., anteriorly protruding lappets); and full midline contact of the elongate xiphiplastra. Gilmoremys getty- spherensis n. sp. can be differentiated from Gilmoremys lan- censis by smaller size; the presence of raised sinusoidal ridges, not grooves; less distally expanded costal II; and less elongate xiphiplastra.
Occurrence.—San Juan County, New Mexico, USA, Fossil Forest Member, Fruitland Formation, late Campanian, Late Cretaceous (type occurrence); San Juan County, New Mexico,
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