Eomylus bayanulanensis, MENG & WYSS & HU & WANG & BOWEN & KOCH, 2005

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 : 3-8

Eomylus bayanulanensis , new species

HOLOTYPE: V14126.1 , a right M2 ( fig. 1 ).

REFERRED SPECIMENS: V 14126.2, a left maxilla with M 1–3 in which the labial edges of the teeth are incomplete; V 14127.1, a fragmentary right mandible with m1–3; V 14127.2, a fragmentary left mandible with partial m2–3; V 14127.3, a fragmentary right mandible with broken m2–3 and a separated anterior portion of a left mandible with incisor (from the same nodule containing V 14126.2); V 14127.4, a left m3 with little wear.

LOCALITY AND AGE: Upper Nomogen formation at Bayan Ulan (within the lower 3 m of the section), late Paleocene.

ETYMOLOGY: The trivial name is after that of the locality, Bayan Ulan.

DIAGNOSIS: Eomylus differs from other genera of basal Glires in having a large metaconule, a distinct hypoconulid on lower molars, and a diagonal wear trough on the talonid. Further differs from Rhombomylus , Matutinia , Mimotona , Heomys , Eurymylus , Gomphos , and Amar by its anteroposteriorly shortened upper cheek teeth and less expanded hypocone shelf. Further differs from Sinomylus in having a stronger hypocone, more marginally positioned paracone and metacone and more lingually positioned protocone. Further differs from Tribosphenomys in having more lophate cheek teeth. The new species differs from Eomylus zhigdenensis in having upper cheek teeth less anteroposteriorly shortened and less unilaterally hyspodont; the hypocone and metaconule more prominent; M2 with the paracone and metacone separated by a labial notch; lower cheek teeth less transversely extended; the mandible proportionally shallower. Differs from E. borealis in being smaller and molars less transversely extended, and in having a partial anterior cingulid on the lower molars and a relatively smaller hypoconulid.

DESCRIPTION: Association of the upper and lower teeth is based on their comparable sizes ( tables 1 , 2 ) and morphology. This association is supported by the fact that the left maxilla with M1–3 (V14126.2) and the fragmentary right mandible with broken m2–3 (V14127.3) were prepared from a single coprolitic nodule. The fragmentary maxilla shows that the posterior border of the hard palate is even with M2. The posterior margin of the palate bears a blunt, rounded lip, the postpalatine torus. This lip continues posteriorly as a ridge that bounds the choanal orifice. Lateral to the ridge is a broad, shallow trough in which a small foramen of unknown function resides. Two small palatine foramina occur within the palatine opposite M1.

Upper premolars are unknown. The alveolus of P4 indicates a large (as wide as M1) double­rooted tooth ( fig. 1 ). The external edges of M1–3 (V14126.2), prepared from a coprolitic nodule in which V14126.2 is also contained, are seemingly etched by acid ( fig. 1 ). The first upper molar resembles M2 except in being slightly shorter lingually. The description of the upper molar is based primarily on V14126.1 (the holotype; fig. 1 ). The tooth is low crowned, wider anteriorly than posteriorly, and has one large lingual and two small labial roots. The protocone is the largest cusp of the tooth and is more lingually extended than the hypocone, although the tips of the two cusps are aligned anteroposteriorly. Lingually, the protocone and hypocone are separated by a shallow valley that deepens toward the hypocone. In occlusal view, the protocone has a large wear facet on its posterior surface; this aligns with the orientation of a wear facet on the posterior surface of the metaconule. The protocone connects with the paracone by a low, strong preprotocrista that forms a curved anterior edge of the tooth. A small, transverse enamel lake is present at the labial end of the preprotocrista and is lingual to the paracone; this suggests the presence of a small paraconule on an unworn tooth. The paracone and metacone are placed on the labial margin of the tooth. The paracone is larger and more labially positioned than the metacone. The two cusps are separated by a notch at the labial edge of the tooth. A small cuspule is at the posterolabial base of the paracone, probably representing the mesostyle. The metaconule is larger than the paracone or metacone. It is connected lingually with the protocone by a short postprotocrista and labially with the metacone by the premetaconule crista. Between the swollen metaconule and the metacone, an enamel bay is formed. The anterior surface of the metaconule is bare dentine, forming part of the concave trigon basin. The hypocone is distinct and separated from the protocone by a narrow valley. It merges labially with the postcingulum, forming the hypocone shelf. The hypocone shelf forms an oblique valley that parallels the plane aligning the wear facets on the posterior sides of the protocone and metaconule. This concave area is covered with thin enamel between the protocone and metaconule, whereas the rest of the shelf is bare dentine. This oblique orientation and the wear facets on the hypocone shelf fit well against the oblique wear facets on the lower molars (see below). The last upper molar is narrower than M1 and M2 and retains the basic molar pattern ( fig. 1 ).

The mandible (V14127.2) is 4.82 mm deep at m1 and 2.42 mm thick at m2. Tooth marks occur on the medial surface of two mandibles (V14127.1–2). Breakages in the mandibles (V14127.1–2), both in cross section and along the lingual surface, reveal no incisor under the molars. The narrow space ventral to roots of molars also indicates that the incisor probably ends anterior to the first molar. The incisor preserved in V14127.3 shows that it probably extends posteriorly to the level of p4. On the lateral surface of the mandible the anterior edge of the masseteric fossa is aligned with the talonid of m3.

No lower premolar is preserved. The molars are all double­rooted. The m1 and m2 differ mainly in m2 being larger ( fig. 2 ). The trigonid is short and has a steep posterior wall. A partial anterior cingulid occurs labially below the protoconid. The protoconid and metaconid are subequal in size, the latter cusp being higher and more anteriorly placed. A wear facet is present on the anterolabial side of the protoconid. A similar facet, having the same orientation, occurs on the hypoconid. The paracristid and protocristid are complete and straight, enclosing the trigonid basin anteriorly and posteriorly. The talonid is longer and wider than the trigonid. All talonid cusps are high. The hypoconid, the largest talonid cusp, is elongated labially. The cristid obliqua is strong and extends anterolingually from the hypoconid to join the posterior wall of the trigonid near its midpoint. In little worn specimens, the mesoconid is distinct. Following wear the cristid obliqua becomes a sharp ridge; the lingual surface of the cristid obliqua forms a flat wear facet that merges posteriorly with the facet on the posterolingual side of the hypoconid, obliterating the mesoconid. The entoconid is anterior to the hypoconid and separated from the hypoconulid by a narrow gap. The gap is deeper on m2 than on m1. The wear facets on the anterolabial surfaces of the entoconid and hypoconulid are extensive, forming the lingual wall of a V­shaped talonid basin that slants from a point between the hypoconid and hypoconulid to the point between the metaconid and entoconid. The valley is oriented diagonally, as indicated by the arrow in figure 2c . Orientations of the valley and the wear facets indicate anterolingual movement of the tooth row during mastication. The hypoconulid is widely and deeply separated from the hypoconid. The cusps are more robust on m3 than on m2; nevertheless, the basic pattern of the two teeth is comparable except that the hypoconulid forms a distinct third lobe on m3. The posterolingual surface of the hypoconulid bears a large wear facet. Unworn m 3 in V14127.4 exhibits a distinct mesoconid. A small enamel peak rises between the hypoconid and hypoconulid in V14127.1 ( fig. 2e ).

COMPARISON: Eomylus was named based on specimens from the late Paleocene of Mongolia ( Dashzeveg and Russell, 1988), including E. zhegdenensis and E. borealis . In the same study, another taxon, Khaychina elongata , was proposed based on a fragmentary mandible with molars from the Naran Member of the Naran­Bulak Formation. Based on a larger sample of specimens including articulated dentitions from the late Paleocene Zhigden Member of the Naran Bulak Formation at Tsagan Khushu, Kondrashov and Lopatin (2003) were able to recognize that the holotype of E. zhigdenensis , a fragmentary maxilla with M1–2, and the holotype of Khaychina elongata (PSS 30­3, Dashzeveg and Russell, 1988) are conspecific; thus, Khaychina and K. elongata are junior synonyms of Eomylus and E. zhigdenensis , respectively.

In the previous study of the Bayan Ulan Fauna, Meng et al. (1998) tentatively referred some specimens that have the incisor ventral to the cheek teeth and ending at the first molar to? Khaychina elongata , although the possibility that these specimens represent a new taxon was also entertained. The diastema on the fragmentary mandible (V14127.3) appears long, which is the main reason for the tentative referral, but the lower cheek teeth resemble those of Khaychina elongata ( Dashzeveg and Russell, 1988) . For instance, as recognized by Dashzeveg and Russell (1988: 143), an oblique groove crosses the talonid basin of Eomylus from the notch between the metaconid and entoconid to another notch between the hypoconid and hypoconulid; the hypoconulid is well separated from both the entoconid and hypoconid and on m3 forms a distinct third lobe. Given the observations of Kondrashov and Lopatin (2003), however, Khaychina should be a junior synonym of Eomylus . With additional specimens, particularly upper cheek teeth collected since 1998, we here regard these Bayan Ulan specimens as a new species.

Eomylus bayanulanensis differs from E. zhigdenensis in having upper cheek teeth less anteroposteriorly shortened and less unilaterally hyspodont. In addition, the molar hypocone and metoconule are more distinct and there is a labial notch between the paracone and metacone on M2. The lower cheek teeth of the two species are similar in size and shape. The m1 and m2 of Eomylus bayanulanensis are less transversely extended. In contrast, these teeth are wider than long in E. zhigdenensis . The partial anterior cingulid on the lower molars of E. bayanulanensis is not seen in E. zhigdenensis . In addition, the mandible of E. bayanulanensis is proportionally shallower than that of E. zhigdenensis .

Eomylus borealis was originally referred to Mimotona and distinguished from other species of Mimotona by the presence of a single incisor in the mandible ( Chow and Qi, 1978). This difference was considered sufficient to place the specimen in the eurymylid genus Eomylus ( Dashzeveg and Russell, 1988) . Eomylus borealis differed from E. zhigdenensis ‘‘by its greater size and by the presence of a deep groove in the lower molars between the hypoconid and the hypoconulid’’ ( Dashzeveg and Russell, 1988: 138). However, the lower dentition referred to E. zhigdenensis is now considered to be from an unnamed species of Sinomylus and the lower jaw originally referred to Khaychina elongata (PSS 30­3) is now transferred to E. zhigdenensis ( Kondrashov and Lopatin, 2003) . Dashzeveg and Russell (1988) did not make comparisons between the lower jaw of E. borealis and PSS 30­3. Although E. borealis is larger than E. zhigdenensis (based on PSS 30­3) in terms of lower cheek tooth dimensions, the ‘‘deep groove in the lower molars between the hypoconid and the hypoconulid’’ is no longer diagnostic of E. borealis . This condition is present in the three species currently recognized in the genus. The features that are unique to PSS 30­3 include a long diastema between the incisor and p3, the proclivity of the incisor, and shallow anterior part of the mandible ( Dashzeveg and Russell, 1988). These features are not preserved in the holotype of E. borealis . Dashzeveg and Russell (1988: 141), however, recognized that PSS 30­3 further differs ‘‘from Eomylus by having lower crowned molars with a smaller hypoconulid’’. Thus, it can be rephrased that E. borealis differs from E. zhigdenensis in being larger and having higher crowned molars with larger hypoconulids.

Eomylus bayanulanensis is the smallest of the three species, although the size differences between these species are not significant. Its m1 and m2 are less transversely extended than those of E. borealis . The partial anterior cingulid on the lower molars and relatively smaller hypoconulid of E. bayanulanensis also distinguish it from E. borealis .

Figures

Fig. 1. Upper cheek teeth of Eomylus bayanulanensis, n.sp. a–d, occlusal, lingual, labial, and anterior views of a right M2 (V14126.1, holotype); e, f, occlusal and lingual views of left M1–3 (V14126.2).

Fig. 2. Lower cheek teeth of Eomylus bayanulanensis, n.sp. a, b, lateral and medial views of a partial right mandible with m1–3 (V14127.1); c–e, occlusal, lingual, and labial views of m1–3. Arrow indicates the direction of trough created by wear.

Tables

TABLE 1 Measurements of Lower Teeth (in mm) L 5 length; W 5 width; tri 5 trigonid; tal 5 talonid; * 5 nonavailable measurement.

L	L/W (tri)	L/W (tal)
Palaeomylus lii
V14132
p3 1.71		1.55
m1 2.27	0.87/1.86	1.35/2.06
m2 2.26	0.85/1.97	1.35/2.38
m3 3.05	0.82/1.99	2.15/2.07
V14129.1
m1 1.87	0.91/1.87	0.93/2.11
m2 2.04	0.89/2.03	1.14/2.41
m3 2.85	0.74/1.93	2.10/2.23
V14129.2
m1 1.98	0.91/*	0.96/1.98
m2 1.93	0.82/*	1.11/2.27
m3 2.92	0.69/1.96	2.28/2.08
V14130.1
p3 1.30	*/1.17
p4 1.93
m1 2.12	1.04/1.77	1.16/1.96
m2 2.44	1.03/2.07	1.37/2.17
V14130.2
p4 2.05	1.01/1.79	1.03/1.66
m1 1.89	1.12/1.83	0.78/*
m3 3.06	0.98/2.21	2.15/2.26
V14129.3
m3 3.11	1.02/2.08	2.06/2.32
V14129.5
m3 3.08	0.90/2.11	2.16/2.26
V14129.4
m2 2.14	0.85/1.94	1.34/2.24
V14131
m1 2.27	0.98/1.88	1.20/2.02
m2 2.19	0.98/1.95	1.18/2.21
Palaeomylus sp.
V14125.1
p4 2.00	1.08/1.73	0.98/1.42
m1 2.25	0.94/*	1.27/*
V14125.2
m1 1.96	0.91/1.65	1.04/1.94
Eomylus bayanulanensis
V14127.1
m1 1.73	0.83/1.18	0.91/1.53
m2 1.74	0.76/1.43	1.02/1.74
m3 2.22	0.75/1.39	1.67/1.63
Eomylus bayanulanensis
V14127.2
m2 1.92	0.82/1.57	1.10/1.76
m3 2.41	0.56/*	1.86/1.56 (est)
V14127.3
m3 2.59	0.69/1.46 (est)	1.88/165

TABLE 2 Measurements of Upper Teeth (in mm)

	Length	Width
Palaeomylus lii V14128
P4 M1	1.43 1.84	3.32 3.75
Eomylus bayanulanensis V14126.1
M2	1.53	2.51
V14126.2
M1 M2 M3	1.44 1.45 1.43	2.70 (est) 2.80 (est) 2.40 (est)

TABLE 1 Measurements of Lower Teeth (in mm) L 5 length; W 5 width; tri 5 trigonid; tal 5 talonid; * 5 nonavailable measurement.

TABLE 2 Measurements of Upper Teeth (in mm)