Merycobunodon? walshi, Murphey & Kelly, 2017

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 : 37-39

Merycobunodon? walshi new species

Figure 15 , Table 3-4

Holotype. RM1 or 2, UCM 67888 .

Type locality. UCM 92189 , 2 m above the base of the Turtle Bluff Member, Bridger Formation , Wyoming .

Distribution and age. Known only from type locality. Earliest Uintan (Ui1a).

Diagnosis. Merycobunodon? walshi differs from the M1-2 of Merycobunodon littoralis Golz, 1976 , by the following: 1) more transverse occlusal outline (more rectangular rather than square, with ap shorter relative to tra); 2) primary cusps slightly less crescentic; 3) an incomplete lingual cingulum (does not extend across lingual base of protocone and hypocone); 4) an incipient entostyle between the protocone and hypocone; and 5) slightly smaller size. It differs from the M1-2 of species of Protylopus Wortman, 1898 , by the following: 1) lower crowned with less crescentic primary cusps (more bunoselenodont); 2) weaker labial ribs on paracone and metacone; 3) weaker development of centrocrista and mesostyle; 4) slightly weaker enamel crenulations; and 5) smaller in size.

Etymology. Patronym for the late Stephen L. Walsh of the Department of Paleontology, San Diego Natural History Museum, California, in honor of his extensive contributions to our understanding of Eocene mammals.

Description. UCM 67888 consists of an upper molar, either M1 or M2, that is well worn and has a small portion of the enamel broken away from the anterior border of the paracone and the posterolabial wall of the metaconule. It has a transversely expanded occlusal outline with bunoseleodont primary cusps. The paracone is nearly equal in size to the metacone and both have weakly developed labial ribs. The protocone is the largest primary cusp and the metaconule is large, only slightly smaller than the protocone, and positioned at the posterolingual corner of the tooth. The centrocrista between the paracone and metacone is moderately developed with a distinct mesostyle at its center. The postmetacrista is moderately strong, extending from the metacone posteriorly to join a small metastyle. The enamel covering the preparacrista is broken away, but a ridge on the underlying dentine indicates that its development was about equal to that of the postmetacrista. The postprotocrista is merged lingually and labially with the premetaconule crista and a distinct inflection is present between their labial termini along with an isolated enamel islet positioned slightly anterior and labial of the inflection. The postmetaconule crista extends labially in a gentle arc from the metaconule to join with the metastyle. The labial cingulum is complete, extending from the parastyle across the labial bases of the paracone and metacone to terminate at the metastyle. The anterior cingulum is moderately strong, extending labially from the anterolingual edge of the protocone to the parastyle. The lingual cingulum extends from the posterior wall of the protocone to the anterior wall metaconule with a central, cuspate swelling (incipient entostyle) present near its center. The enamel is finely crenulated. A hypocone is lacking. Remarks. When the molar dental characters of UCM 67888 are compared to those of other small, middle Eocene artiodactyls, it exhibits the most similarity to those of the Oromerycidae ( Table 4). However, the presence or absence of a small protoconule cannot be determined for UCM 67888 because of its advanced wear stage. In homacodonts, a distinct protoconule is present, which, even in advanced wear, can be identified by distinct indentations anteriorly and posteriorly along the preprotocrista that outline the position of the protoconule ( Gazin, 1955; Prothero, 1998a; Stucky, 1998). In most homacodonts, the metaconule is moderately large and positioned labial of a distinct hypocone, whereas in UCM 67888, hypocone is not present and the metaconule is greatly enlarged forming a primary cusp positioned at the posterolingual corner of the tooth, like those of the Oromerycidae ( Gazin, 1955; Prothero, 1998a; Stucky, 1998). The primitive oromerycid Merycobunodon littoralis from the early Uintan (Ui1b) of southern California is only known from a small number of upper cheek teeth including a dP4, an associated, moderately worn P4-M1 and an associated, unworn M1-2 ( Golz, 1976). In M. littoralis , a small, but distinct, protoconule is present on the unworn M1-2, but in the referred specimen with a moderately worn M1, the protoconule is almost completely obliterated by wear ( Golz, 1976). In UCM 67888, there is no indentation along the anterior border of the preprotocrista indicating that a distinct protoconule was present, but along the worn labial border of the anterior crescent outline there is a slight, convexity just posterior of the level of the paracone apex. However, whether this convexity represents a remnant of a protoconule cannot be determined because in well worn upper molars of Protylopus , which lack a protoconule, a similar convexity may also occur due to advanced wear of the protocone. In UCM 67888, the postprotocrista and premetaconule crista are merged lingually and labially due to wear, but there is a distinct lingual inflection between their labial termini along with an oval, isolated enamel islet positioned lingually and slightly anteriorly of the inflection. The inflection represents the remnant of the labial portion of the valley between the postprotocrista and premetaconule crista. The enamel islet appears to represent the remnant of the bifurcation of the postprotocrista and the anterior border of the premetaconule crista. A similar precursor to this enamel islet can also be observed in the bifurcation of the postprotocristae of moderately worn teeth of Protylopus and Oromyerx plicatus ( Gazin, 1955, plates 13-15). Thus, it appears that UCM 67888 possessed a bifurcated postprotocristae, which is diagnostic for the Oromerycidae .

UCM 67888 exhibits the following shared characters with Merycobunodon and Protylopus ( Table 4): 1) mesostyle present; 2) hypocone absent; 3) enlarged, posterolingually positioned metaconule; 4) postprotocrista bifurcation; 5) paracone and metacone ribs; and 6) crenulated enamel. UCM 67888 is definitely specifically distinct from all other known middle Eocene oromeryicids and homacodontids. Of all recognized oromerycids, UCM 67888 is most similar to the M1- 2 of Merycobunodon by being low crowned and bunoseleodont. UCM 67888 could represent a new oromeryicid genus. However, without additional cheek teeth to better characterize it, including ones that are not as heavily worn, we cannot establish enough diagnostic characters to generically separate it from Merycobunodon . Furthermore, all of the differences between UCM 67888 and the M1-2 of M. littoralis listed above in the diagnosis only rise to the level of specific separation. Therefore, UCM 67888 is questionably assigned to Merycobunodon , as M.? walshi . In this systematic scenario, earliest Uintan M.? walshi could have given rise to early Uintan Merycobunodon littoralis , which then gave rise to middle to late Uintan Protylopus .

Figures

FIGURE 15. Merycobunodon? walshi new species from TBM. RM1 or 2 (UCM 67888, holotype, with 1 original specimen, 2-4 cast showing detail better) in occlusal (1- 2), posterior (3), and anterior (4) views. Scale bar equals 1 mm.