Distylomys burqinensis Bi & Meng & Wu & Ye & Ni 2009, n. sp.
Creators: name: Bi, Shundong
name: Meng, Jin
name: Wu, Wenyu
name: Ye, Jie
name: Ni, Xijun
Publisher: name: Zenodo
Year: 2009
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Mammalia
Rodentia
Ctenodactylidae
Distylomys
Distylomys burqinensis

description: Distylomys burqinensis, n. sp.
Figures 2–5, table 1
DIAGNOSIS: Intermediate sized species, slightly larger than D. tedfordi and smaller than D. qianlishanensis. Differing from D. tedfordi in having p4 longer than m1, a more elongate anterolophid and absence of posterolophid on p4, m1 relatively wider with a more acute lingual apex of the trigonid. Differs from D. qianlishanensis in having a more acute anterolophid and subtriangular talonid on p4, lower molar trigonid triangular, and mental foramen higher on the mandible.
HOLOTYPE: IVPP V16014.1, anterior partial skull with P4–M3 and associated mandible.
REFERRED MATERIAL: IVPP V 16014.2, palate with dentition; V 16014.3, palate with broken left P4–M2 and right M1–M3; V 16014.4, palate with left M1–M2 and right P4–M2; V 16014.5, palate with left M1 and right M1–M2; V 16014.6, partial premaxilla with incisors; V 16014.7, left premaxilla with broken I2; V 16014.8–9, 2 right premaxillae with broken I2; V 16014.10–14, 5 left M1; V 16014.15, left M2; V 16014.16–21, 6 right M1; V 16014.22, right M2; V 16014.23–26, 4 right M3; V 16014.27–32, 6 left fragmentary mandibles with broken i2, and p4–m3; V 16014.33–34, 2 left fragmentary mandibles with broken i2 and p4–m2; V 16014.35–39, 5 left fragmentary mandibles with p4–m1; V 16014.40, left fragmentary mandible with p4; V 16014.41, left fragmentary mandible with broken p4–m1; V 16014.42, left fragmentary mandible with m1–m2; V 16014.43, left fragmentary mandible with m2–m3; V 16014.44, left m1; V 16014.45–46, 2 left m2; V 16014.47, left m3; V 16014. 48, right fragmentary mandible with broken i2 and p4–m3; V 16014.49, right fragmentary mandible with broken p4 –m1 and complete m2–m3; V 16014.50–51, 2 right fragmentary mandibles with p4–m2; V 16014.52– 55, 4 right fragmentary mandibles with p4; V 16014.56–57, 2 right fragmentary mandibles with m1–m2; V 16014.58, right fragmentary mandible with m2–m3; V 16014.59, right fragmentary mandible with broken m1; V 16014.60–61, 2 right p4; V 16014.62–64, 3 right m1; V 16014.65–66, 2 right m3.
ETYMOLOGY: the species name refers to Burqin County, where site XJ200601 is located.
LOCALITY AND AGE: Locality XJ200601 (47 ° 23.198 9 N, 86 ° 47.981 9 E), Burqin County, Xinjiang, China. Suosuoquan Formation, early Miocene.
REPOSITORY: The specimens are stored in the collections of the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing.
DESCRIPTION: The holotype IVPP V16014.1 is a partial skull with articulated mandible. The skull preserves the anterior half, including the rostrum, palate, and partial frontal (fig. 2) and has been crushed and distorted, with the premaxilla shifted slightly lateroventrally. The incisive foramen is obscured due to distortion. Judging from what is preserved in the holotype and V16014.2 (fig. 4), it is very slender and narrow and ends posteriorly at the level of the center of the anterior zygomatic root. The base of the zygomatic root is preserved and shows that the zygomatic plate is not developed; its anterior margin is located far in front of P4 and its posterior margin is between P4 and M1. The tooth rows are convergent anteriorly. The palate is wider than the length of m1. The palatine-maxillary suture starts from anterior to M1. No palatine foramen can be recognized.
In dorsal view, the nasal bones are narrow and extend posteriorly as far as the premaxillae. Laterally, the premaxillary-maxillary suture extends above M1. The infraorbital foramen, although broken, is evidently large and hystricomorphous. The frontals are flat and broadened relative to the width of the rostrum.
The diastema of the mandible is shallow and shorter than the tooth row (fig. 3). The mental foramen is small and near the dorsal surface of the diastema. The mandible has a robust ventral masseteric crest, but there is no trace for the dorsal crest. The ventral crest terminates slightly anterior to the talonid of p4. Very little of the ascending ramus and angular process is preserved, but what remains suggests that the angular process is in the plane of the incisor within the horizontal ramus, as in other distylomyids.
The dental formula is 1/1, 0/0, 1/1, 3/3. All cheek teeth are hypselodont and at least moderately worn, so that little is known about tooth cusps. The upper incisors are broad and flattened anteriorly. Upper cheek teeth are strongly recurved toward the root, and the curvature increases posteriorly (fig. 4).
P4 is nonmolariform, unilobate, and somewhat trapezoidal in occlusal outline, with the posterior border slightly wider than the anterior one (fig. 4). Enamel is thinner at posterior margin.
All upper molars are strongly bilophodont without the mure, so that the trigon and talon form two separated enamel loops. The trigon and talon are straight and parallel to each other. They are separated by a transverse central flexus, which is almost completely filled with cement. In side views, lateral and medial flexi persist to the base of the tooth crown. The trigon is trapezoidal with a flattened lingual wall and a gently convex labial one. The talon is subrectangular, slightly wider than long. Given the tooth curvature, the upper tooth crown inclines posteriorly. Each molar bears four transverse enamel lophs that form the cutting edges of the tooth.
M1 is on average larger than M2 (table 1). In M1, the talon is wider but anteroposteriorly more compressed than the trigon. In M2 the talon is equal to or narrower than the trigon. M3 is larger than the preceding molars. In M3, the talon is reduced and much narrower, but slightly longer than the trigon.
The lower incisors extend posteriorly in the jaw to below m3 (fig. 3). The incisor enamel is thick, relative to the depth of the incisor (fig. 5D–F). It has a flat buccal surface and similarly wraps around to cover the medial and lateral sides of the tooth to a significant extent. The enamel microstructure is multiserial (Korvenkontio, 1934; Wahlert, 1968, 1989; Martin, 1992, 1997), and more specifically it consists of multiserial Hunter-Schreger bands (HSB) with rectangular interprismatic matrix (IPM), a more derived type than the parallel IPM that has an acute angle to the bands (Martin, 1992; 1997). The enamel is 150– 165 Mm thick, of which the portio interna (PI) and portio externa (PE) account for about 85% and 15%, respectively. Within the PI, each band consists mostly of four prisms. The HSB has a 50 ° –60 ° angle to the enamel-dentine junction, as measured following Martin (1992).
Unlike P4, which is nonmolariform, p4 is fully molariform and the largest of lower cheek teeth (fig. 4). The p4 trigonid is narrower than the talonid, with an acute anterior projection; its anterolingual edge is concave. The talonid is shorter but wider than the trigonid. No posterolophid is present except for three specimens that bear a shallow one. In 11 mandibles with check teeth, the premolar is less worn or in the same stage of wear as m1. Therefore we identify it as permanent p4, although Wang (1997) suggested that it may be deciduous p4.
All lower molars are similar in overall morphology. The m1 is longer than m2, but shorter than m3. All molars are bilophodont but have the mure; therefore, the trigonid and talonid are confluent through a narrow neck. The trigonid is triangular, with a tapered and acute lingual apex and straight anterolingual edge. The mesoflexid and hypoflexid intrude transversely halfway across the crown and are filled by thin cement. In side view, both flexids extend to the base of the crown. In m1, the talonid is wider than the trigonid with gently convex posterior edge. In m2, the talonid is equal to or slightly narrower than the trigonid. In m3, the talonid is much narrower than the trigonid.
COMMENT: The type species D. tedfordi was described by Wang (1988) on the basis of a single mandible, which was collected in 1928 by the Third Asiatic Expedition, AMNH, from the middle Miocene Tunggur Formation of the Tairum Nor Basin of Inner Mongolia. No additional material has since been referred to the species, restricting the comparison of the new materials to the lower jaw only.
D. burqinensis is similar to D. tedfordi and is clearly closely related to it. The type specimen of D. tedfordi from Tunggur lies within the lower size range of D. burqinensis from Burqin. Morphologically, however, D. tedfordi differs from D. burqinensis in several aspects: p4 shorter than m1, having a less elongate anterolophid and subelliptical talonid, m1 relatively narrower with an inward curved anterolingual edge and more blunt lingual apex. In D. tedfordi, the posterolophid is distinct on p4; in Burqin specimens, it is distinct in three, and absent in eight.
D. burqinensis is smaller than D. qianlishanensis and differs in having a more acute anterolophid and subtriangular talonid on p4; triangular trigonid on lower molars, and higher position of the mental foramen on the mandible.
Wang (1988) thought D. tedfordi was morphologically more primitive than D. qianlishanensis because it possesses a well-developed posterolophid, a condition considered primitive. This, as Wang noted, is inconsistent with the stratigraphic occurrences of the species because D. tedfordi was found in later deposits (middle Miocene) than D. qianlishanensis (late Oligocene). D. burqinensis, with an early Miocene age, also lacks the posterolophid or has only an incipient one. We therefore offer an alternative interpretation: the presence of the posterolophid is a derived character within the genus, appearing in species with a younger age. If this is true, D. burqinensis is morphologically intermediate between D. qianlishanensis and D. tedfordi, but is more similar to the latter. It should be noted that size decreases in this lineage.
description: Published as part of Bi, Shundong, Meng, Jin, Wu, Wenyu, Ye, Jie & Ni, Xijun, 2009, New Distylomyid Rodents (Mammalia: Rodentia) from the Early Miocene Suosuoquan Formation of Northern Xinjiang, China, pp. 1-20 in American Museum Novitates 3663 on pages 4-11, DOI: 10.1206/665.1, http://zenodo.org/record/4735501