The stratigraphic importance of the brontothere (cf. Diplacodon elatus) in the Brennan Basin Member of the Duchesne River Formation of Utah

We report on the first occurrence of an early horned brontothere in the Brennan Basin Member of the Duchesne River Formation in northeastern Utah. This is the first record of a brontothere from the Brennan Basin Member. Previously, brontotheres have been reported from the higher stratigraphic La Point Member ( Duchesneodus uintensis ) and the lower stratigraphic Uinta Formation ( Sphenocoelus uintensis, Fossendorhinus diploconus, Metarhinus fluviatilis, Metarhinus abbotti, Sthenodectes incisivum, Metatelmatherium ultimum, Protitanotherium emarginatum, Pollyosbornia altidens, Diplacodon elatus ). The recovered specimen consists of an upper third molar, which is comparable to the speciesDiplacodon elatus . The specimen supports the continued presence of brontotheres throughout the deposition of the Duchesne River Formation across the late Uintan to early Duchesnean North American Land Mammal Age. The previous lack of brontotheres within the lower beds of the Duchesne River Formation is likely a result of poor sampling and the relative rarity of fossils from this unit.

Early workers, such as Osborn (1929), viewed the evolution of brontotheres as a stepwise progression, culminating in forms such as the large horned Megacerops.Osborn's progressive gradualistic view of the evolution of brontotheres depended on what Simpson (1961) called Osborn's extreme use of vertical classification and lack of regard for monophyly.Osborn's preconceived idea hinged on his generalized use of stratigraphic information, which was often limited among early fossil collections.But his greatest error was erecting numerous taxonomic names with separate points of origin.Much of Osborn's (1929) brontothere classification has been clarified by recent work of Mader (1989, 1998, 2008) and Mihlbachler (2008). Both Mihlbachler (2008) and Mader (2008) reviewed the systematics of the family, and Mihlbachler (2008) offered a cladistic study of the group clearing up much of the confusing taxonomy of Osborn (1929).Mihlbachler et al. (2004) (Mihlbachler, 2008).Mader (2008) grouped Dolichorhinus within Sphenocoelus, and later Mader (2009b) grouped Fossendorhinus within Metarhinus, while Mihlbachler (2008) grouped Mader's (2000) Pseudodiplacodon and Eotitanotherium within Diplacodon.Despite these differences in the taxonomy of brontotheres, nearly half of brontothere genera known from North America occur during a very narrow time interval during the early Uintan Land Mammal Age (about 46 to 44 million years ago).Rock formations of this age are well exposed in the Uinta Basin of northeastern Utah, the Sand Wash Basin of northwestern Colorado, and the Washakie Basin of southwest Wyoming.This zone of high diversity is followed by a gap in the fossil record during the late Uintan Land Mammal Age (Uinta C2 of Osborn, 1929).During the Duchesnean Land Mammal Age, three genera -Duchesneodus (Peterson, 1931(Peterson, , 1934;;Lucas and Schoch, 1982;De-Blieux et al., 2011) from Utah, Eubrontotherium from Oregon (Mihlbacher, 2007) and Parvicornus occidentalis from southern California (Mihlbacher and Deméré, 2009) -are known after this gap in the fossil record.
In this article we describe the first occurrence of an early horned brontothere in the lower portion of the Brennan Basin Member of the Duchesne River Formation in northeastern Utah.This is the first record of a brontothere from the Brennan Basin Member (see Rasmussen et al., 1999;Kelly et al., 2012, for a fauna list), and supports the continued presence of brontotheres in the Uinta Basin of northeastern Utah during the middle and late Eocene.

Materials and methods
The fossil described in this paper was discovered from an ongoing Utah State University project to sample fossils from the poorly represented middle Eocene Duchesne River Formation in northeastern Utah and to study in detail the mammalian fauna from this previously poorly sampled stratigraphic interval.The specimen described in this article was collected on public land managed by the Bureau of Land Management and deposited at the Utah Field House Museum located in Vernal, Utah.Detailed locality information is on file with the Utah Geological Survey and the Utah Field House Museum.Research was conducted under permit #UT12-001S issued to the senior author.
The recovered fossil specimen (FHPR 11763) is a complete right upper third molar (Fig. 1).The central molar fossa was fractured and repaired with polymer clay, but all the anatomical features of the occlusal surface can be discerned from original recovered material.The tooth features a large W-shaped ectoloph characteristic of brontotheres.The parastyle is prominent on the anterior buccal edge of the tooth, with a slightly bent anterior projection.The paracone and metacone are well preserved, with steep cutting edges on the lingual margins.The mesostyle is long, forming the mid- dle crest of the W-shape.The tooth lacks any distinct labial ribs along the buccal slope of the paracone and metacone.
The lingual heel of the tooth features a well-developed knobby protocone, flanked by two projections on the cingulum representing the hypocone on the posterior side, and a crest on the anterior cingulum, which is nearly, if not as large as, the hypocone.The tooth measures 60.0 mm in length and 57.0 mm in width.
The fossil was discovered in a smectic yellowish brown mudstone located near the head of Antelope Draw, close to the contact between the Uinta and Duchesne River Formations as mapped by Sprinkel (2007).The fossil was recovered below the sandstone interval that forms Glen Bench, a prominent topographic ridge in the eastern Uinta Basin.Thus, the fossil was recovered stratigraphically in the lower part of the Brennan Basin Member of the Duchesne River Formation.Kelley et al. (2012) place the Brennan Basin Member of the Duchesne River Formation in the late Uintan North American Land Mammal Age, and this age assessment seems consistent with fossils recovered from the Brennan Basin Member.This is the first occurrence of a brontothere from the Brennan Basin Member.Previously the upper La Point Member of the Duchesne River Formation was the only unit to produce brontotheres -a number of individuals of Duchesneodus uintensis (Peterson, 1931(Peterson, , 1934;;Lucas and Schoch, 1982).Currently, Duchesneodus uintensis is an index fossil for the later Duchesnean North American Land Mammal Age (Lucas, 1992;Robinson et al., 2004).

Discussion
It is not surprising to find a specimen closely resembling Diplacodon occurring in the Brennan Basin Member of the Duchesne River Formation, given the occurrence of Diplacodon in the upper units of the underlying Uinta Formation (Osborn, 1929).However, this specimen is important in helping to establish a continuous record of brontotheres in North America, and by filling in a previously noted gap in the fossil record (Figs. 3,4).Diplacodon elatus belongs to the monophyletic clade (subtribe Brontotheriina) which includes later Eocene brontotheres such as Megacerops, as well as the earlier Protitanotherium.Brontotheriina appears to have arisen in Asia during the late Uinta B time with a number of taxa entering into North America in the late Uintan Age, but the exchange of genera between the continents likely continued into the upper Eocene (Mihlbachler, 2008).It is interesting to note that the Brontotheriina likely replaced the earlier more primitive brontothere fauna, replacing such genera as Dolichorhinus, Sphenocoelus, Metarhinus, Sthenodectes, and Metatelmatherium, which are known from the stratigraphically lower Uinta Formation, but have not been reported from higher stratigraphic units in the Uinta Basin.This change may reflect a significant ecological shift among the larger mammals during the transitional time between the Uintan to Duchesnean Land Mammal Ages, or it is a factor of poor sampling.Nevertheless, the new specimen is important for refuting any notion of the absence of brontotheres in North America during the late middle Eocene.Rather North America continued to foster brontotheres until their extinction at the end of the Eocene, and continued collection of fossil vertebrates from the Brennan Basin Member of the Duchesne River Formation will lead to a better picture of how the mammalian faunas in North American changed during the second half of the Eocene Epoch.
The Supplement related to this article is available online at doi:10.5194/fr-17-69-2014-supplement.

Figure 1 .
Figure 1.(a) Photograph of location of fossil discovered in situ.(b) Terminology used in the description of the upper molar.(c) Image of the specimen of cf.Diplacodon elatus collected from the Brennan Basin Member of the Duchesne River Formation.See the Supplement.

Figure 2 .
Figure 2. Graphs showing length and width of the upper third molar of brontothere species.Data come from published measurements from Mihlbachler (2008) andMihlbacher and Deméré (2009).Blue diamond represents length and width of the recovered specimen from the Brennan Basin Member.All units in millimeters.

Figure 4 .
Figure 4. Stratigraphic position of brontotheres in northeastern Utah.Measure section of Uinta Formation based on Townsend et al. (2006).

Published by Copernicus Publications on behalf of the Museum für Naturkunde Berlin.
advocated grouping late Eocene brontotheres from the Chadronian into two species of Megacerops, and a single species of Protitanops, but still left 18 genera of brontotheres in North America during the middle Eocene