Yuanotherium minor, Hu & Meng & Clark, 2009

Hu, Yaoming, Meng, Jin & Clark, James M., 2009, A new tritylodontid from the Upper Jurassic of Xinjiang, China, Acta Palaeontologica Polonica 54 (3), pp. 385-391 : 386-388

Yuanotherium minor sp. nov.

Figs. 1 , 2 .

Etymology: From Latin minor , small.

Holotype: IVPP V15335 , a partial left upper jaw with anterior three postcanines ( Figs. 1 , 2 ).

Type locality: Wucaiwan area of Junggar Basin in northeastern Xinjiang, China.

Type horizon: The upper part of the Shishugou Formation, radiometrically dated as between 161.2±0.2 Ma and 158.7±0.3 Ma ( Eberth et al. 2001, 2006; Clark et al. 2006), within the Oxfordian stage of the Late Jurassic.

Diagnosis.—Upper cheek tooth with a cusp formula 2:4:3 on the second and third postcanines. Differing from other tritylodontids in having posteriormost two cusps of the median row on upper postcanines closely placed; cusps high and slender (potentially owing to lack of wear), buccolingually compressed; posterobuccal cusp with a prominent posterior crest.

Description.—The specimen is preserved in matrix, with only the palatal side being exposed ( Fig. 1 ), and represents the middle section of the left upper jaw, including much of the palatal portion of the highly reduced maxilla and palatine, a small portion of the premaxilla, the root of the zygomatic arch, and three anterior upper postcanines, denoted as PC1, PC2, and PC3. In ventral view, the maxilla comprises only a thin bone forming the alveoli of postcanines, as in derived tritylodontids such as Bienotheroides . A narrow band of the maxilla forms the lingual margins of the alveoli and separates them from the premaxilla and palatine; the latter two elements contact each other along a transverse, interdigitating suture at a level medial to the anterior portion of PC2. Only a small portion of the premaxilla medial to the suture is preserved. The preserved portion of the palatine is a thin, slightly concave plate bearing two small foramina, one being about 1.5 mm medial to the middle lingual cusp of PC2 and the other 2.0 mm medial to the middle lingual cusp of PC3. Watabe et al. (2007) identified similar foramina on a specimen of Bienotheroides shartegensis Watabe, Tsubamoto, and Tsogtbaatar, 2007 as the greater and lesser palatine foramina, respectively. Due to the damage of the premaxilla, the maxilla anterior and medial to PC1 is exposed ventrally. The medial edge of the bone does not contact its counterpart on the other side of the skull. It appears that the maxilla is reduced in size, but is not completely excluded from the secondary palate. Instead, the premaxilla ventrally overlaps the reduced palatal process of the maxilla and contacts the palatine. The stout zygomatic process of the maxilla forms the anterior root of the zygomatic arch laterodorsal to PC 2 and 3. The surface of the maxilla between the postcanines and the zygomatic arch is concave. A triangular but uneven facet on the ventral side of the maxilla root of the zygomatic arch is probably for the jugal.

Upper postcanines of Yuanotherium have a sub−quadrangular outline in occlusal view with rounded and convex buccal and, to a less extent, lingual margins ( Figs. 1 , 2 ). Measurements of these teeth are (length/width in mm): PC1, 2.63/2.76; PC2, 2.82/3.07; and PC3,?/3.48, respectively. Each tooth has three anteroposterior rows of cusps, which are separated by two deep longitudinal furrows. Apices of cusps are knob−like ( Fig. 2A, B ). Cusps of the median row are nearly symmetrical relative to the longitudinal axis, and those of lingual and buccal rows are asymmetrical. All cusps are slightly compressed linguobuccally and most of them form crescents facing anteriorly ( Fig. 2D ). The posterior buccal cusp is the tallest and anteroposteriorly longest cusp on the crown.

In crown view, PC1 has a rounded anterior edge and a transversely straight posterior one. The PC1 cusp formula is 2:3:3. Three cusp rows are subequal in length although the anterior cusp on the median row is more anteriorly positioned than those of the other two rows. The anterior cusps of all three rows are small and similar in size. The buccal and lingual anterior cusps are somewhat crescentic and strongly asymmetrical in that the buccal cusp has a sharp lingual crest and a blunt buccal one, whereas the lingual one has the reverse condition. The median anterior cusp is nearly symmetrical but is not crescentic. There are four accessory cuspules along the anterior margin of the crown, two of which are at the anterior ends of the longitudinal furrows and are smaller than other cuspules. Within the buccal row, the posterior cusp is similarly asymmetrical to, but much taller than, the anterior one. Its lingual crest extends anteriorly to the base of the anterior cusp but does not embrace it. In buccal and lingual views, the cusp is triangular in outline, with the apex of the cusp being positioned almost at the transverse axis of the tooth. The buccal surface of the cusp is convex, contrasting the flat lingual one that forms most of the lateral wall of the buccal furrow. The cusp possesses a distinct posterior crest that extends from the apex to the posterobuccal corner of the tooth, forming the posterobuccal margin of the tooth crown. The middle and posterior cusps of the median row are similar in size and shape; each is slightly shorter than the posterior cusp on the buccal row. The middle median cusp has symmetric buccal and lingual crests that enclose a steep crescent surface anteriorly and form a convex posterior surface. The crests reach anteriorly to the base of the anterior median cusp and slightly embrace it. In buccal or lingual view each crest is steep near the apex of the cusp and gently sloped approaching the base of the preceding cusp. The posterior median cusp is somewhat asymmetrical due to wear, deeper on its buccal side than on the lingual. Its crests extend to the base of the preceding cusp but do not embrace it. The posterior surface of the cusp protrudes slightly posteriorly beyond the posterior edge of the tooth and partially overlaps the anteromedian cusp of PC2. The middle lingual cusp is strongly asymmetrical, somewhat crescentic, and slightly smaller than the posterior two cusps of the median row. Its apex levels with that of the posterior cusp on the buccal row and its buccal crest extends anteriorly to a point lingual to the base of the anterior cusp on the lingual row. The buccal and lingual furrows are deep, roughly V−shaped in transverse cross section, anteroposteriorly straight, and widened posteriorly. The buccal furrow is wider and deeper than the lingual one, probably owing to deeper wear. The crests of crescentic cusps do not reach the bottom of either furrow, nor were they eliminated by wear.

PC2 is about 10% longer and wider than PC1 and has the cusp formula 2:4:3. The anterior and posterior edges of its crown are nearly straight and parallel to each other. The tooth is similar to PC 1 in general morphology. The most obvious difference is that its median row has four rather than three cusps, of which the two posterior cusps are closely packed so that they share a common base. PC2 also differs from PC 1 in several detailed aspects: (i) the anterior cusp on the median row is at the anterior edge of the tooth and buttresses the corresponding cusp of PC1; (ii) the notch separating the posterolingual and mediolingual cusps is wider than that on PC1; and (iii) there are only two cuspules, instead of four, on the anterior edge of the tooth crown.

PC3 was broken at its posterior end. The preserved tooth is almost identical to PC 2 in crown morphology, except for its slightly larger size. Trivial differences from PC2 include that the lingual crest of the anterior cusp on the buccal row almost reaches the anterior edge of the tooth and that the buccal crest of the middle lingual cusp embraces the anterolingual cusp.

IVPP V15335 probably belongs to a young adult individual because PC1–3 display some wear. All wear facets are limited to the anterior edges of crescents, suggesting that the crescents functioned as a cutting apparatus, as commonly seen in tritylodontids.

Stratigraphic and geographic range.—The upper part of the Shishugou Formation, Oxfordian of the Late Jurassic.

Figures

Fig. 1. Tritylodontid cynodont Yuanotherium minor gen. et sp. nov. from the upper part of the Shishugou Formation (Oxfordian, Late Jurassic), Junggar Basin, northwestern Xinjiang, China. IVPP V15335 (holotype), fragment of upper jaw and three anterior postcanines in occlusal view. Stereo pair (A) and explanatory drawing (B). 1–3, the first, second and third left upper postcanines.

Fig. 2. Tritylodontid cynodont Yuanotherium minor gen. et sp. nov. from the upper part of the Shishugou Formation (Oxfordian, Late Jurassic), Junggar Basin, northwestern Xinjiang, China. IVPP V15335 (holotype), showing details of PC1–3 in occlusolingual (A), occlusobuccal (B), occlusodistal (C), and occlusomesial (D) views.