Triplopus implicatus (Cope, 1873)

Murphey, Paul C. & Kelly, Thomas S., 2017, Mammals from the earliest Uintan (middle Eocene) Turtle Bluff Member, Bridger Formation, southwestern Wyoming, USA, Part 2: Apatotheria, Lipotyphla, Carnivoramorpha, Condylartha, Dinocerata, Perissodactyla and Artiodactyla, Palaeontologia Electronica (Cambridge, England: 2003) 20 (2), pp. 1-51 : 41-43

Triplopus implicatus 17, 28

Helohyus sp. 28

Leptoreodon pusillus 19

from the TBM probably represent new species restricted to the TBM ( Kelly and Murphey, 2016a; this paper), and thus potentially additional Ui1a index species, but assigning formal names to them has been deferred until additional material of each is available to better characterize them. Along with the above species, the first appearances of Metanoiamys , Triplopus and Epihippus occur in the TBM, genera that are generally regarded as having their first appearances in the Uintan (e.g., Prothero, 1998b; Flynn, 2008; MacFadden, 1998; Robinson et al., 2004; Gunnell et al., 2009). Assuming our referral of the brontothere from high in the TBM section is correct, the first appearance of Wickia also occurs in the TBM. Although the marsupials of the TBM have not yet been formally described, four taxa are present, which we will document in a future report. Currently, at least 26 mammal species in the TBM can be recognized as holdovers from lower in the Bridger Formation of which 10 are range-through species into Ui1b or later ( Table 5). In summary, the mammals of the TBM include at least five index species (probably more), the first appearances of five genera and at least 15 species, and at least 26 species that are holdovers from lower in the Bridger Formation.

Flynn (1986) proposed a new subage, the Shoshonian, for the earliest Uintan based on transitional faunas primarily from the Bone Bed A (Horizon D) of the type section of the Tepee Trail Formation of Wyoming and what he regarded as correlative faunas from the lower Adobe Town Member of the Washakie Formation of Wyoming and the Friars and Mission Valley Formations of San Diego County, California. The Shoshonian has been controversial and has not been accepted by most recent investigators (e.g., Walsh, 1996a; Robinson et al., 2004). Robinson et al. (2004) recognized a single biochron for the early Uintan, Ui1. Subsequently, Gunnell et al. (2009) proposed dividing the Ui1 into two biochrons, Ui1a and Ui1b. Gunnell et al. (2009) regarded the fauna from the TBM as representing the oldest transitional Uintan interval and assigned it and the Basal Tertiary Local Fauna of the Devil's Graveyard Formation of Texas along with possibly all of the informal unit A of the eastern Uinta Basin and the lower part of unit B of the western Uinta Basin of Uinta Formation, Utah, to biochron Ui1a. Gunnell et al. (2009) did not designate a stratotype section for biochron Ui1b, but referred the following sections to it: 1) the Washakie Formation of the Sand Wash Basin of Colorado; 2) the middle unit of the Adobe Town Member of the Washakie Formation of Wyoming; 3) the lowermost Tepee Trail Formation of the East Fork Basin of Wyoming; 4) the Friars Formation and the upper part of Member B of the Santiago Formation of southern California; 5) the portion of the lower member of the Devil's Graveyard Formation yielding the Whistler Squat Local Fauna of Texas; and 6) tentatively all or a portion of the informal units A and B1 of the Uinta Formation of the eastern part of the Uinta Basin and the lowermost part of unit B in the western part of the Uinta Basin, Utah. Thus, the areas referred by Flynn (1986) to the Shoshonian were subsumed into the Ui1a and Ui1b of Gunnell et al. (2009).

The Ui1a is now better characterized with at least five index species, and probably more, along with at least 15 taxa with first appearances in the stratotype section of the TBM ( Table 5). However, the fauna of the TBM also includes a large number of Bridgerian holdover taxa and is lacking certain characteristic later Uintan taxa including Amynodon Marsh, 1877 , and seleonodont artiodactyls ( Gunnell et al., 2009; Campisano et al., 2014). These facts strongly support Gunnell et al.'s (2009) recognition of an earliest Uintan Ui1a biochron for the stratotype section of the TBM.

Wilson (1986) documented the faunas from the Devil's Graveyard Formation (DGF) of Texas. Two local faunas are recognized from the lower member of the DGF; the Basal Tertiary Local Fauna (Ui1a) from Junction (TMM Locality 41443), 0.6 miles east of Junction (TMM Locality 41444) in the basal Tertiary conglomerate and the laterally equivalent Hen Egg Mountain localities in the lower member of the DGF; and the Whistler Squat Local Fauna (Ui1b) from stratigraphically higher in the lower member of the DGF ( Wilson, 1986; Walton, 1992; Gunnell et al., 2009; Campisano et al., 2014). Recently, Campisano et al. (2104) reevaluated the geochronology and taxonomy of the fauna from the Whistler Squat Quarry wherein they provided high precision 40 Ar/ 39 Ar dates of 45.04 ± 0.10 Ma for the lower tuff and 44.88 ± 0.04 Ma for upper tuff at the quarry. They also noted that the Whistler Squat Quarry is ~2 Ma younger than the Hen Egg Mountain localities based on an 40 Ar/ 39 Ar date of 46.80 ± 0.08 Ma (corrected, see also Miggins, 2009) for a basalt that overlies the localities on the southeast side of Hen Egg Mountain.

Gunnell et al. (2009) regarded the fauna from the basal Tertiary conglomerate (= Basal Tertiary Local Fauna of Walton, 1992) as correlative with the TBM fauna and assigned it to biochron Ui1a. Similar to the TBM fauna, the Basal Tertiary Local Fauna also contains a large number of Bridgerian holdover taxa and is lacking Amynodon ( Table 5). However, there are minor faunal differences between them. The TBM fauna contains a larger number of typical Bridgerian hold over taxa and has a proportionally smaller number of typical Bridgerian taxa that range-through to the Ui1b or later. Although Wilson (1984) referred three teeth from the basal Tertiary conglomerate at TMM Locality 41443 to the selenodont artiodactyl Leptoreodon pusillus, Gunnell et al. (2009) did not include it in their account of the Ui1a taxa. After years of collecting in the TBM, no seleondont artiodactyls have been discovered, which could be a result of paleoenviromental/geographical factors or the possibility that they had not yet evolved. In addition, cf. Pareumys sp. from the TBM appears to be slightly less derived than Pareumys boskeyi of the Basal Tertiary and Whistler Squat local faunas and could be ancestral to P. boskeyi ( Kelly and Murphey, 2016a). The above facts suggest the possibility that the TBM fauna could be slightly older than the Basal Tertiary Local Fauna.

Murphey et al. (1999) reported an 40 Ar/ 39 Ar date of 47.13 ± 0.45 Ma (corrected) for a tuff that occurs 8 m below the base of the TBM (= the Basal E tuff of Murphey and Evanoff, 2007; see also Smith et al., 2003). Smith et al. (2008) provided an 40 Ar/ 39 Ar date of 47.45 ± 0.15 Ma (corrected, see Smith et al., 2010; Tsukui, 2016) for the Sage Creek Mountain tuff (SCMT), reported to occur in the TBM at about 44 m above the base (= Basal E limestone) and 14 m below the unconformably overlying Oligocene Bishop Conglomerate. Kelly and Murphey (2016a) incorrectly stated that the SCMT of Smith et al. (2008) was equivalent to the Basal E tuff of Murphey and Evanoff (2007). Based on four single-grain zircon analyses, Tsukui (2016) provided 206 Pb/ 238 U dates for the SCMT at 47.192 ± 0.068, 47.287 ± 0.038, 47.344 ± 0.041, and 47.381 ± 0.057 Ma. Because of the small sample size of zircons (four) whose population could not be rigorously defined, Tsukui (2016) selected the youngest single zircon analysis as the best approximation of the depositional age for the SCMT. Tsukui and Clyde (2012) determined that the SCMT is of normal magnetic polarity, which they assigned to Chron C21n of the Global Polarity Time Scale. As noted above, a basalt overlying the Hen Egg Mountain localities, which are laterally equivalent to the basal Tertiary conglomerate localities in the DGF, was 40 Ar/ 39 Ar dated at 46.80 ± 0.08 Ma. The new 40 Ar/ 39 Ar date and 206 Pb/ 238 U dates for the SCMT ( Smith et al., 2008, 2010; Tsukui, 2016) indicate an age of about 47.45– 47.19 Ma for the TBM fauna, which also suggests that it might be slightly older than the Basal Tertiary Local Fauna.

Additional data on the geochronology of the TBM are being analyzed and will be presented in our third report on the mammals from the TBM, including magnetostratigraphy and 206 Pb/ 238 U dating of the "tuffaceous white sandstone bed" and DMNH Locality 4673 ( Figure 1 ) in the stratotype section on Cedar Mountain. These new data will further clarify the age of the TBM fauna relative to that of Basal Tertiary Local Fauna and the Bridgerian-Uintan transition. Until then, we follow Gunnell et al. (2009) and regard the TBM fauna and its correlative, the Basal Tertiary Local Fauna, as the principal faunas characterizing the Ui1a.

Figures

FIGURE 1. Schematic stratigraphic column of type section Turtle Bluff Member on southwest flank of Cedar Mountain, Bridger Formation, showing relative stratigraphic positions of SDSNH, DMNS and UCM localities (along with the locality names in parentheses) that yielded the fossils described in this paper (reproduced from Kelly and Murphey, 2016a, figure 1). MPM Locality 2970 occurs on southwest flank of Sage Creek Mountain, and its approximate relative stratigraphic position is projected onto type section. Abbreviations are: Lithostr., lithostratigraphic; NALMA, North American Land Mammal age.