Palaeomylus sp.

MENG, JIN, WYSS, ANDRÉ R., HU, YAOMING, WANG, YUANQING, BOWEN, GABRIEL J. & KOCH, PAUL L., 2005, Glires (Mammalia) from the Late Paleocene Bayan Ulan Locality of Inner Mongolia, American Museum Novitates 3473, pp. 1-26 : 17-18

Palaeomylus sp.

Eomylus borealis ; Dashzeveg and Russell, 1988: 138.

Eomylus borealis ; Meng, Zhai and Wyss, 1998: 168.

REFERRED SPECIMENS: V14125.1, fragmentary left mandible with erupting p4 and partial m1; V14125.2, an isolated right m1; V14125.3, a talonid of left m3; V14125.4, a talonid of right m2; V14125.5, a fragmentary right mandible with broken incisor and roots of p3–m2 are preserved.

LOCALITY AND AGE: Upper part of the Nomogen Formation at Bayan Ulan (within the lower 3 m of the section), Inner Mongolia; late Paleocene .

DESCRIPTION: The fragmentary mandible (V14125.5) has two mental foramina, one below p3 and the other below the trigonid of m1. The mandible is 5.84 mm deep at m1. The incisor extends posteriorly to at least below m2. Breakage near m2 reveals that the incisor lies ventromedial to the root of the tooth. Because of the position of the incisor, the mandible is quite thick (3.65 mm at m2).

The p 4 in V14125.1 is well preserved ( fig. 10a–c ). It is partially erupted and is unworn. The tooth is nonmolariform. The trigonid consists of two cusps of roughly equal size. The lingual cusp is higher than the labial one. Anterior and posterior ridges extend from the labial cusp to the anterior and posterior bases of the lingual cusp, defining a closed trigonid basin. In the center of the basin, the bases of the two cusps merge. The talonid is narrower and lower than the trigonid. The only distinct cusp occurs lingually; from it a narrow ridge extends across the talonid to the labial border of the tooth. The lateral and posterior borders of the talonid are ridgelike and do not form a cusp.

The trigonid of m1 (V14125.2) is anteroposteriorly compressed, and the metaconid is higher than the protoconid ( fig. 10d–f ). The anterior and posterior arms of the protoconid are weak, and join the anterior and posterior bases of the metaconid to enclose a small, crescentic trigonid basin. The talonid is longer and wider than the trigonid. The hypoconid is the largest talonid cusp and projects labially. A strong cristid obliqua extends from the hypoconid to the posterior base of the trigonid at the midpoint. There seems to be no mesoconid. The entoconid is conical and is connected to a low, transverse hypoconulid. The talonid of m3 (V14125.3) is heavily worn. It has the third lobe formed by the hypoconulid, similar to that of Palaeomylus lii (V14130.1).

COMMENTS: These specimens were collected from the lowest 3 m within the Bayan Ulan section in the 1970s. They were previously referred to Eomylus borealis ( Dashzeveg and Russell, 1988; Meng et al., 1998). Meng et al. (1998) cautioned, however, that referral of the specimens from Bayan Ulan to Eomylus was provisional, inasmuch as the diagnosis for Eomylus ( Dashzeveg and Russell, 1988) was based exclusively on the upper dentition, while the Bayan Ulan specimens then available (to Dashzeveg and Russell) consisted solely of lower teeth. These specimens are certainly different from those assigned to Eomylus bayanulanensis in being larger and with the incisor extending posteriorly beyond and ventrolateral to the roots of the molars. The p 4 in V14125.1 is similar to that of Palaeomylus lii but is larger and more robust. The general morphology of these specimens is comparable to that of P. lii , but they are too fragmentary to warrant more precise identification.

Figures

Fig. 10. a–c, Occlusal, lingual, and labial views of left p4–m1 of Palaeomylus sp. (V14125.1). d– f, Occlusal, lingual, and labial views of a right m1 of Palaeomylus sp. (V14125.2