Hyopsodus lepidus Matthew, 1909

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 : 26-29

Hyopsodus lepidus Matthew, 1909

Figure 11.2-3 , Table 3

Referred specimens. From UCM Locality 92189: RM 1 or 2, UCM 67956; Rm1 or 2, UCM 67958.

Description. The two molars from the TBM have typical hyopsodontid morphology. The upper molar (M1 or 2) has a quadrate occlusal outline with a subselenodont paracone and metacone that are connected centrally by a strong centrocrista. The hypocone is robust and only slightly smaller than the protocone with these cusps connected labially by a cristid. A distinct parastyle, protoconule, and metaconule are present. The anterior and posterior cingula are well developed. The ectocingulum is distinct and shelf-like, extending between the bases of the paracone and metacone.

The m1 or 2 has a rectangular occlusal outline. The trigonid is two-cusped (paraconid lacking), consisting of a conical protoconid and metaconid that are obliquely orientated and connected both anteriorly and posteriorly by a paracristid and postprotocristid, respectively. The entoconid and hypoconid are large and slightly obliquely orientated to one another. The hypoconulid is relatively large and positioned along the posterior border of the talonid, slightly lingual of its

PALAEO- ELECTRONICA.ORG center. A moderately tall cristid obliqua extends from the hypoconid apex to terminate on the posterolabial wall of the metaconid. The anterior, labial, and posterior cingulids are joined as a continuous cingulid.

Remarks. Gazin (1968) provided the first comprehensive review of Hyopsodus wherein he recognized five species from the Bridger Formation based on size. West (1979) reevaluated the systematics of Hyopsodus from the Bridger Formation using a stratophenetic analysis ( Gingerich, 1974a, 1974b, 1976) on large, stratigraphically controlled samples, and provided evidence that only three species occur in the formation ( H. paulus Leidy, 1870 [= H. despiciens Matthew, 1909 , and H. marshi Gazin, 1968 ]; H. minusculus Leidy, 1873 ; and H. lepidus Matthew, 1909 ). In West's (1979) phylogeny, H. paulus and H. minusculus are the only species of Hyopsodus present low in the Bridger (Br2, or Bridger B of Matthew [1909]). Hyopsodus minusculus , the smallest species, is not known from higher in the Bridger Formation. However, samples of H. paulus progressively exhibited increased size through the upper Bridger (Br3, or Bridger C-D of Matthew [1909]). Moreover, West (1979) regarded H. paulus to have given rise to the smaller H. lepidus during the upper Bridger (lower part of Bridger C of Matthew [1909]), with the last occurrence of H. lepidus occurring in the lowermost part of Matthew's Bridger D 1.

The two Hyopsodus specimens from the TBM agree well with the dental measurements provided by West (1979, table 9) for H. lepidus . West (1979) also utilized the Log 10 of the M1-2 and m1-2 area (ap x greatest transverse width) to differentiate H. paulus from H. lepidus from equivalent stratigraphic levels in the lower half of the upper Bridger (Bridger C-D 1 of Matthew [1909]). The measurements of the M1 or 2 from the TBM are very near the means for the M1 of H. lepidus and only slightly smaller than those of the means for the M2 of H. lepidus ( West, 1979, table 13). The mean Log 10 for the area of the M1 of H. lepidus is 1.25 and for M2 is 1.36, and that of the TBM M1 or 2 is 1.29, whereas the Log 10 of the M1 and M2 for samples of H. paulus from stratigraphically equivalent samples from the upper Bridger (Bridger C-D of Matthew [1909]) are 1.34 and 1.44, respectively. Similarly, the m1 or 2 from the TBM differs from those of stratigraphically lower samples of H. paulus from high in the Bridger D (D 4 -D 5 of Matthew [1909]) by being smaller (over three standard deviations smaller than the mean m1 ap and over seven standard deviations smaller than the mean m2 ap). The Log 10 of the area of the m1 or 2 from the TBM is 1.05, similar to that of H. lepidus (mean m1 Log 10 = 1.02, mean m2 Log 10 = 1.08), but significantly transverse (trigonid) width; trp = posterior transverse (talonid) width.

smaller than that of stratigraphically equivalent samples of H. paulus (mean m1 Log 10 = 1.16, mean m2 Log 10 = 1.25). The above data strongly supports referral of the two TBM molars to H. lepidus .

Additional species of Hyopsodus have been recognized from the Bridgerian and Uintan of North America as follows: 1) early Uintan (Ui1b) H. despiciens Matthew, 1909 , from the Washakie Formation, Sand Wash Basin, Colorado (= H. markmani Abel and Cook, 1925 , vide West and Dawson, 1975); 2) middle to late Uintan (Ui2-3) H. uintensis Osborn, 1902 , from the Uinta Formation, Utah; 3) late Uintan (Ui3) H. fastigatus Russell and Wickenden, 1933 , of the Swift Current Creek Local Fauna of Saskatchewan (= H. uintensis vide Krishtalka [1979], but see also Storer [1984]); 4) middle Bridgerian (Br2) Hyopsodus tonksi Eaton, 1982 , from the Wapiti Formation of Carter Mountain, Absaroka Range, Wyoming; and 5) early Uintan (Ui1b) H. lovei Flynn, 1991 , from the type Tepee Trail Formation, East Fork Basin, Wyoming. Although it should be noted that West (1979) regarded H. despiciens as a junior synonym of H. paulus , whereas Archibald (1998) and Tsukui (2016) retained H. despiciens as a distinct species. West (1979) further regarded H. marshi as a junior synonym of H. paulus , whereas Tsukui (2016) retained H. marshi as a distinct species. Tsukui (2016) also recognized H. despiciens as occurring in the Bridger Formation from the Bridger B 2 through D 4 of Matthew (1909). Archibald (1998) followed Krishtalka (1979) and regarded H. fastigatus as a junior synonym of H. uintensis , despite the fact that Storer (1984) resurrected H. fastigatus based on a larger sample from the Swift Current Creek Local Fauna. Gazin (1968), Krishtalka (1979), and Storer (1984) noted that H. uintensis and H. fastigatus are more derived than representatives of Hyopsodus from the Bridger Formation by having the following: 1) a greater degree of molarization of the upper premolars; 2) a greater tendency towards formation of transverse lophs in the upper molars with a deeper valley between the protocone and hypocone (less connection of the protocone and hypocone) and the protoconule and metaconule more strongly connected to the protocone and hypocone, respectively; 3) more obliquely orientated crescentic lower molar cristids (the trigonid and talonid cristids more obliquely angled relative to the anteroposterior axis of the tooth); and 4) a relatively larger m1-3 entoconid. The TBM molars are significantly smaller than those of H. uintensis and H. fastigatus , and like those of H. lepidus , lack the derived states seen in these species wherein the upper molar protocone and hypocone are connected by a cristid labially with a shallower valley between them, the lower molar transverse cristids are less obliquely orientated, and the lower molar entoconids are not as enlarged. Flynn (1991) reported that H. lovei shares some of the derived dental characters seen in H. uintensis and H. sholemi Krishtalka, 1979 , including well-developed molar lophodonty, high crowned upper molars, M1-2 with a squared occlusal outline, M1-2 with a large hypocone, a deep lingual valley separating the M1-2 protocone and hypocone, and a reduced hypoconulid on m1-3. The TBM molars are significantly smaller than those of H. lovei and lack the derived dental characters seen in the species. The TBM molars differ from those of H. despiciens and H. marshi by their smaller size ( Matthew, 1909; Abel and Cook, 1925; West and Dawson, 1975; West, 1979; Tsukui, 2016). The TBM lower molar further differs from the lower molars of H. despiciens by lacking a cingulid between the metaconid and entoconid ( Tsukui, 2016). The TBM molars can be easily distinguished from those of H. tonksi by the following ( Eaton, 1982): 1) less lophidont; 2) much more transverse, with the width larger relative to the length (in H. tonksi , molars extremely narrow relative to their lengths); and 3) much less obliquely orientated lower molar cristids.

In summary, based on size and dental proportions, the TBM molars cannot be distinguished from those of H. lepidus , but can be eliminated as representing H. paulus , H. despiciens , H. marshi , H. uintensis , H. fastigatus , and H. lovei . They can be further distinguished from those of H. uintensis , H. fastigatus , H. tonksi , H. lovei , and H. sholemi by lacking the derived molar character states seen in these species. Thus, the TBM molars are referred to H. lepidus . In his study, West (1979) did not find any specimens representing H. lepidus above the lower level of Bridger D 1 of Matthew (1909) and considered its lineage had gone extinct. However, Tsukui (2016) reported H. lepidus from as high as the Bridger D 4 of Matthew (1909). Assuming our referral of TBM molars to H. lepidus is correct, then the geochronologic range of the species is extended into the earliest Uintan (Ui1a).

Figures

FIGURE 11. Carnivoramorpha and Condylartha from TBM. 1, Viverravus gracilis, LM2 (UCM 95809). 2-3, Hyposodus lepidus: RM1 or 2, UCM 67958 (2); Rm1 or 2, UCM 67956 (3). All occlusal views. Scale bar = 1 mm.