Triplopus sp.

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 : 32-35

Triplopus sp. , cf. T. obliquidens ( Scott and Osborn, 1887)

Figure 13.1-4 , Table 3

Referred specimens. From UCM Locality 92189: partial left dentary with p2, partial p3, and p4-m1, UCM 69053. From DMNH Locality 5064: partial left dentary with p1 alveolus, roots of p3, complete p4, partial m1-2, and unerupted m3, DMNH 75320. From DMNH Locality 4673: partial m1, DMNH 75315.

Description. UCM 69053 consists of a partial dentary with the alveolus for p1, a complete p2 and p4, a p3 that is missing a portion of the labial edge of the tooth, including the protoconid and hypoconid, and a partial m1 that is missing the labial half of the hypoconid. DMNH 75320 is a partial dentary with a complete p4, the posterior half of m2 and an unerupted m3 positioned deep in the alveolar crypt, indicating that it represents a juvenile. The enamel surfaces on the teeth of DMNH 75320 are somewhat eroded and etched, especially along the labial side of the p4. DMNH 75315 is a partial m1 that is missing a portion of the enamel along the metalophid.

The dentary is moderately deep with a depth below m1 of 19.6 mm for DMNH 75320 and 22.6 mm for UCM 69053. Three mental foramina are present with the largest one positioned below the posterior root of p3, whereas the other two are smaller with one positioned below the anterior root of m1 (or just slightly posterior of the anterior root) and one below the anterior root of p2. An alveolus is present anterior to p2 on both specimens, indicating that a p1 was present.

The p2 is premolariform. The protoconid is the tallest and largest primary cuspid positioned slightly anterior to the center of the tooth. The paraconid is a distinct, relatively tall cuspid that is separated from the short paralophid by a small, but distinct notch. The protolophid extends posterolingually from the protoconid apex to an incipient metaconid (slight bulge) and then descends rapidly to terminate at the anterolingual edge of the talonid. The hypoconid is anteroposteriorly elongated and connected anteriorly to the protoconid forming a nearly straight, continuous lophid. The talonid basin is small and open at the posterolingual corner. A hypolophid is lacking.

Although the p3 is damaged, certain characters can still be discerned. It is submolariform. The paralophid extends from the protoconid in a tall, gentle arc to the anterolingual corner of the tooth. The protolophid is strong and extends as a slightly oblique lophid from the protoconid to join a tall metaconid. A slight swelling (incipient entoconid) is present on the lingual border of the talonid. The talonid is open posteriorly.

The p4 is submolariform. The paralophid is moderately well developed, descending in an arc from the protoconid apex to terminate just labial of the anterolingual corner of the trigonid. The protoconid is the tallest primary cusp and is connected to the metaconid by a strong, tall protolophid. The hypoconid is anteroposteriorly elongated and connected anteriorly to the posterior wall of the trigonid by a relatively straight, tall metalophid (= cristid obliqua). A small, oval entoconid is present on the labial edge of the talonid. A hypolophid is lacking resulting in the talonid being open posteriorly.

The m1 has a rectangular occlusal outline. The protoconid, metaconid, hypoconid, and entoconid are tall and equal in height. The paralophid is well developed and extends from the protoconid as a wide, continuous, slight arc to terminate at the anterolingual corner of the trigonid. The protolophid and hypolophid are tall, complete lophids connecting the protoconid to the metaconid and the hypoconid to the entoconid, respectively, and are inclined slightly posteriorly and nearly parallel to each other. The metalophid is well developed, forming a nearly straight, tall lophid extending anteriorly from the hypoconid to terminate on the posterior wall of the trigonid, slightly below the protolophid. A very small hypoconulid is present, positioned centrally on a weak posterior cingulid. The occlusal morphology of the posterior half of the m2 is nearly identical to that of the m1 except that it has a slightly better developed, but still weak, hypoconulid.

The only specimen with an m3 is DMNH 75320, where it is unerupted and buried within the alveolar crypt. Radiographs were taken of DMNH 75320. Although detailed occlusal morphology could not be discerned because of the mineralization of the enamel and surrounding bone, the radiographic outline of the m3 showed the bilophodont pattern and lack of a well-developed, protruding hypoconulid, characters that are typical of Triplopus .

Remarks. The taxonomic history of Triplopus and certain other middle Eocene tapiroid and rhinocerotoid taxa from the Bridger, Washakie, and Uinta formations is complicated. Cope (1873a, 1873b) described Hyrachyus implicatus based on two specimens, but neither were designated as a holotype; specimen number one (AMNH FM 5072) consists of a partial skull with partial (all teeth damaged) RP3-M3 and LP4-M3 from Cottonwood Creek, Bridger Formation, and specimen number two (AMNH FM 5078) consists of a partial skull with a partial P1, P2, partial P3, P4, and partial M1- 3 and associated partial right dentary with Rp2-m3 from the Washakie Formation south of Bitter Creek. Cope (1884) transferred AMNH FM 5072 to Hyrachyus agarius Leidy, 1871 , and restricted H. implicatus to AMNH FM 5078, making it the lectoholotype. Contrary to Cope (1884), Wood (1934) reidentified AMNH FM 5072 as representing H. affinis Marsh, 1871 , which Radinsky (1967) synonymized with Hyrachyus modestus Leidy, 1870 . Wood (1934) erected a new genus Ephyrachyus based on AMNH FM 5078 along with a referred lower dentition (AMNH FM 12672) from the upper Bridger Formation (Bridger D, Br3), making E. implicatus the type species. Radinsky (1967) synonymized Ephyrachyus with the early hyracodontid genus Triplopus , which made Triplopus cubitalis Cope, 1880 , from the Washakie Formation the type species and transferred E. implicatus to Triplopus . Based on an upper dentition with the alveolus for P1 and P2-M3 (PU 10166), Scott (1883) described Desmatotherium guyotii , which he regarded as closely related to Hyrachyus Leidy, 1871 . The provenance of the holotype of D. guyotii is uncertain and could be either the Washakie Formation or Bridger Formation ( Scott, 1883; Granger, 1909; Matthew and Granger, 1925; Matthew in Osborn, 1909; Gazin, 1956; Radinsky, 1963). Radinsky (1963) believed the differences cited by Scott (1883) to differentiate D. guyotii from Hyrachyus are characters similar to those that define Helaletes Marsh, 1872b . Based on a partial maxilla with M1-3 (PU 10095), Osborn et al. (1878) described Hyrachyus intermedius from the Bridger Formation. Wood (1934) regarded H. intermedius as generically distinct from Hyrachyus and referred it to a new genus Chasmotheriodes , which he questionably considered a member of the Helaletidae . Radinsky (1963) synonymized Lophiodon nanus Marsh, 1871 , with Helaletes boops Marsh, 1872b , as Helaletes nanus and regarded Chasmotheriodes and Desmatotherium as junior synonyms of Helaletes . Radinsky (1963) also regarded D. guyotii as a junior synonym of Helaletes intermedius and reassigned the lower dentition (AMNH FM 12672), previously assigned by Wood (1934) to E. implicatus from the upper Bridger Formation, to Helaletes intermedius , which was followed by Schoch (1984). Most recently, Colbert and Schoch (1998) resurrected Desmatotherium as a valid genus with D. intermedius as the type species and questionably regarded D. guyotii as a junior synonym of D. intermedius . Based on a partial upper dentition (PU 10403) from the Uinta Formation, Scott and Osborn (1887) erected a new genus and species, Prohyracodon intermedium , and, based on a partial lower dentition (PU 10402) from the Uinta Formation, named Hyrachyus obliquidens . Scott and Osborn (1889) later regarded these species as conspecific and also cogeneric with Triplopus , and transferred them to a single species as T. obliquidens . Peterson (1919) resurrected Prohyracodon and reassigned T. obliquidens to the genus as P. obliquidens , which was also followed by Wood (1927). Radinsky (1967) placed Prohyracodon back in synonymy with Triplopus and reassigned P. obliquidens back to Tripolpus. Wood (1927) erected the genus Eotrigonias , to which he assigned two new species, the type species E. rhinocerinus from the Uinta Formation and E. petersoni , from the Washakie Formation, and described a new species of Triplopus , T. grangeri , from the Uinta Formation. Radinsky (1963) placed Eotrigonias in synonymy with Triplopus , which transferred E. rhinocerinus to Triplopus . Radinsky (1967) also placed E. petersoni in synonymy with T. implicatus and T. grangeri in synonymy with T. obliquidens . Stock (1936) described a new species from the latest Uintan Strathearn Local Fauna of the Sespe Formation of California (see also Kelly, 1990; Kelly et al., 2012), which he questionably assigned to Triplopus , as T.? woodi . McCarroll et al. (1996) documented the occurrence of T. cubitalis and T. implicatus in the Washakie Formation, wherein these species are restricted the early Uintan middle unit of the Adobe Town Member. Most recently, Prothero (1998b) regarded Triplopus as the most common rhinocerotoid in the Uintan of North America with a restricted range from the early Uintan to the Duchesnean.

In summary, four species and one questionably referred species of Triplopus are currently recognized ( Prothero, 1998b); T. cubitalis and T. implicatus from the Washakie Formation of Wyoming, T. obliquidens and T. rhinocerinus from the Uinta Formation of Utah, and T.? woodi from the Sespe Formation of California.

The lower teeth of UCM 69053 from the TBM are very similar in size to those of the only known lower dentition of Desmatotherium intermedius (AMNH FM 12672). However, they can be easily distinguished from those of D. intermedius by the following: 1) p1 present (absent in D. intermedius ); 2) the p2 lacks an entoconid, p3 with a slight bulge (incipient entoconid) at the posterolingual corner of the tooth, and p4 with a low, very small, round entoconid (in D. intermedius , p2 with a small entoconid, p3 with a large entoconid, and p4 with a very large entoconid, latter nearly as tall as the hypoconid); 3) the p2 and p4 talonid widths (p3 broken in UCM 69053) significantly less transversely expanded relative to the trigonid widths (p2 tra/trp = 0.97 and p4 tra/trp = 0.90 in UCM 69053 versus p2 tra/trp = 0.72 and p4 tra/trp = 0.79 in D. intermedius ); 4) p2 with distinct, tall paraconid, about 80% of the height of the protoconid (in D. intermedius , paraconid shelf-like and lower, about 40% of the height of the protoconid); 5) the p4-m1 paralophid slightly better developed and taller, extending anterolingually from the protoconid as a descending arc; 6) p4 metalophid extends anteriorly as a relatively straight ridge from the hypoconid to terminate on the posterior wall of the protoconid just lingual of the protoconid apex (in D. intermedius , p4 metalophid more obliquely orientated, extending anterolingually from the hypoconid to terminate further lingually on the posterior wall of the trigonid, resulting in V-shaped hypoconid occlusal outline); and 7) the m1 metalophid is well developed, extending from the hypoconid to the posterior wall of the trigonid, closing off the valley between the protoconid and hypoconid (in D. intermedius , m1 metalophid essentially lacking, only represented by a rapidly descending low ridge not extending to the posterior wall of the trigonid, resulting in an open valley between the protoconid and hypoconid). The lower teeth of UCM 69053 are also similar in size to those of Uintan Isectolophus annectans Scott and Osborn, 1887 , but differ by the following ( Troxell, 1922; Radinskyi, 1963, 1967; Schoch, 1983; Colbert and Schoch, 1998; Prothero, 1998b; Lucas et al., 2003): 1) relatively higher crowned (more hypsodont) resulting in greater lophodonty; 2) p2-4 less anteroposteriorly expanded relative to m1, especially p2 (p2 ap 10% shorter relative to m1 ap); 3) p2 lacks distinct fold at junction of metalophid with protoconid; 4) p4-m1 labial cingulids more weakly developed; 5) p4-m1 metalophids better developed and straighter; and 6) m3 lacking a large, protruding hypoconulid.

All of the above dental characters that differentiate UCM 69053 from D. intermedius and I. annectans are also diagnostic for Triplopus ( Radinsky, 1967; McCarroll et al., 1996; Prothero, 1998b), so the TBM specimens can be confidently assigned to the genus. Species of Triplopus are differentiated primarily by size ( McCarroll et al., 1996; Prothero, 1998b). The lower teeth of the UCM 69053 are larger than those of T. cubitalis , slightly smaller than those of T. implicatus and Triplopus? woodi , much smaller than those of T. rhinocerinus , and within the observed size ranges for those of T. obliquidens (except p2, which is slightly smaller). However, until more complete dentitions including upper teeth from the TBM are available for further comparisons, UCM 69053 is only compared to T. obliquidens , as Triplopus sp. , cf. T. obliquidens .

Figures

FIGURE 13. Perissodactyla from TBM. 1-4, Triplopus sp., cf. T. obliquidens, partial left dentary with alveolus for p1, p2, partial p3, p4 and partial m1 (UCM 69053 with 1, 3-4 original specimen, 2 cast showing detail better) in occlusal (1- 2), labial (3) and lingual (4) views. 5-6, Epihippus sp., cf. E. gracilis, Rp3 (SDSNH 110408) in occlusal (5) and lingual (6) views. Scale bars for 1-2 and 3-4 equals 5 mm. Scale bar for 5-6 equals 1 mm.