Coelodontomys, WANG, 2001

WANG, BANYUE, 2001, On Tsaganomyidae (Rodentia, Mammalia) of Asia, American Museum Novitates 3317, pp. 1-52 : 32-47

Coelodontomys , new genus

Tsaganomys Matthew and Granger, 1923 (partim). Vinogradov and Gambaryan, 1952 (partim). Shevyreva, 1974a. Bryant and McKenna, 1995 (partim). McKenna and Bell, 1997 (partim). Cyclomylus Matthew and Granger, 1923 (partim). NEW SYNONYMY.

DESCRIPTION: The rostrum is slender and long. The anterior end of the zygomatic arch is formed by a broad vertical maxillary root. On the ventral side there is a scar for the masseteric muscle. The infraorbital foramen is small and oval in shape. The major axis is nearly vertical and perpendicular to the anterior margin of the ventral surface of the anterior zygomatic root. One of the particular features is that the infraorbital foramen is separated by a small bony plate into two foramina. Between P4 and the incisive foramen no indication of antepremolar crest is present. The two cheek tooth rows are nearly parallel to each other. The upper incisor extends lateral to M1–2. The P4 grows from below the posterior part of the upper incisor alveolus, and the M1–2 grow medial to the upper incisor. The lower cheek teeth grow from above the lower incisor.

The cheek teeth are highly crowned with great unilateral hypsodonty and have only one main closed root and thick cement on walls. The occlusal surface is oval in shape, having a core when worn. As in other Cyclomylus , the dentine part is very short and

TYPE SPECIES: Coelodontomys asiaticus , new and only known species.

GEOLOGICAL RANGE: late early Oligocene.

GEOGRAPHIC DISTRIBUTION: North China, Mongolia, and Kazakhstan.

ETYMOLOGY: Coelodontomys, Greek ; koilos, hollow; odontos, tooth; and mys, mouse. The name indicates that the cheek teeth have developed pulp cavity.

DIAGNOSIS: Medium­sized tsaganomyid, nasal extending slightly posteriorly, with nasal­frontal suture in one line with premaxillary­frontal suture; small oval infraorbital foramen with nearly vertical major axis, almost perpendicular to anterior margin of ventral surface of anterior zygomatic root; left and right antepremolar crests convergent forward but not merging; premaxillary­maxillary suture intersects the two antepremolar crests; angle between dorsal surface of posterior part of upper incisor alveolus and occlusal surface of cheek teeth near 60°; P4 concave buccally, passing by ventral and reaching buccal side of upper incisor; upper molars extending to orbital region passing by upper incisor on the medial side; lower cheek teeth concave medially or anteromedially; p4 extends from above lower incisor, but lower molars buccal to lower incisor; cheek teeth strongly unilarterally hypsodont, with open roots, very short dentine part, well­developed cylindrical pulp cavity, and covered by cement; enamel on occlusal surface slightly upper part of Ulantatal Formation, Hsanda thick, occlusal pattern lost early and usually Gol Formation and Buran Formation).

with core when worn; lower cheek teeth with GEOGRAPHIC DISTRIBUTION: Saint Jacques hypertrophic hypoconid, oval p4 with narrow ( IVPP Locs. 77045 [ IVPP V 11427], 77046 trigonid; PE of lower incisor enamel thin, [V 11428], 77047 [V 11429], 77049 [V with near 30° inclined prisms; HSB inclined 11430, V 11431], 77049.1 [V 11432], at nearly 30° and composed of 4–5 prisms. 77049.2 [V 11433], 77049.3 [V 11434],

77049.7 [V 11435], 78020 [V 11436]), Qian­ Coelodontomys asiaticus , new genus and lishan District ( IVPP Loc. 78018: V 11437),

species Ulantatal (V 11438) of Nei Mongol, China;

Grand Canyon and other localities of Tsagan

Figures 13C , 28–31 ; table 5. Nor Basin, Mongolia (V 11439, V 11440, Tsaganomys altaicus Matthew and Granger, 1923 and other specimens in AMNH); and K 15, (partim): 2–4, figs. 1 (partim), 2 (partim), 3, 4A. Podorpzhnik, Zaysan Basin, Kazakhstan Vinogradov and Gambaryan, 1952 (partim): 23, [ K 35(15)/425].

24. Bryant and McKenna, 1995 (partim): 5–16, DIAGNOSIS: As for genus.

figs. 3 (partim), 4, 5, 10A, 11A. Shevyreva, ETYMOLOGY: Asiaticus, Latin , Asian.

1974a: 46, figs. 1v, g, 6, 7, 8. NEW SYNONYMY. DESCRIPTION: Bryant and McKenna (1995) Cyclomylus lohensis Matthew and Granger, 1923 carefully described Tsaganomys altaicus but, (partim): 5.

NEW SYNONYMY

.

unfortunately, based their description on a Tsaganomys sp. Emry et al. 1998: 308–310, fig.

mixture of specimens, some of which are re­ 8B. NEW SYNONYMY.

ferred to Tsaganomys ( AMNH 19037), Cy­ HOLOTYPE: A nearly complete skull clomylus ( AMNH 81927, 82183, 84545), and ( AMNH 21675), from Hsanda Gol Forma­ others to Coelodontomys ( AMNH 21675, tion, Field. no. 531, Grand Canyon, north of 19021, and 19023), respectively, in the pre­ Tsagan Nor, Mongolia, figured by Bryant and sent paper. Their description of the skull is McKenna, 1995: figure 4. (Note that this fig­ mainly based on AMNH 21675 and 19021. ure is actual size. The scale is incorrect.) These two specimens are now referred to REFERRED SPECIMENS: IVPP V 11427– Coelodontomys asiaticus , of which AMNH 11440; AMNH 19021–19025, 19028, 19029, 21675 is the holotype. To some extent, what 19033–19035, 19038, 19099, 21659, 21676– they described is the skull of Coelodontomys . 21678, 21691, 22085, 56625, 81219, 81221, A few additional remarks are given below.

81222, 81235, 81273, 81335, 81347, 81396, Taken as a whole, the skull of Coelodon­ 81397, 81468, 81469, 81472, 81482, 81484, tomys is similar to that of Tsaganomys . How­ 81485, 81990A, 81993, 81995–81998, 82251, ever, there are some important differences 82255, 82266, 82271, 82278–82281, 82284, between them. In Coelodontomys the range 82400, 83502–83504, 83506, 83507, 83521, of variation in size is not as large as in Tsa­ 83541, 83548–83554, 83556, 83557, 83559– ganomys. It is usually medium sized. The na­ 83561, 83565, 83579, 83583, 83589, 83599, sals extend more posteriorly than in Tsagan­ 83601–83604, 83611–83614, 83616–83620, omys. The nasal­frontal suture is nearly 83625, 83627, 83628, 83630–83632, 83635, straight or convex posteriorly and almost in 83642, 83645, 83648, 84329–84331, 84333– one line with the premaxillary­frontal suture. 84340, 84342, 84344–84346, 84497–84500, As in Tsaganomys the temporal fossae are 84502–84504, 84506, 84507, 84509, 84511, large and the distinct temporal crests arise 84512, 84514, 84515, 84518–84521, 84523, from the prominent anterior process medial 84526, 84535, 84546, 85183, 85185–85188, to the orbit and turn posteromedially to 85194, 85239, 85242, 85244, 85245, 85248, merge into the sagittal crest, which, in turn, 85254, 85259, 85262, 85267–85269, 85274, meets the occipital crest. On the dorsal view 85277–85281, 85287, 85318, 85538, 85544, the narrowest part of the skull is in the post­ 85738, 85741, 85743, 85745; PIN 475­3878, orbital constriction area rather than in the in­ 47540; and K 35(15)/425. terobital constriction. The strong zygoma is GEOLOGICAL RANGE: Late early Oligocene much arched outward.

(upper member of Wulanbulage Formation, The antepremolar crests from P4 to the incisive foramen are nearly straight, are convergent toward the incisive foramen, but do not merge. The premaxillary­maxillary suture crosses the two crests. In anterior view the small oval infraorbital foramen is slightly inclined, with a nearly vertical major axis that is almost perpendicular to the anterior margin of the ventral surface of the anterior zygomatic root. The pterygoid fossa is small, shallow, and not perforated.

As in Tsaganomys the pocket formed by lachrymal, jugal, and maxillary is interpreted here as the eye socket. The orbit is very small. The procumbent upper incisor extends backward to the buccal side of the M1–2. The dorsal line of the posterior part of the upper incisor alveolus and the occlusal surface of the upper cheek teeth form an angle of nearly 60°, which is larger than in Tsaganomys . The upper molars extend to the orbital area, passing by the medial side of the upper incisor. Unlike in Tsaganomys , however, P4 is strongly concave buccally, passing ventrally to reach the buccal side of the upper incisor (fig. 28).

The mandible is similar to that to Tsaganomys , and the lower cheek teeth are concave lingually or anterolingually. The lower molars originate buccal to the lower incisor. However, p4 grows from above the lower incisor, which is similar to that of Cyclomylus but different from that of Tsaganomys .

The cheek teeth, as in Tsaganomys , are cylindrical, strongly unilaterally hypsodont, and with open roots. The enamel, extending walls. On the occlusal surface the enamel is from occlusal surface to the base, covers the thicker than in Tsaganomys but thinner than anterolingual side on the upper cheek teeth in Cyclomylus . With wear the occlusal patbut the posterobuccal side on the lowers. tern is lost early; the occlusal surface be­ There is always cement covering the side comes smooth and concave and usually has a core, which is slightly convex and usually similar to the contour of the tooth in shape. Different from Tsaganomys but similar to Cyclomylus , the dentine part is very short in height and the pulp cavity is well developed. The ratio of the dentine part to the pulp cavity is very small, even smaller than in Cyclomylus . The primary dentine seems to stop growing early. As the primary dentine is worn away, the void is replaced by the secondary dentine, which forms a core. The pulp cavity is usually cylindrical in shape, with its walls parallel to those of the tooth and with a flat or concave top.

The occlusal pattern is more similar to that of Cyclomylus than Tsaganomys . It is weaker than in the former, with more slender crests and shallower valleys. The cheek teeth are four crested. The p4 is oval in occlusal view, with a very narrow trigonid. The protoconid and metaconid are close to each other, and the metalophid II is very short. The metalophid I is absent. The trigonid basin forms a very narrow fissure opening anteriorly. The hypertrophic hypoconid is the largest of the main cusps, extending anterobuccally. The hypolophid is weak. The molars are similar to p4 but with a wider trigonid.

As in Tsaganomys , the incisors are triangular in cross section, with distinct or indistinct longitudinal ridges on the labial surface. The incisor enamel is multiserial with relatively thin PE; its ratio to total enamel thickness is near 17%. Its total enamel thickness is near 110–139µm, thinner than in Tsaganomys . The HSB is composed of 4–5 prisms and has 25–30° inclination. The inclination of the prisms of PE is near 30°.

DIMENSIONS: See table 5.

DISCUSSION: While describing Tsagano­

TABLE 4 Measurements (mm) of Cheek Teeth of Cyclomylus biforatus , new species

mys altaicus, Matthew and Granger (1923) mixed the specimens presently referable to both Tsaganomys altaicus and Coelodontomys asiaticus . Of their paratypes only AMNH 19020 and 19037 belong to T. altaicus . The other specimens, including AMNH 19029, 19033, and 19038 (I have not seen AMNH 19030), belong to Coelodontomys asiaticus . Figure 1 of Matthew and Granger (1923) is a reconstructed skull based on both T. altaicus (holotype, AMNH 19019) and C. asiaticus ( AMNH 19021 and 19033). Their figures 3 and 4 showed the specimens belonging to C. asiaticus . As mentioned above, Shevyreva (1972, 1974a) distinguished two forms among the strong hypsodont tsaganomyids. Unfortunately Beatomus bisus is a synonym of Tsaganomys altaicus , and her Tsaganomys altaicus is actually Coelodontomys asiaticus . Bryant and McKenna (1995) lumped all the known tsaganomyid taxa into one species Tsaganomys altaicus . However, Coelodontomys is generically different not only from Cyclomylus but also from Tsaganomys . The differences are listed in the table 6.

DISCUSSION cavity is well developed and high. As the tooth

aged, it does not seem the primary dentine THE STRUCTURE OF CHEEK TEETH OF THE

grew continuously as in Tsaganomys , but the TSAGANOMYIDAE

pulp cavity grew higher and higher and the The cheek teeth of the Tsaganomyidae are tooth walls became thin. This kind of tooth all unilaterally hypsodont, and the occlusal pat­ seems physically weak, even if it is very high tern is lost early in wear. However, the tooth crowned. When considerably worn, the short structure of Tsaganomys is quite different from primary dentine in the center of the tooth was that of Cyclomylus and Coelodontomys . In lost, and the pulp cavity became uncovered on Tsaganomys the dentine part is well developed, the occlusal surface so that a hole can be seen always deeper than the pulp cavity in height. extending from the surface to the pulp cavity The primary dentine grew continuously with in some teeth. To reinforce the tooth secondary the growth of the tooth. With wear, the occlusal dentine filled this hole, forming a core.

surface became smooth, lacking a core of sec­

ondary dentine. Therefore, it is made totally of P3 Or dP3 OF TSAGANOMYIDAE primary dentine. In Cyclomylus and Coelodon­ In the diagnosis of Tsaganomys, Matthew tomys the dentine part is short, but the pulp and Granger (1923: 2–4) mentioned that the

TABLE 5 Measurements (mm) of Cheek Teeth of Coelodontomys asiaticus , new genus and species

dental formula of the cheek teeth was 4/4 and dP3 and dP4 were present. In the diagnosis of Cyclomylus lohensis they stated (1923: 5), ‘‘The permanent premolar is preceded by a minute, simple dP3 and a large round molariform dP4, as in Tsaganomys .’’ Obviously they thought that in both Tsaganomys and Cyclomylus two upper deciduous teeth, dP3 and dP4, were replaced by only one premolar, P4, and P3 was never present. In his diagnosis of the Tsaganomyinae Wood (1974: 50) mentioned ‘‘milk teeth dP3–4/dp4; permanent cheek teeth reduced to P4/p4, M1–3/ m1–3).’’ Later, however, misinterpretations of dP3 as P3 by some paleontologists have led the classification of the tsaganomyids into confusion. Shevyreva (1972) considered the presence of P3 one of the main features of her new genus, Sepulkomys , because she thought dP3 and dP4 of PIN 475­50 as P3 and P4. Kowalski (1974: 159) even thought that Cyclomylus lohensis had a P3. This was accepted as the main feature of Cyclomylus lohensis distinct from Tsaganomys by Huang (1993). Bryant and McKenna (1995: 8) mentioned that ‘‘the tooth in question is actually

TABLE 6 Comparison of Characters among Tsaganomys , Cyclomylus , and Coelodontomys

a dP3 that is lost early in life and not replaced.’’ They came to the same conclusion as Matthew and Granger (1923). As far as I know the specimens, which preserved dP3 (see Bryant and McKenna: 19, table 2), belong to either Cyclomylus or Coelodontomys . So far no specimen of Tsaganomys altaicus preserving dP3 has ever been found. In Cyclomylus and Coelodontomys dP3 occurred but it was lost early in life and was not replaced, whereas in Tsaganomys it is uncertain whether dP3 was present or not.

ANALYSIS OF SOME FEATURES OF SKULL AND MANDIBLE OF THE TSAGANOMYIDAE

Vinogradov and Gambaryan (1952) analyzed the masticator muscle and the muscle of the anterior limb of the tsaganomyids. Some supplementary analysis is given below. The Tsaganomyidae have many particular features, which can indicate something about their ecology and relationships with other rodents. These features are

(1) protrogomorphous skull with small infraorbital foramen, which is not the passage for the masseter muscle;

(2) temporal fossa tremendous, sagittal and occipital crests very prominent, with pronounced anterior process;

(3) zygomatic arch very robust and strongly flared laterally, with rough area for masseteric muscle on the ventral surface;

(4) jugal enlarged and extending anteriorly to meet the premaxilla;

(5) pterygoid fossa small, shallow, not perforated;

(6) glenoid fossa broad, short, and shallow;

(7) hystricognathous mandible with large, flared, but not very much posteriorly extend­ ed angular process;

(8) coronoid process high;

(9) condyloid process with wide, short, triangularly convex articular surface;

(10) postcondyle expansion absent;

(11) masseteric fossa large and deep, with weak upper and lower masseteric crests;

(12) small fossa present anterior to the masseteric fossa;

(13) lower margin of angular process is rough;

(14) distinct longitudinal crest separating medial fossa from flat lower part on the lingual side of angular process;

(15) occlusal surface of cheek tooth row with steplike profile; and

(16) cheek teeth cylindrical, unilaterally hypsodont, with occlusal pattern lost early in wear, and the worn occlusal surface concave bordered by high margin.

Among these features, character (2) and (8) indicate that the tsaganomyid temporal muscle was voluminous. The anterior process of the sagittal crest is interpreted as the anterior origin of the temporal muscle. It is situated anterior to P 4 in position, and a distinct anteroventral crest is present posteromedial to the orbit. These features indicate that the temporal muscle had a well­developed orbital part in the tsaganomyids. The primary function of this muscle was to close the jaw. This voluminous temporal muscle may have been correlated with the specialized mode of life in the tsaganomyids, characterized by digging and crushing rather than by propalinally chewing food.

Probably the extremely large temporal muscle needed a very large space. Therefore the zygoma forms a strong laterally convex arch. To strengthen the zygoma the jugal is enlarged and extends anteriorly to meet the premaxillary. Corresponding to the strongly laterally convex zygoma, the angular process of the lower jaw becomes very large and flared. This is apparently associated with the strongly convex zygoma.

Character (1) indicates that the masseter muscle did not pass through the infraorbital foramen. The small pocket formed by the jugal, lachrymal, and maxillary is the eye socket rather than a depression for the attachment of the anterior part of the masseter muscle. As in other protrogomorphs, the masseter medialis of the Tsaganomyidae was not elongate forward and its origin did not shift forward. It may have originated from the medial side of zygoma and inserted on the masseteric fossa of the lower jaw. It was strong and pulled the lower jaw in almost a vertical direction. Generally, the horizontally lying posterior part of masseter lateralis profundus is often associated with the posterior expansion of the mandible (= postcondyloid process) (Woods, 1972). However, character (10) indicates that the masseter lateralis in the tsaganomyids was not as elongated and of the glenoid fossa and condyloid process did not lie as horizontally as in hystrico­ in an anteroposterior direction. However, the morphs but was short, strong, and mainly condition of the tsaganomyids is contrary vertically directed. (see characters [6] and [9]). This indicates

As in the Hystricognathi there is a groove that the mouth of the animal could open very between the angular process and the lower widely and the mandible could move rather incisor alveolus for the masseter superficialis freely in different directions, but the propalin the Tsaganomyidae . However, character inal movement was limited. (14) indicates that the groove was restricted The flatness of the worn cheek tooth surby a distinct longitudinal crest, which may face in many rodents is an indication that the represent the upper margin of the insertion animal chews propalinally. In the tsaganoof the pars reflexa of the superficial masseter. myids the steplike occlusal surface of the As so interpreted, the insertion of this part of cheek tooth row, character (15), obviously masseter muscle was low and restricted on hindered the propalinal chewing of the lower the flat lower part and rough ventral margin jaw when the upper and lower cheek teeth of the angular process. came into contact.

According to Woods (1972) the elongated It seems that the tsaganomyids were adapt­ and horizontally lying masseteric muscle is ed for a fossorial mode of life. They had a an important component of the propalinal low, broad skull with a narrow rostrum and masticatory apparatus of the rodents. Its strong and procumbent incisors and could function is to force the lower jaw anteriorly open their mouth widely. The well­develin propalinal chewing. In the tsaganomyids oped temporal muscle and the short, vertithe main function of the masseter muscle was cally oriented masseter and pterygoid musnearly vertical, and the muscles were mainly cles could close the mouth quickly. They associated with crushing and transversely may have dug with their lower jaw and chewing rather than with anteroposterior crushed and chewed food transversely rather movement of the lower jaw. than propalinally. In addition, their tooth

The pterygoid muscles are important com­ morphology (character [16]) is similar to ponents of the rodent jaw musculature. To some of the edentates. Possibly they had simaccommodate the propalinal movement the ilar feeding habits, adapted to feed on insects pterygoid muscles increase in length and lie and worms. in a more horizontal plane. This is accomplished by the movement of the origin of the PHYLOGENETIC RELATIONSHIPS AMONG THE internal pterygoid muscle deep into the pter­ GENERA OF THE TSAGANOMYIDAE ygoid fossa, through movement of the insertion of the internal pterygoid muscle distally The Tsaganomyidae comprises five speonto the posteriorly extended angle of the cies in three genera. Among the genera, Cymandible, and by movement of the insertion clomylus seems the most primitive. Tsaganof external pterygoid muscle onto the post­ omys and Coelodontomys , sharing highcondyloid process. As a result, the pterygoid crowned, unilaterally hypsodont cheek teeth, fossa becomes enlarged or even open into the have been confused for a long time. Howorbit or braincase, and a prominent postcon­ ever, tooth structure, occlusal pattern, and the dyloid process is present. In the tsagano­ relation between the incisor and the forth myids, however, characters (5), (10) and (13) premolar are quite different between them. indicate that as in other protrogomorphs their On the contrary, Cyclomylus and Coelodonpterygoid muscles did not increase in length tomys have a closer relationships than either and did not lie horizontally (Wood, 1965). do with Tsaganomys . They share such de­ Obviously the main function of these mus­ rived features as an infraorbital foramen with cles in the tsaganomyids was to pull the low­ a nearly vertical major axis, a larger angle er jaw mediad and serve a transverse motion between the dorsal surface of the posterior in chewing food. part of upper incisor alveolus and the occlu­

In rodents a greater anteroposterior shift of sal surface of the cheek teeth, cheek teeth the lower jaw is allowed by the elongation having a reduced dentine part, a well­developed pulp cavity, a secondary dentine core, and incisor enamel having inclined prisms of PE. Although the cheek teeth of Tsaganomys and Coelodontomys have a similar evolutionary tendency, becoming strongly unilaterally hypsodont, their evolutionary direction is quite different. In Tsaganomys the dentine grows continuously with tooth growth and the premolar growth mimics that of the molars: P4 reaches the medial side of I2 and p4 extends to buccal side of i2 as do the lower molars. In Coelodontomys , when cheek teeth grow the pulp cavity grows but the dentine part stops growing. The premolars proceed in a different path from the molars: P4 passes by the ventral side of I2, reaches the buccal side of I2, and is separated from the upper molars by the posterior part of the upper incisor, and p4 extends to dorsal side of the incisors, not to the buccal side as do the low­ er molars. Obviously, Tsaganomys and Coelodontomys represent two different lineages initially. The cylindrical, strongly unilaterally hypsodont cheek teeth common to both Tsaganomys and Coelodontomys are the result of parallel evolution. Therefore, Coelodontomys and Cyclomylus may form a sister group to Tsaganomys . Within Cyclomylus , C. lohensis is the most primitive species, whereas C. biforatus is the most advanced, with C. intermedius lying in between.

BIOSTRATIGRAPHIC DISTRIBUTION OF THE TSAGANOMYIDAE

The Tsaganomyidae ranged through a very short geological time in central Asia (from early Oligocene through early late Oligocene, perhaps to early Miocene). They are index fossils for the Asian Oligocene. With their relationships now resolved, the geological range of the genera and species of the Tsaganomyidae becomes clear (fig. 32). The most primitive taxon, Cyclomylus lohensis , appeared in the early early Oligocene. Cyclomylus intermedius made its first appearance in early early Oligocene as well but may be slightly later than C. lohensis . Cyclomylus lohensis made its first appearance in the 13th level of the Wulanbulage Formation ( IVPP Loc. 79010 and the 4th level of section XIIIH), whereas C. intermedius appears in the 16th level, just below the boundary of the upper and lower members of Wulanbulage Formation. Cyclomylus biforatus and Coelodontomys asiaticus were restricted to late early Oligocene. Tsaganomys altaicus made its first appearance in late early Oligocene. By the early late Oligocene Cyclomylus and Coelodontomys were extinct, and only Tsaganomys survived. The cheek tooth structure may be one of the major reasons why Tsaganomys survived longer than did Coelodontomys and Cyclomylus . The cheek teeth of Tsaganomys were stronger than those of Coelodontomys and Cyclomylus and could have lasted longer in life.

RELATIONSHIPS OF TSAGANOMYIDAE WITH OTHER RODENTS

The relationships of the Tsaganomyidae with other rodents have long posed a puzzling problem. There have been four different hypotheses: (1) the tsaganomyids were an Asian ancestor of the bathyergids, without relation to cylindrodontids (Matthew and Granger, 1923; Landry, 1957); (2) the tsaganomyids were derived from the cylindrodontids, and the relation with bathyergids is obscure (Burke, 1935, 1936; Wood, 1937, 1970, 1974); (3) the bathyergids originated from the tsaganomyids, which, in turn, were derived from Asian cylindrodontids (Wood, 1980, 1985; Patterson and Wood, 1982); and (4) Tsaganomys and Hystricognathi form a sister group to the Ctenodactylidae (Bryant and McKenna, 1995) .

RELATIONSHIP BETWEEN THE TSAGANOMYIDAE AND BATHYERGIDAE : Matthew and Granger (1923) considered the tsaganomyids as the fossil record of the Bathyergidae and put them in a separate subfamily of the Bathyergidae . They suggested that Tsaganomys might be the Asiatic ancestor of the Bathyergidae . Landry (1957) agreed with them and further thought the tsaganomyids to be more closely related to bathyergids and put them in the Bathyerginae with Bathyergus . The main similarities he mentioned between these taxa are protrogomorphy, hystricognathy, and hypsodonty. However, Landry (1957: 73) was very careful when he said, ‘‘ Tsaganomys should be reexamined to see if the pterygoid fossa communicates with the orbit in this genus. Its incisors should be sectioned to check the pattern of the enamel Mellett, 1968; Shevyreva, 1971, 1972, 1974a, prisms, and, if necessary, its tympanic bullae 1974b, 1994; Kowalski, 1974; Huang, 1993). should be opened to see whether the malleus The similarities of the skull between the two and incus are fused.’’ Now it is evident that taxa were listed as follows: an antorbital crest, in the tsaganomyids the pterygoid fossa is widely expanded zygomatic region, oval insmall and not open into the orbit, and the fraorbital foramen, and the extension of the malleus and incus are not fused. These de­ base of the incisor into the orbit (Burke, 1936, rived traits of Bathyergidae are absent in Tsa­ Wood, 1970, 1974). Bryant and McKenna ganomyidae. In addition, the Tsaganomyidae (1995) analyzed the similarities and differenchave more different derived features than the es between the two taxa in more detail. I agree Bathyergidae , including horizontal position with them that the similarities mentioned of fenestra ovalis, partly covered stepedial above are superficial or plesiomorphous. The muscle origin, as well as other features (see occlusal pattern of the cheek teeth of the Tsa­ Parent, 1993, and Bryant and McKenna, ganomyidae is strikingly similar to that of 1995: table 4). The protrogomorphy of the some cylindrodonts. However, the microstruc­ Bathyergidae has been showed to be second­ ture of the incisor of the Tsaganomyidae is ary (Maier and Schrenk, 1987). Obviously different from that of the Cylindrodontidae . In the tsaganomyids and bathyergids do not the Tsaganomyidae it is multiserial, whereas have a close relationship. in the Cylindrodontidae ( Cylindrodon and Ar­

RELATIONSHIP BETWEEN THE TSAGANOMYI­ dynomys) it is uniserial (Wahlert, 1968: 15; DAE AND CYLINDRODONTIDAE : Burke (1935, Martin, 1992: 43). According to presently 1936) first pointed out the possible relation­ available data, the Tsaganomyidae and Cylinship between the tsaganomyids and Ardyno­ drodontidae developed in different directions. mys. Wood (1937, 1970, 1974, 1980) agreed RELATIONSHIPS AMONG THE TSAGANOMYI­ with Burke that the tsaganomyids were de­ DAE, HYSTRICOGNATHI, AND CTENODACTYLO­ rived from Ardynomys and formally put the IDEA: Bryant and McKenna (1995) suggested tsaganomyids in the Cylindrodontidae . This that the Ctenodactylidae is the sister taxon to viewpoint was widely accepted (Simpson, the Hystricognathiformes, including Tsagan­ 1945; Vinogradov and Gambaryan, 1952; omys and Hystricognathi. They stated that ‘‘in addition to the morphological characters from our analysis, this relationship is supported by embryological (Luckett, 1985), reproductive and chromosomal (George, 1985), and dental characters (Hussain et al., 1978; Jaeger et al., 1985; Flynn et al., 1986).’’ (Bryant and McKenna, 1995: 34).

It is necessary to point out that the analysis of the authors mentioned above on the relationship of the Ctenodactylidae with the Hystricognathi did not include the tsaganomyids. Luckett (1985: 269) pointed out the difficulties involved in corroborating any hypotheses for the affinities of Sciuridae , Aplodontidae , Castoridae , Anomaluridae , Pedetidae , and Ctenodactylidae with either of Hystricognathi and Myomorpha, based on the available developmental data. Wang (1997b) stated that the evidence from the fossils is not enough to support the sister group hypothesis of the Ctenodactyloidea with the Hystricognathi. The dental characters used in the works of some authors (Hussain et al., 1978, Jaeger et al., 1985, and Flynn et al., 1986) were based on isolated cheek teeth of the Chapattimyidae and Protophiomys , whose status is still problematic. In addition, all the other ctenodactyloids have a sciurognathous mandible, and we have not seen any material in the Ctenodactyloidea showing the change from the sciurognathy to hystricognathy.

No matter whether the relationship between the ctenodactyloids and the Hystricognathi is as close as hypothesized by the authors mentioned above, it is difficult to corroborate the affinities of the Tsaganomyidae with these two taxa because the Tsaganomyidae lack the derived features common to these taxa, including hystricomorphous skull, large and deep pterygoid fossa, well­developed lower masseteric crest, fused malleus and incus, large and anteriorly expanded malleus head, proximal fusion of the tibia and fibula, and large hypocone on the upper cheek teeth. On the other hand, the Tsaganomyidae have derived features of their own (autapomorphies), such as voluminous temporal fossa, prominent sagittal and occipital crests, robust and strongly convex laterally zygomatic arch with well­developed jugal, which meets the premaxillary to form a plate with the lachrymal, high ascending ramus with high coronoid process, triangularly convex surface of condyloid process, and absence of a hypocone on upper cheek teeth. The Tsaganomyidae also have short masseter and pterygoid muscles, well­developed temporal muscle, and steplike occlusal surface of the cheek tooth row. These features of the Tsaganomyidae can be associated with crushing and chewing food transversely. On the contrary, in the Hystricognathi and the Ctenodactyloidea the masseter and pterygoid muscles are elongated and the temporal muscle is very much reduced. These features are associated with propalinal chewing (Landry, 1957; Woods, 1972). Obviously the Tsaganomyidae evolved in an evolutionary direction distinct from both the Ctenodactyloidea and Hystricognathi.

The sister group hypothesis of the Tsaganomyidae with the Hystricognathi suggested by Bryant and McKenna (1995) was based on such derived features as hystricognathous mandible, reduced lachrymal, and multiserial enamel microstructure. As mentioned above the orbital wing of the lachrymal of the Tsaganomyidae is wide. As for the multiserial enamel, it evidently evolved at least twice in the Ctenodactyloidea (Martin, 1992), and in the Tsaganomyidae the enamel structure of Cyclomylus represents a primitive stage of the multiserial structure. Possibly the multiserial enamel microstructure arose independently in the Tsaganomyidae and the Hystricognathi.

The hystricognathous lower jaw is an important character for classification in Rodentia . Although the tsaganomyids and Hystricognathi share the hystricognathous mandible, the two taxa are different in the form of the angular process. In the Hystricognathi the lower masseteric crest is well developed, the coronoid process is reduced, the condyloid process is narrow and long, the postcondyloid process is usually present, and the angular process usually extends posteriorly with the insertion of pars reflexa of the superficial masseter high up near and posterior to the condyle (Woods, 1972: 124). All these features of the Hystricognathi are associated with propalinal chewing (Woods, 1972; Landry, 1957).

However, the characters in the tsaganomyids are different: the angular process is large, strongly flared, and extends ventrally hystricognathous mandible of the Hystricorather than posteriorly; the lower masseteric gnathi. crest is weak; the coronoid process is high; (4) Biostratigraphic distribution of the Tsathe articular surface of the condyloid process ganomyidae shows that Cyclomylus lohensis is wide, triangularly convex, and without appeared earliest in the early early Oligopostcondyloid process; and the insertion of cene. Cyclomylus intermedius appeared in pars reflexa of masseter superficialis is low. early early Oligocene but a little later than The Tsaganomyidae did not develop propal­ C. lohensis . Cyclomylus biforatus and Coelinal chewing but mainly crushed and chewed odontomys asiaticus were limited to the late food transversely. Obviously the laterally early Oligocene. Tsaganomys altaicus apflared angular process in the Tsaganomyidae peared also in late early Oligocene, but suris due to the strongly laterally convex zy­ vived into the early late Oligocene (perhaps goma forced by the strong temporal muscle. into early Miocene). After that the Tsagano­ Therefore, the origin of the hystricognathous myidae disappeared in Asia. mandible of the Tsaganomyidae is indepen­ (5) The Tsaganomyidae represent a special dent from the typical hystricognathous man­ independent evolutionary group in the Rodible of the Hystricognathi. For simplicity’s dentia and have nothing to do with other sake, I tentatively call it tsaganomyid­type known rodents based on the present available hystricognathous mandible. data. It seems that the Tsaganomyidae represent a quite independent lineage. Up to now no ACKNOWLEDGMENTS other rodents have been shown to have a

It would have been impossible for me to close relationship with it. Thus, I temporarily

finish this work had I not had the opportunity consider the Tsaganomyidae as Rodentia in­

to observe so large a portion of the tsagancertae sedis.

omyid collection of the Asiatic Expedition

held in the AMNH. I wish to express my

CONCLUSIONS deep appreciation to Dr. R. H. Tedford of

AMNH for kindly inviting me to visit the (1) Cyclomylus lohensis is not a junior

AMNH, his encouragement and manifold synonym of Tsaganomys altaicus but a valid help during my stay in New York, and for genus and species. The concept of Tsagan­ assistance in improving my written English. omys altaicus of some authors includes at I owe a great deal to Dr. M. C. McKenna of least two taxa: Tsaganomys altaicus and the AMNH for his permission to make casts Coelodontomys asiaticus , new genus and and his loan of specimens to me, and to Drs. species. Altogether the Tsaganomyidae com­ X.­m. Wang, J. Meng, and K.­q. Gao of the prises three genera and five species: Tsagan­ AMNH for their help during my stay in New omys altaicus , Cyclomylus lohensis , C. inter­ York and work on this paper. I am deeply medius, new species, C. biforatus new spe­ gratful to Profs. Z.­x. Qiu, C.­k. Li, S.­h. cies, and Coelodontomys asiaticus , new ge­ Zheng, W.­y. Wu, X.­s. Huang, Z.­d. Qiu, nus and species. and Y.­s. Tong of IVPP and Prof. R.­y. Li of (2) Among the Tsaganomyidae Cyclomy­ Beijing Stomatological Hospital for their lus lohensis is the most primitive genus and stimulating discussion, from which some of species. Cyclomylus and Coelodontomys rep­ the ideas expressed in this paper were deresent one lineage and form a sister group to veloped. Many thanks to Mr. E. Pederson of Tsaganomys . the AMNH for his excellent skill in prepa­ (3) The Tsaganomyidae are suggested to ration of some specimens and some casts, have had a fossorial habit with a diet of in­ Prof. W.­l. Sheng of IVPP for his good illussects and worms. The mandible of the Tsa­ trations, Mr. J. Zhang for his photographs, ganomyidae has a large flared angular pro­ and Mrs. L. Ouyang for preparing the SEM cess. It is associated with crushing and chew­ photomicrographs of the incisors. I express ing food transversely and represents a special my heartfelt thanks to two reviewers for their type of mandible distinct from the typical helpful reviews. This project was supported by NSF of China (Nos. 49472083 and 49872011) and my travel was supported by the Carter Fund of the AMNH.

Figures

Fig. 13. Microstructures of longitudinal sections of lower incisors of tsaganomyids. A. Tsaganomys altaicus. B. Cyclomylus lohensis. C. Coelodontomys asiaticus, new genus and species. Scale = 20 µm.

Fig. 28. Coelodontomys asiaticus, new genus and species, showing the relations between incisors and cheek teeth. A. Left upper jaw with I2, P4–M2 (AMNH 85277). A1. Medial view. A2. Anterior view. A3. Buccal view. B. Anterior view of transverse section across p4 of right lower jaw (IVPP V 11434). C. Right lower jaw with i2, p4–m3 (AMNH 83521). C1. Buccal view. C2. Anterior view.

Fig. 29. Coelodontomys asiaticus, new genus and species A. Occlusal view of left lower jaw with p4–m3 (AMNH 84344). B. Occlusal view of upper jaw with left and right P4–M3 (AMNH 83541) (in stereo). Scale = 1 cm.

Fig. 30. Coelodontomys asiaticus, new genus and species. A. Occlusal view of right P4–M3 (AMNH 83541). B. Left lower jaw with i2, p4–m3 (AMNH 84344). B1. Occlusal view. B2. Buccal view. B3. Posterior view.

Fig. 31. Cheek teeth of Coelodontomys asiaticus, new genus and species. A. Right upper molar (IVPP V 11428). A1. Occlusal view. A2. Anterior view. A3. Buccal view. A4. Posterior view. B. Buccal view of longitudinal section of cheek tooth (IVPP V 11433). Abreviation as in figure 10.

Fig. 1. Tsaganomyid cheek teeth showing reference points for measurements, A, B. Lateral view of cheek teeth. A. p4. B. Other cheek teeth except p4. C–E. Occlusal view of cheek teeth. C. Upper cheek teeth. D, E. Lower cheek teeth. D. Less worn. E. More worn. Abbreviations are: h, enamel height; h1, enamel height on lingual side of upper teeth (buccal side of lower teeth); h2, on buccal side of upper teeth (lingual side of lower teeth); h3, enamel height between two roots on anterobuccal side on p4; L, Length; and W, width.