Ticholeptus sp.

Smith, Kent S., Czaplewski, Nicholas J. & Coombs, Margery C., 2022, Ungulates of the middle Miocene Monarch Mill Formation, Churchill County, Nevada, USA, Acta Palaeontologica Polonica 67 (1), pp. 239-256 : 243-247

Ticholeptus sp.

Fig. 3 .

Material.—UCMP 141873, left dentary fragment with a complete m1 and an incomplete m2; OMNH 77341, left DP3; OMNH 77348, left astragalus; OMNH 77479, right astragalus; from localities UCMP V70142 and OMNH V974,

Eastgate, Churchill County, Nevada, USA; Monarch Mill Formation, early Barstovian, middle Miocene.

Measurements.—UCMP 141873 m 1, AP = 14.50 mm; T = 10.73 mm: m2, AP = 15.95 mm; T = 11.89 mm.

Description.— Dentary and cheek teeth: The dentary fragment has no foramina or other features present. The molar teeth present include an m1 and m2, which are semi-hypsodont and completely selenodont. The m1 has a well-developed protoconid and hypoconid with an intercrescentic valley closed by a low, labial cingulum. The paraconid is missing. An anterior cingulum is present but is not continuous with the labial cingulum. A posteromedially-directed metastylid occurs midway between the metaconid and entoconid. The hypoconulid is distinct. Internal ribs (vertical ribs along the lingual aspects of the lingual cusps) of the metaconid and entoconid and posterior cingulid are weak. The m2 is longer and wider than the m1, has a paraconid, and the anterior cingulum is continuous with the labial cingulum. The hypoconulid and posterior cingulum of the m2 are missing.

Remarks.— Lander (1998) listed several characteristics of the m1–m3 for the Merycoidodontidae that are similar to those of the Eastgate specimen (UCMP 141873). These structures include completely selenodont lingual cusps (metaconid and entoconid) with a posteromedially-directed metastylid that occurs midway between the metaconid and entoconid, reduced stylids, and the reduced metaconid and entoconid ribs. The molar teeth of the Eastgate specimen are similar to those of Ticholeptus cf. obliquidens (UO 22798; see Shotwell 1968: 56, fig. 27) from the Red Basin fauna, Oregon in Shotwell 1968). The specimen (UO 22798) described by Shotwell 1968) has lower molars with a labial cingulum closing off the intercrescentic valleys, no protostylid, and other stylids present but not distinct. Morea (1981) identified 11 dentaries of Ticholeptus cf. zygomaticus from the Massacre Lake LF, Washoe County, Nevada; these specimens also are similar to the Eastgate fossil specimen. No measurements were given by Shotwell (1968) for UO 22798 or UO 26798. The greatest length (AP) of the Eastgate m1 occurs within the range of those listed by Morea (1981) for the m1s of T. cf. zygomaticus m1 AP = 12.70–14.60 mm, T = 8.30–10.10 mm); however,

the greatest width (T) is slightly larger for the Eastgate specimen. In addition to size, the specimen from Eastgate is similar in characteristics to those listed above by Shotwell (1968) for T. cf. obliquidens . Teeth are not particularly useful in identifying various taxa of merycoidodontids, and deciduous teeth in the group are little described in the literature. Thin enamel and a low crown height suggest that OMNH 77341 is a deciduous tooth, which is tentatively referred to this genus because no other merycoidodontid is known in the Eastgate LF. Similarly, we tentatively refer the two astragali listed above to Ticholeptus sp. because of their relatively large size, morphology, and differences from the only other artiodactyl astragali in the fauna, which likely pertain to the ruminant palaeomerycid described below. In addition, the specimens extend the known age for Ticholeptus in the Great Basin from the late Hemingfordian (Massacre Lake LF) to the early Barstovian (Eastgate LF).

Family Palaeomerycidae Lydekker, 1883

Subfamily Dromomerycinae Frick, 1937

Tribe Cranioceratini Frick, 1937

Genus Barbouromeryx Frick, 1937

Type species: Barbouromeryx trigonocorneus ( Barbour and Schultz, 1934) ; Antelope Creek , Nebraska, USA; early Hemingfordian .

Barbouromeryx trigonocorneus ( Barbour and Schultz, 1934)

Figs. 4–6 .

Material.— OMNH 54976, left dentary fragment with p2– m3; OMNH 54977, right dentary with dp3, partial dp4, alveolus with posterior root of m1, and developing-erupting m2; OMNH 77355, right dentary fragment with dp2 or p2 and alveoli for?dp1; OMNH 77369, partial C1; OMNH 77344, right M1 or M2; OMNH 77368, associated right m1, m3, and fragments of dentary; UCMP 141499, right dentary fragment with m1; UCMP 141518, right dentary fragment with m1 fragment and m2–m3; UCMP 141524, seven isolated cheek teeth and a maxilla fragment with M1-M2; UCMP 141527, median phalanx; UCMP 141650, right dentary fragment with p3–p4; UCMP 141654, right dentary fragment with p3–p4; UCMP 141655, left maxilla fragment with P2-M1; UCMP 141669, left dentary fragment with p3–m3; UCMP 141702, right dentary fragment with p2–m3; UCMP 141863, left maxilla fragment with M2–M3; UCMP 141664, right dp4; UCMP 141667, left DP3; UCMP 141693, left m2 fragment. From localities OMNH V974; UCMP V70138, V70142, and V70140, Eastgate, Churchill County, Nevada; Monarch Mill Formation, early Barstovian, middle Miocene.

Measurements.—See Table 2.

Description.— Upper teeth: All maxillae of Barbouromeryx trigonocorneus at Eastgate are very fragmentary. The upper premolars have three roots, an ectoloph with a low anterior style, external rib, and a posterior style. The upper molars are brachydont and increase in length and width from M1 to M3; however, the M2 is only slightly shorter and narrower than the M3. Other than size, the molars are similar in the following characteristics: four primary cusps (protocone, paracone, metacone, and metaconule); parastyle, mesostyle, metastyle, and entostyle; strong, anteriorly-inflected external rib on the labial surface of the paracone that forms a small extra loph that will shorten with increasing wear; strong, thin anterior cingulum with deep gutter; and posterior cingulum weak (or absent on M3). In all upper molars the postprotocrista curves anteriorly at its labial end and is bifurcated in little-worn teeth, with the anterior branch approaching or contacting the base of the paracone and the posterior branch contacting the premetaconulecrista. The premetaconulecrista may also be bifurcated at its labial end and in contact with the postprotocrista but not in contact with the paracone or metacone. An external rib is absent on the metacones of all upper molars. The metaconule folds are weak.

Lower cheek teeth: All dentaries are fragmentary; thus, we provide no information on the coronoid process, condyle, ramus, or foramina. The cheek teeth are brachydont and p1 is absent. One specimen ( OMNH 77355) contains two small alveoli that might have held a dp1. The premolars are unreduced relative to the molars and are not molariform. In two specimens ( UCMP 141669 and OMNH 54976), the premolar row is long relative to the molar row length and entire cheek tooth row length ( Table 2); the specimens fit within the ranges of these dimensions provided for Barbouromeryx trigonocorneus by Prothero et al. (2014: fig. 5). The p3 and p4 each have five transverse lophids. The p4 anterior valley is not closed by an anterolingual cristid of the mesolingual conulid (“metaconid” of previous authors), as there is no anterolingual cristid. In p4 the posterolingual conid and posterior stylid are crestlike and converge posterolingually, but a vertical groove remains between their lingual ends. In one dentary ( OMNH 54977) of a juvenile are teeth that we interpret as dp3, partial dp4, alveolus with posterior root of m1, and m 2 in the crypt; the deciduous premolars of this specimen differ in morphology from the permanent premolars in other Eastgate B. trigonocorneus dentaries. The dp3 is complete and has five lophs: anterior stylid, anterior conid, mesolingual conid, posterior cristid, and posterior stylid. These lophs are similar to those of the available permanent p3s except for minor differences. The distal (lingual) end of the anterior conid sweeps posteriorly as it descends to the base of the crown, partly closing off the bottom of the anterior valley. The mesolingual conid has its posterolingual cristid angled posterolingually. The posterior cristid is confluent with the posterior stylid, closing off the back valley with no notch or vertical groove on the posterolingual corner of the tooth. The crown of the dp4 is mostly broken away except for an anterior portion, but the alveolus and root fragments indicate that it had three lobes like a normal ruminant dp4. There is a portion of the posterior root remaining in a socket that is confluent with the labially situated socket and its small root that supported the second lobe. The first (anterior) lobe has a large low anterolabial conid with a tiny anterior stylid that juts forward; with increasing wear this stylid would have joined the anterior cingulid. The anterior cristids of the anterolabial and anterolingual conids are confluent. A low anterior ectostylid is present. The m1 is completely broken away and is represented only by its posterior root. The erupting m 2 in this specimen is identical to those of the other specimens described below. In its unworn state it bears a short external postmetacristid that will disappear into an external rib of the metaconid with wear.

All lower molars have strong external postprotocristids the “ Palaeomeryx -folds” of previous authors) that form a small extra lophid. The m1 and m2 have prominent anterior and posterior cingulids and ectostylids. The m3 has a strong anterior cingulid, a moderate ectostylid in the anterior external valley, a weak ectostylid in the posterior external valley, and a weak or absent posterior cingulid on the posterior extremity of the tooth. The m3 posterior lobe imitates the shapes of the trigonid and talonid lobes, having a lingual preentoconulidcristid–entoconulid–postentoconulidcristid and labial selene formed by the prehypoconulidcristid–hypoconulid–posthypoconulidcristid, and the back fossa is closed posteriorly.

Remarks.—We follow Janis and Scott (1987) and Prothero and Liter (2008) in placing Dromomerycinae in the family Palaeomerycidae . Janis and Manning (1998) provided characters for the identification of the subfamilies and tribes, including dental characters. Members of this family are largely diagnosed on cranial appendages ( Janis and Manning 1998; Prothero and Liter 2007, 2008), none of which were found with the Eastgate fauna.

The ungulate upper canine ( OMNH 77369) from Eastgate was unassociated with cheek teeth or other remains. It is tentatively referred to Barbouromeryx trigonocorneus because the type specimen for the species, a cranium, was found with a large saber-like tusk associated near the palate and assumed to belong to the cranium ( Barbour and Schultz 1934). Although no measurements of the tusk were provided by Barbour and Schultz (1934), reviewing published photographs aided in determining the size of the C1 alveolus of the cranium of the type specimen published in Prothero and Liter (2008). The C1 anteroposterior length in the type is about 1cm. OMNH 77369 has an anteroposterior length of 8.5 mm (see Table 2). The Eastgate tusk is missing the tip of its crown as well as the base of the root. The crown in cross-section has a “teardrop” shape with the anterior edge rounded and the posterior edge pointed and covered with extremely thin enamel ( Fig. 4 ).

Among the Palaeomerycidae , only members of the tribe Aletomerycinae ( Aletomeryx and Sinclairomeryx ) and the tribe Cranioceratini (only the Barbouromeryx ) possess upper canines ( Prothero and Liter 2008). The palaeomerycids Aletomeryx and Sinclairomeryx have upper canines that are reduced, but the Eastgate dentaries bear molars having strong external postprotocristids (“ Palaeomeryx -folds”), so the dentaries cannot be referred to Aletomerycinae , in which these folds are weak to absent ( Janis and Manning 1998; Prothero and Liter 2008). The Eastgate premolars are unreduced relative to molar size, are not molariform, and are mesodont; dp1 is not retained. These characteristics do not match those of aletomerycines or dromomerycines as diagnosed by Janis and Manning (1998) and Prothero and Liter (2008). Thus, based on the cheek teeth, the Eastgate jaws pertain to a member of the Cranioceratini , either Bouromeryx or Barbouromeryx . As revised by Prothero and Liter (2008), these two genera are essentially identical dentally except that Barbouromeryx has upper canines while Bouromeryx lacks canines ( Prothero and Liter 2008). Based on known temporal ranges of these two genera, Bouromeryx extends into the Barstovian while Barbouromeryx is only known prior to the Barstovian. The Eastgate palaeomerycid could possibly represent Bouromeryx unless the isolated canine belongs to the same taxon as the multiple dentaries. If the Eastgate palaeomerycid represents Bouromeryx , the presence of an unassociated but enlarged artiodactyl canine is difficult to explain unless more than one palaeomerycid taxon is present, one of which bears upper canines. Given the currently available sample, we believe it is more parsimonious to assume that a single dromomerycine taxon is present at Eastgate. If the Eastgate canine represents the tusked genus Barbouromeryx , with one recognized species, B. trigonocorneus , ranging from latest Arikareean to middle Hemingfordian ( Prothero and Liter 2008), then the temporal range of B. trigonocorneus must be extended into the Barstovian. Alternatively, the Eastgate isolated canine might represent a moschid or gelocid for which no cheek teeth or other skeletal elements have been found (Prothero 2008; Webb 1998, 2008).

Stratigraphic and geographic range.—Latest Arikareean to middle Hemingfordian ( Janis and Manning 1998; Prothero and Liter 2008). The Eastgate LF record extends the range to early Barstovian ( Smith 2002). Geographic range: USA: Nebraska (Anderson Ranch Formation, Runningwater Formation, Box Butte Formation), South Dakota (Batesland Formation), Colorado (Martin Canyon Formation), Texas (Oakville Formation), and Wyoming (Split Rock Formation) ( Prothero and Liter 2008); and Nevada (Eastgate LF).

Figures

Fig. 3. Merycoidodontid artiodactyl Ticholeptus sp. indet. from locality UCMP V70142, Eastgate Churchill County, Nevada, USA; early Barstovian, middle Miocene. UCMP 141873, partial left dentary fragment with complete m1 and partial m2 in lingual (A1), labial (A2), and occlusal (A3) views.

Fig. 4. Palaeomerycid artiodactyl tentatively referred to Barbouromeryx trigonocorneus (Barbour and Schultz, 1934), from locality OMNH V974, Eastgate, Churchill County, Nevada, USA; early Barstovian, middle Miocene. OMNH 77369, partial upper tusk (C1) in labial view with remnant strips of very thin enamel (A1), lingual view (A2), cross-section at natural break in crown (A3).

Fig. 5. Palaeomerycid artiodactyl tentatively referred to Barbouromeryx trigonocorneus (Barbour and Schultz, 1934), from locality UCMP V70138, Eastgate Churchill County, Nevada, USA; early Barstovian, middle Miocene. A. UCMP 141518, right dentary fragment with m1 fragment and m2–m3. B. UCMP 141524, maxilla fragment with M1–M2. In labial A1, B1), lingual (A2, B2), and occlusal (A3, B3)views.

Fig. 6. Dental terminology for palaeomerycid artiodactyl Barbouromeryx trigonocorneus (Barbour and Schultz, 1934), from Eastgate, Churchill County, Nevada, USA; early Barstovian, middle Miocene. A. UCMP 141518 from UCMP V70138; right dentary fragment with m1 fragment and m2–m3 in occlusal (A1) and labial (A2) views. B. UCMP 141669 from UCMP V70140; left dentary fragment with p3–m3 in occlusal view (only p3–p4 figured).