Pseudotomus sp.

Samuels, Joshua X. & Korth, William W., 2017, The first Eocene rodents from the Pacific Northwest, USA, Palaeontologia Electronica 53 (4), pp. 1-18 : 6-8

Pseudotomus sp.

( Figure 4 ; Table 1)

Referred Specimen. JODA 8900, right m3.

Occurrence. JDNM-83, Whitecap Knoll, Wheeler County, Oregon, Big Basin Member, John Day Formation.

Age. Duchesnean (early late Eocene), between Member A Ignimbrite dated 39.22 ± 0.03 Ma ( Bestland et al., 1999) and Whitecap Knoll Tuff dated 38.4 ± 0.7 Ma ( Manchester, 2000).

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Description. The m3, JODA 8900, is heavily worn and pitted in places, including the enamel surfaces along the margin of the talonid basin and the posterior portion of the posterolophid. Distinct from the wear and pitting, there is some slight wrinkling of the enamel in the talonid basin. The metaconid is the highest cusp and is more anterior than the protoconid. Due to wear, there is no indication of a trigonid, anterior cingulid, or metalophid. The anterobuccal margin of the protoconid is broken and missing a section of enamel. The protoconid is continuous with the hypoconid via a short ectolophid that is just buccal to the center of the tooth. There is no distinct mesoconid, though there is a swelling within the ectolophid between the protoconid and hypoconid. The tooth is narrower posteriorly, and the posterior cingulid connects the hypoconid and entoconid, forming the posterior cingulid. The entoconid is obliquely compressed and a continuation of the posterior cingulid. The anterior transverse width of the tooth is nearly equal to its length ( Table 1).

Remarks. JODA 8900 is referred to Pseudotomus based on its large size, simple occlusal morphology, and mesodont crown-height. This specimen is as large as the largest previously described species of Pseudotomus , approximately the same size as P. timmys from the Duchesnean age Lac Pelletier Lower Fauna of Saskatchewan (see tables in Wood, 1962, 1974; Korth, 1985; Storer, 1988; Dunn and Rasmussen, 2007). Unlike JODA 8900, P. timmys has a distinct mesoconid and hypolophid on the m3 ( Storer, 1988). A distinct m3 mesoconid is also seen in P. californicus , P. littoralis , and P. robustus ( Wood, 1962) . The m3 hypolophid is also present in P. californicus , P. eugenei , and P. littoralis ( Wood, 1962; Dunn and Rasmussen, 2007). Though several taxa are represented by material that is too worn or fragmentary to discern these characters, P. petersoni is the only known species in the genus that lacks both a distinct m3 mesoconid and hypolophid, as in JODA 8900. Unlike JODA 8900 and other members of the genus, the Bridgerian P. robustus has a rhomboid-shaped m3, which is wider posteriorly than anteriorly ( Table 1).

Though this specimen is similar in size to Manitsha tanka , it is not referable to that genus because it lacks the distinct and complete hypolophid present within the m3 talonid basin of the latter ( Simpson, 1941). Interestingly, the proportions are also distinct from Manitsha ; while having nearly identical length, the anterior portion of the tooth is substantially wider in JODA 8900 ( Table 1). JODA 8900 is actually wider relative to its length than any described manitschine ischyromyid. JODA 8900 also differs from the large Duchesnean Quadratomus gigans ( Wood, 1974) in being higher-crowned and not as squared in occlusal outline.

Contemporaneous species of Pseudotomus include P. johanniculi and P. petersoni from the Duchesnean Porvenir Local Fauna of Texas ( Wood, 1974), and P. timmys from the Duchesnean Lac Pelletier Lower Fauna from Saskatchewan ( Storer, 1988). JODA 8900 predates the appearance of Manitsha by several million years, M. tanka is known only from the Orellan Scenic Member of the Brule Formation in South Dakota ( Simpson, 1941). Manitsha sp. was reported from the mid- Chadronian (Ch2) Airstrip Local Fauna of Texas ( Wood, 1962), but material was limited to a single partial incisor, which was similar in size to M. tanka but differed in proportions.

Figures

FIGURE 4. Pseudotomus sp. right m3, JODA 8900, from the Big Basin Member of the John Day Formation, Oregon. Scale bar equals 1 mm.