Introduction

Tritylodontidae are a group of highly derived nonmammalian cynodonts living in the middle Mesozoic (Kemp 1982, 2005; Watabe et al. 2007). Fossil remains of definite tritylodontids have been found in terrestrial sediments of Early Jurassic to Early Cretaceous age from England, South Africa, Antarctica, North America, China, western Siberia, Japan, and Mongolia (Watabe et al. 2007; Hammer and Smith 2008). The family Tritylodontidae Cope, 1884 are characterized by an aberrant and stereotyped dentition that has enlarged incisors, no canine but a distinct diastema, and squarish postcanines with three rows of crescent cusps on upper teeth and two rows on the lowers. During mastication, leading edges on cusps of the lower postcanines moved posteriorly relative to opposing surfaces of the uppers, producing shearing action (Crompton 1972; Sues 1986a). The peculiar dental structures of tritylodontids are somewhat multituberculate-like and, to a less extent, rodent-like, and were presumably suitable for an herbivorous diet (Kemp 1982). Because tritylodontids possess some mammal-like dental, cranial and postcranial features, they were considered a group within mammals in early studies (see Simpson 1928 for a summary). The group was later removed from Mammalia because a mammalian jaw joint is lacking (Kühne 1956). Some workers recognize a close relationship between tritylodontids and mammals (Kemp 1982; Rowe 1988), whereas most studies of advanced synapsids favor the hypothesis that tritylodontids belong to a clade of largely herbivorous cynodonts, of which no branch led to mammals (Crompton and Ellenberger 1957; Hopson and Crompton 1969; Crompton 1972; Sues 1985a; Hopson 1991; Sidor and Hopson 1998; Hopson and Kitching 2001; Bonaparte et al. 2003, 2005). Regardless of their affinities, tritylodontids have a cosmopolitan distribution and are very abundant in terms of specimens in many Jurassic localities, indicating a degree of evolutionary success for the group in the middle Mesozoic.

Here we report a new genus and species of Tritylodontidae, represented by a partial left upper jaw with three postcanines. The specimen was collected from the Upper Jurassic part of the Shishugou Formation in the Wucaiwan area of the Junggar Basin in Xinjiang, northwestern China (Clark et al. 2006). Other tritylodontids, e.g., Bienotheroides zigongensis Sun, 1986, B. ultimus Maisch, Matzke, and Sun, 2004, and B. shartegensis Watabe, Tsubamoto, and Tsogtbaatar, 2007, have been reported from the Middle and Upper Jurassic of the Junggar Basin (Sun and Cui 1989; Maisch et al. 2004) and neighboring southwestern Mongolia (Watabe et al. 2007).

Materials and methods

The fossil described in this paper is housed at the IVPP. Although the canines were absent in tritylodontids, we follow the convention of referring to cheek teeth of tritylodontids as postcanines. Cusps on the tooth crown are described based on their relative positions (buccal, median and lingual; anterior, middle and posterior). The photographs were taken using a Nikon digital camera mounted to Nikon SMZ-U microscope.

Systematic paleontology

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.

Comparison and discussion

As in Bienotheroides, Stereognathus, Dinnebitodon, and Bocatherium (Sun 1984; Clark and Hopson 1985; Sues 1986c; Watabe et al. 2007), the premaxilla and palatine of IVPP V15335 contact each other on the palate and the maxilla is highly reduced. IVPP V15335 further shows that the premaxilla ventrally overlapped the reduced maxilla, although the latter is not completely excluded from the secondary plate by the expanded premaxilla. The condition is unclear in the other taxa with reduced maxillae.

IVPP V15335 displays the typical tooth morphology of tritylodontids and unquestionably belongs to the family. Yuanotherium minor is one of the smallest tritylodontids. Its PC2 is comparable in size to that of Oligokyphus minor Kühne, 1956 and smaller than all other tritylodontids except for Lufengia and Yunnanodon. The presence of closely packed posterior cusps in the median row of PC2-3 (and probably in more distal teeth as well) is unique among tritylodontids. Judging from moderate tooth wear and small size gradient of teeth, the specimen was probably from a young adult and thus the packed posterior cusps on the median row are not juvenile features. These morphological differences warrant establishment of the new genus and species within Tritylodontidae.

The early Jurassic Oligokyphus has a wide distribution and is known by abundant fossil remains (Simpson 1928; Kühne 1956; Sues 1985b; Luo and Sun 1993). Given its basal position in tritylodontid phylogeny (Clark and Hopson 1985; Sues 1986b; Setoguchi et al. 1999; Watabe et al. 2007), Oligokyphus may be regarded as approximating the ancestral morphotype of tritylodontids. Overall, the upper postcanine of Oligokyphus is longer than wide, which is different from all other tritylodontids. Oligokyphus differs from Yuanotherium in many aspects. In Oligokyphus, the cusp formula of a typical upper postcanine (PC2-5) is 3:4:4; the mesial and distal edges of the tooth are convex instead of being truncated; the median cusp row is longer than other two; the buccal row is placed mesially relative to the lingual row; the posterior median cusp projects posteriorly well beyond the margin of the tooth crown; the cusps are low, with minor differences in height between neighboring cusps, and are not linguobuccally compressed; the crests of most cusps (except for the anterior cusps) embrace the bases of their respective preceding cusps. The anterior cusp on the median row is crescentic. A similarity between PCs of Oligokyphus and those of Yuanotherium is that their buccal and lingual margins are somewhat convex rather than straight.

The early Jurassic Tritylodon from Africa (Owen 1884; Simpson 1928) has the same PC cusp formula (2:4:3) as in Yuanotherium, but as in Oligokyphus the dental morphology of Tritylodon differs from Yuanotherium in having similar cusp shape, small height difference between cusps, crescentic shape of the anteromedian cusp, and anterior shifting of the buccal cusp row. Tritylodon resembles Yuanotherium in having straight anterior and posterior margins and convex lingual margins on upper postcanines. In addition, the posterior median cusp does not project posteriorly beyond the tooth margin.

A PC cusp formula of 2:3:3 exists in several tritylodontid genera, including Bienotherium, Dianzhongia, Lufengia, Kayentatherium, and Bienotheroides, of which Bienotherium, Dianzhongia and Lufengia are from the early Jurassic of Yunnan, China (Young 1940, 1947, 1974; Chow and Hu 1959; Chow 1962; Hopson 1965; Cui 1981; Luo and Wu 1994). In Bienotherium a small cuspule is present in front of each cusp row (Hopson 1965; Sun 1984; Luo and Wu 1994) so that the total cusp number of a typical PC can be as many as eleven (Sun 1984). As in Oligokyphus the upper tooth of Bienotherium is square-shaped and has low cusps and a short buccal cusp row that is more anterior than the lingual row. As in Yuanotherium the upper tooth cusps of Bienotherium have weak crests.

Dianzhongia and Lufengia are similar to each other in upper postcanine morphology but differ from each other in size. Lufengia is by far the smallest known tritylodontid, with a skull length only about 40 mm (Chow and Hu 1959; Cui 1981; Cui and Sun 1987; Luo and Wu 1994; see also Fig. 3). The buccal cusp row in these two genera is short but is not shifted anteriorly relative to the lingual row. In the two genera, the anterior edge of the tooth is transversely straight but the posterior edge is convex, due to the protruding of the posterior median cusp, and most cusps have well-developed crests. The two genera are similar to Yuanotherium in having a reduced posterolingual cusp but in Lufengia the reduced posterolingual cusp extends more anteriorly to its preceding cusp, a condition absent in Dianzhongia and Yuanotherium.

Kayentatherium Kermack, 1982 (including Nearctylodon Lewis, 1986, as a junior synonym; Sues 1986a, c; Sues and Jenkins 2006), is from the Lower Jurassic Kayenta Formation of southwestern North America, whereas Bienotheroides, with four named species, comes from the middle to late Jurassic of eastern and central Asia (Young 1982, Sun 1984, 1986; Sun and Li 1985; Sun and Cui 1989; Maisch et al. 2004; Watabe et al. 2007). Both genera have postcanine tooth cusps buccolingually compressed and trenchant with steep flanks, resembling the condition of Yuanotherium, but crests on the cusps are more conspicuous than in Yuanotherium. The tooth crowns of the two taxa are somewhat oval in occlusal view due to the rounded tooth corners, in contrast to a more angular appearance in Yuanotherium. The anterior and posterior edges of the postcanines are straight in Bienotheroides, except for B. ultimus, but are convex in Kayentatherium. Kayentatherium is more comparable to Yuanotherium in tooth crown shape, whereas those of Bienotheroides are much wider than long. Kayentatherium is further similar to Yuanotherium in having a cuspule in front of the anterobuccal cusp and in the presence of a distinct posterior crest (“posteromedial crest” of Sues 1986) attached to the posterobuccal cusp, although the crest appears shorter and steeper than that in Yuanotherium. However, the buccal cusp row is the shortest cusp row in Kayentatherium, differing from both Bienotheroides and Yuanotherium. In Bienotheroides, a vestigial cuspule is developed in front of the anterobuccal cusp only in the genotype (Bienotheroides wansiensis), and the posterior crest from the posterobuccal cusp is not developed (Watabe et al. 2007: fig. 7).

Fig. 3.

Size distribution of upper postcanines in tritylodontids. Line indicates where length and width are equal. Unless indicated, all teeth are second upper postcanines. Data source: 1, Bienotherium yunnanense: Young (1947); Chow (1962); 2, Bienotherium magnum: Chow (1962); 3, Bienotherium minor: Young (1947); Chow (1962); 4, Bienotherium elegans: Young (1947); 5, Bienotheroides wansienensis: Sun (1984); 6, Bienotheroides zigongensis: PC3 measured from Sun (1986); 7, Bienotheroides shartegensis: Watabe et al. (2007); 8, Bienotheroides ultimus: isolated tooth, Maisch et al. (2004); 9, Bocatherium mexicanum: Clark and Hopson (1985); 10, Dianzhongia longirostrata: Cui (1981); 11, Dinnebitodon amarali: Sues (1986c); 12, Kayentatherium wellesi: MCZ8811, Sues (1986a); 13, Lufengia delicata: PC3, Chow and Hu (1959); 14, Oligokyphus major: R7030, Kühne (1956); 15, Oligokyphus minor: R7025, Kühne (1956); 16, Polistodon chuannanensis: He and Cai (1984); 17, Stereognathus ooliticus: a middle tooth, Simpsom (1928); Savage (1971); 18, Tritylodon longaevus: Butler (1939); 19, Xenocretasuchus sibiricus: isolated upper tooth, Tatarinov and Matchenko (1999); 20, Yuanotherium minor gen. et sp. nov.: this study (holotype, IVPP V15335); 21, Yunnanodon brevirostre: Cui (1976); 22, Unnamed Japanese specimen (a right upper tooth): measured from Setoguchi et al. (1999: fig. 1–1).

Two early Jurassic genera, Yunnanodon from Yunnan, China (Cui 1981) and Dinnebitodon from the Kayenta Formation, western North America (Sues 1986c), share an upper cusp formula of 2:3:2. In addition to the difference in the cusp formula, their upper postcanines further differ from those of Yuanotherium in having a quadrangular outline, cusps with more distinct crests that embrace the bases of preceding cusps, and cusps less transversely compressed. They are similar to Yuanotherium in having a reduced anteromedian cusp. Dinnebitodon is further similar to Yuanotherium in having a posterior crest extending posteromedially from the posterobuccal cusp, but the cusp is more posteriorly situated and the crest is shorter. Dinnebitodon also has a crest extending posterobuccally from the posterolingual cusp, in contrast to Yuanotherium in which a small third lingual cusp is developed at the posterolingual corner of the tooth. In contrast, the posterolingual and posterobuccal cusps in Yunnanodon have rounded posterior flanks (Cui 1976: fig. 4; Luo and Wu 1994: fig. 14.4D).

Stereognathus from the Middle Jurassic of England has a cusp formula of 2:2:2 on upper postcanines (Charlesworth 1854; Simpson 1928; Evans and Milner 1994). It further differs from Yuanotherium in having quadrangular upper postcanines, cusps with distinct and long crests extending from apices and extending to the lingual/buccal flanks of the preceding cusps, and any cusp on the lingual or buccal row having only a single crest.

Xenocretosuchus from the Lower Cretaceous of Siberia is similar to Stereognathus in upper tooth cusp formula and morphology of lingual and buccal cusps (Tatarinov and Matchenko 1999). However, it has small cuspules in front of the principal cusps, which are absent in Stereognathus. An unnamed tritylodontid from the Lower Cretaceous of Japan has upper postcanines similar to those of Stereognathus, although they probably differ in the development of cuspules along the anterior edges of the teeth (Setoguchi et al. 1999: figs. 1, 3). Polistodon from the Middle Jurassic of Sichuan, China (He and Cai 1984) and Bocatherium from the Lower Jurassic of Mexico (Clark and Hopson 1985; Clark et al. 1994; Fastovsky et al. 2005) have the same cusp formula as Stereognathus, on upper postcanines, but they also have a small cuspule in front of each cusp row, which is a condition more similar to Yuanotherium than to Stereognathus. Polistodon is unique among tritylodontids in having thirteen upper postcanines, with each tooth being relatively wide and short compared to those of other tritylodontids, Yuanotherium included. Bocatherium is similar to Yuanotherium in having an oval coronal shape of the upper postcanine.

To sum up, the new specimen differs from those of previously known tritylodontids and represents a new genus and species within Tritylodontidae. The interrelationships among tritylodontid genera have been explored in several studies (Clark and Hopson 1985; Sues 1986b; Setoguchi et al. 1999; Watabe et al. 2007). Based primarily on dental and cranial characters, these studies concluded that Oligokyphus was probably the most basal member of the tritylodontid clade and that more derived members of the clade evolved a relatively short and broad snout with a highly reduced maxilla and upper postcanines bearing fewer cusps in each row. Though uncertainties remain on several aspects of the tritylodontid morphology, such as the cusp homologies (Setoguchi et al. 1999; Watabe et al. 2007), it is notable that tritylodontid postcanines are highly conservative and variations among different taxa are small, if not trivial.

In IVPP V15335 the first upper postcanine has a rounded anterior margin decorated with minor cuspules, in contrast to more posterior teeth whose anterior margin is truncated. Evidence suggests that in tritylodontids the postcanines were not replaced and that new teeth were added sequentially at the posterior end of the tooth row, while anterior ones were shed after severe wear (Crompton 1955; Kühne 1956; Sues 1986a; Matsuoka and Setoguchi 2000). The morphology and the degree of wear of the first preserved tooth (PC1) of IVPP V15335 suggests that there has not been a tooth shed anterior to PC1, and that PC1 is probably the first upper postcanine this individual ever developed in life. PC1 of V15335 is only slightly smaller than more posterior teeth, suggesting that the animal probably grew to a substantial size before the first cheek tooth erupted. This appears also the case for several other tritylodontids with well-preserved upper dentitions, such as Bocatherium, Bienotherium, Bienotheroides, Dianzhongia, Lufengia, Oligokyphus, Tritylodon, and Yunnanodon. Timing of the first tooth eruption could be demonstrated with fossils from earlier ontogenetic stages, which would shed light on tritylodontid biology, such as parental care of neonates.

Tritylodontids have been inferred to have been herbivorous (Kühne 1956; Kemp 1982, 2005; Sues 1986a). The opposing crescent cusps of upper and lower postcanines are considered to be efficient for cutting vegetable fibers (Kühne 1956), and the cranial morphology is also consistent with an herbivorous masticatory apparatus (Sues 1986a). However, the postcanine morphology of Yuanotherium suggests some deviation from a purely herbivorous diet. The cusps in Yuanotherium are slender, with relatively weak crests, and the posterolingual cusp being high and blade-like. These features indicate more trenchant and probably more fragile postcanines in Yuanotherium than in other tritylodontids. The postcanines of Yuanotherium appear not well adapted for processing tough plants but suitable for puncturing, tearing and slicing soft food items, suggesting that this tritylodontid may have been omnivorous rather than herbivorous. This is consistent with the observation that some rodents and other extant mammals with teeth seemingly adapted to plant eating are actually omnivorous (Landry 1970; Nowak 1999 and references therein).