SYSTEMATIC PALEONTOLOGY

Order RODENTIA Bowdich, 1821
Superfamily MUROIDEA Illiger, 1811
Family incertae sedis
PARARHIZOMYS Teilhard and Young, 1931

Type species: Pararhizomys hipparionum Teilhard and Young, 1931

Type locality: Loc. 10, Chenchiangpao, Fugu County.

Geographic distribution: Fugu, Shaanxi Province; Qin’an, Gansu Province; Altan Teli, Mongolia; Lantian, Shaanxi Province.

Diagnosis: Large muroid rodent with the origin of the anterior part of the lateral masseter muscle on the maxillary portion of the zygomatic arch, the infraorbital foramen is restricted in a high position with the ventral slit partially closed. Upper tooth rows diverge somewhat posteriorly. Molars high crowned but still rooted when mature. M1/m1 have one lingual and two labial reentrants, M2/M3/m3 have only one lingual and one labial reentrant, while m2 has one lingual reentrant but two on the labial side.

Discussion: In their description Teilhard and Young (1931, p. 11), listed the type specimen erroneously as from Loc. 1, but corrected this in errata to Loc. 10. Kowalski (1968) did not notice this change, citing the type locality as Loc. 1, near Chinglo in Shanxi, northern China, and thus associating all the fossils from Chinglo together with Pararhizomys. The Chinglo (Jingle in Chinese spelling) fauna is now considered late Pliocene, equivalent to MN16 according to the small mammals (Zhou 1988). Paleomagnetic data restrict it to the 2.5~3.0 Ma interval (Yue and Zhang 1998). In contrast, Loc. 10 is a very poor locality with (in addition to Pararhizomys) only a partial lower jaw of Yangia omegodon (current identification of Zheng 1997). All the more confusingly, the latter rootless siphneid is typical of forms found no earlier than the late Pliocene Loess. The occurrence and geological age of fossils at the type locality should be verified by future work.

The type specimen of Pararhizomys, a left lower jaw with both ends broken, was only briefly described in 1931 and superficially discussed in subsequent literature. Thanks to Li Chuankui, Everett Lindsay was able to mold the holotype (about the year 1980), and we have a cast. To the original description, we add the following observations. The masseteric fossa is indeed prominent as Teilhard and Young (1931) indicated. It is delimited by a heavy masseteric crest projecting laterally from the horizontal ramus. The ascending ramus is thus well separated from the tooth row. The lingual surface of the mandible is depressed. The mental foramen is exceptionally low on the jaw, under the anterior root of m1 and below the level of the front of the masseteric crest.  Molar patterns are simple, m2 shows an anterolabial reentrant behind the labial arm of its anterolophid, and m3 is the smallest molar. The incisor enamel is flattened and does not extend far laterally. A faint ridge runs the length of the incisor, somewhat lateral to its midline. Incisor width is 3.47 mm.

Pararhizomys hipparionum Teilhard and Young, 1931

Referred Material: Partial left maxilla with M1-M3 (V 14178); partial left mandible with m1-m3 (V14179).

Locality: V 14178 from Lamagou gully, Fugu, eastern Shaanxi Province, 50 km north of Baode; V 14179 from Qin’an, 200 km southeast of Lanzhou, Gansu Province.

Description: V 14178 has the maxillary process of the zygomatic arch originating from a position about opposite the middle part of M1. The maxilla anterior to M1 slopes strongly. Molars are high crowned, and in both M1 and M2, the base of the enamel undulates and descends to its greatest depth anteriorly (Figure 1.4). M1, the largest molar, has a slight inflection on the lingual surface of the anteroloph. There is one lingual reentrant, spanning about half the tooth, and angled slightly anteriorly. Of the two labial reentrants, the anterior one is slightly shorter and transverse; the other angles slightly posteriorly. The posterior wall of M1 is rounded.

M2 is wider anteriorly than posteriorly and has gently rounded anterior and posterior walls. The labial reentrant bends gently anteriorly. Posterolingual to it is a slightly longer diagonal lingual reentrant. M3 is the smallest molar in the tooth row. It also has only one labial and one lingual reentrant. However, the labial reentrant is much shallower and extends across about one third of the occlusal surface. The deeper lingual reentrant, strongly anteriorly oriented, crosses the central longitudinal axis. Anteriorly M3 is flattened, but its posterior end is round.  Two roots are visible on M3.

The mandible from Qin’an (V 14179) has the ascending ramus arising well lateral to m2. There is a wide valley between the tooth row and the ascending ramus. The specimen has well worn teeth, but is not in advanced wear. The occlusal surface of m1 is oval shaped with the anterior part narrower. The one lingual reentrant extends transversely and then bends strongly anteriorly from about its middle point. One small enamel island is situated on the anterolabial side. The transverse posterior labial enamel loop spans about half the molar width.

There are three (two labial and one lingual) reentrants on m2. The anterior labial reentrant is much shorter, and slightly anteriorly oriented, while the posterior labial reentrant is about twice its size and is basically transverse. The lingual reentrant curves slightly anteriorly. The anterior surface is nearly straight, with very thin enamel at the contact wear facet. The posterior surface is rounded. The occlusal surface of m3 is generally rectangular, with only one lingual and one equal labial reentrant.