gyuan, western Liaoning, China, which preserves clear Sinobaatar gen. nov.: First impressions of both dorsal and ventral sides of a subadult individual. The specimen clearly shows morphologies of multituberculate from the dentition, skull, and fore- and hindlimbs. It is the first Jehol Biota of Liaoning, representative of multituberculates in the Jehol Biota, and 1) the most complete pre-Late Cretaceous multituberculate Northeast China specimen. Most groups of Mesozoic mammals are poorly HU Yaoming1,2,3 & WANG Yuanqing1 documented; only fossil multituberculates are relatively 1. Institute of Vertebrate Paleontology and Paleoanthropology, Chinese abundant and their dentitions display distinct evolutionary Academy of Sciences, Beijing 10044, China; trends. Therefore, the dental features of the new multitu- 2. American Museum of Natural History, New York, NY 10024, USA; berculate specimen can be used as evidence in estimating 3. Biology Program (Ecology, Evolutionary Biology, and Behavior), the geological age of the Jehol Biota through correlation Graduate School and City College, City University of New York, NY 10016, USA of mammalian fossils. Correspondence should be addressed to Hu Yaoming (e-mail: yhu@ 1 Systematic paleontology amnh.org) or Wang Yuanqing (e-mail: [email protected]) Class Mammalia Linnaeus, 1758 Abstract A multituberculate skeleton from the Lower Order Multituberculata Cope, 1884 Cretaceous Yixian Formation at Dawangzhangzi, Lingyuan Family Eobaataridae Kielan-Jaworowska, City, Liaoning Province, Northeast China, provides new Dashzeveg and Trofimov, 1987 morphological information for early multituberculates. The Sinobaatar gen. nov. specimen is the holotype of Sinobaatar lingyuanensis gen. et sp. nov. It has a narrow skull that lacks the superorbital crest Type species. Sinobaatar lingyuanensis gen. et sp. or postorbital process. The dental formula is 3·?·5·2/1·0·3·2. nov. The dental morphology, especially that of cheek teeth, of S. Diagnosis. As for the type species. lingyuanensis is similar to that of Eobaatar, which places it in Etymology. “Sino-”, Latin, China; “-baatar”, the family Eobaataridae. The postcranial skeleton of Sino- Mongolian, hero, which has been used as the suffix of baatar is similar to that of other multituberculates. As in generic names of many Asian Cretaceous multitubercu- Holotheria, Metatarsal V of Sinobaatar articulates only with lates. the cuboid and has no contact with the calcaneus, which probably represents the primitive condition of multituber- Sinobaatar lingyuanensis gen. et sp. nov. culates. Nine carpals of Sinobaatar resemble those of Zhang- heotherium, except the centrale being larger than the trape- Holotype. Skeleton of a subadult individual, pre- zoid. The dental features of Sinobaatar show again that eo- served mainly in impressions of both dorsal and ventral baatarids are obviously intermediate between Late Jurassic sides on the slate and its counterpart, respectively (Insti- multituberculates and the later forms. Because eobaatarids tute of Vertebrate Paleontology and Paleoanthropology are only known from the Early Cretaceous, the finding of catalogue number: IVPP V12517) (fig. 1; Plate ĉ). All Sinobaatar, therefore, supports that the age of the Jehol Bi- the pictures were taken from the casts. ota is most likely Early Cretaceous. Diagnosis. Narrow skull; with two infraorbital Keywords: Lingyuan, western Liaoning, Yixian Formation, Early foramina; no superorbital crest or postorbital process; Cretaceous, Jehol Biota, multituberculates, Sinobaatar. dental formula 3g?g5g2/1g0g3g2; both I1 and I2 small; I3 larger than I1-2; cusp formulae for upper cheek Multituberculates, an extinct mammalian group liv- teeth: P4, 3Ή4; P5, 3Ή5Ή4; M1, 3Ή4Ή1; M2, 1Ή3Ή41); ing from the Late Jurassic through the Paleogene, have lingual cusps of M1 obtusely conical; labial cusps of M2 morphological features intermediate between the primitive relatively low; anterior two cusps of lingual row on M2 mammaliaformes, such as the Jurassic morganucodontids, not completely separate; lower incisor conical and com- and the living therians. Their evolutionary history has pressed with complete enamel; P4 roughly rectangular in been one of the important topics about the early evolution lateral view with a slightly convex dorsal edge; P4 having [1ü3] of mammals . So far, however, most multituberculate 11 serrations and 10 ridges; a basal cuspule present on the fossils were known from the Late Cretaceous and the labial side of the crown; cusps on lower molars tending to early Cenozoic. Not much about early multituberculates close with each other; cusp formulae for lower molars: M1, has been known, and their phylogeny remains controver- 4Ή?; M2, 3Ή2; labial cusps relatively low with fine [4] sial . A multituberculate specimen was recently recov- ridges; the centrale larger than the trapezoid; metacarpal V ered from the Early Cretaceous Yixian Formation in Lin-

1) The Chinese version of this was published on Chinese Science Bulletin (2002, 46(5): 372ü386), which should be referred for citation of the naming of the genus and species.

Chinese Science Bulletin Vol. 47 No. 11 June 2002 933 NOTES not contacting the triquetrum; proximal end of metatarsal lar in crown view. Its main cusp row extends from the V articulating only with the cuboid, and not in contact anterolingual corner to the posterolabial corner. The mid- with the calcaneus. dle cusp is the largest among five cusps on the main cusp Locality and age. Dawangzhangzi, Lingyuan City, row. There are three cusps labial to the posterolabial part Liaoning Province, China; Yixian Formation; Early Cre- of the main cusp row. Lingual cusps are lower than the taceous[5]. main cusps and are present at the posterolingual margin of Etymology. The species is named after Lingyuan, the tooth crown. M1 is roughly rectangular in crown view the name of the city where the type specimen was col- with a slightly convex posterior margin and a low cusp at lected. the posterolingual corner. There are four cusps on the me- Skull and lower jaw. The skull of the holotype dial row, and three on the labial row. The crown of the was crushed (Plate ĉ(a)ü(c)). The nasal is large and erupting left M2 is trapezoid in crown view, with the ante- extends to anterior margin of orbit. A large lacrimal is rior part wider than the posterior one. The labial cusp is situated at the anteromesial corner of the orbit. No postor- low and ornamented with ridges. A deep groove separates bital process is present. The lambdoid crest is profound, the lingual and medial cusp rows. while the sagittal crest and superorbital crest are not de- The lower incisor is conical and compressed trans veloped. In the ventral view, the anterior end of palatine versely. Its crown is completely covered by enamel. The 5 1 reaches the level between P and M . The suture between P2 is reduced and single-rooted. The crown of P3 is spear- the palatines and the maxillae is U-shaped. Internal nares head-like and supported by two roots. The crown of P4 is are located between the M2s. The base of zygomatic proc- 4.0mm long and 2.1mm high with a slightly convex dorsal ess of maxilla is wide, with its anterior margin external to edge. There are 11 serrations along the dorsal edge. Each P2 and posterior margin external to the middle of P5. There of them has a corresponding ridge that extends anteroven- are two infraorbital foramina external to P3 and P4. trally, except the first serration. An isolated basal cusp is The horizontal ramus of the dentary is robust with present on the labial side above the posterior root. The ventral margin of the exodaenodont lobe of P is distinct the maximum depth beneath P4 and M1. The ascending 4 ramus is slender and extends posterodorsally at about 45° and V-shaped. The left M1 is partially visible and the right to the horizontal ramus. The base of its anterior margin is one is missing. The anterior and labial margins of M1 are roughly straight. There are four cusps on the labial row. lateral to M2. The condyle is short and sturdy. Only one mental foramen is present on the lateral surface of the The anterior two cusps are pyramidal in shape. The poste- dentary. The masseteric fossa is shallow. The pterygoid rior two cusps are relatively low. The most anterior cusp shelf is relatively wide and is positioned at the ventrome- of the lingual row is at the position corresponding to the dial margin of the posterior part of the lower jaw. The groove between the anterior two cusps of the labial row. The posterior part of the lingual cusp row is invisible on mandibular foramen is below the M2 and above the ante- rior tip of the pterygoid shelf. the specimen, but it is deduced that there is one or two Dentition. The dental formula is 3g?g5g2/ more cusps. Partially exposed M2 from each side makes it possible to reconstruct its crown structure. Both the lin- 1g0g3g2 (Plate ĉ(b)ü(e)). Ornamentation on cheek gual and labial margins of its crown are straight, with the teeth is weak. labial margin being slightly longer. Its anterior margin is The crown of I1 was broken, and its root is slightly transversely straight, while the posterior margin intersects smaller than that of I2. The crown of I2 is conical, while 3 with the direction of the tooth row. The cusps on the lin- that of I expands transversely with a small posterior tu- gual row are higher than those on the labial one. There are bercle. Connecting area between premaxilla and maxilla is two cusps on the lingual row; the anterior one is higher damaged on the left side and not exposed on the right side, than the posterior. Three cusps are on the low labial row. and it is thus unknown if there is upper canine or not. All The labial cusps of M2 are low, and the cusps on lower upper premolars are double-rooted. Both P1 and P2 are 3 molars tend to close with each other. damaged, and indicated by roots. The crown of P is ob- The postcranial morphology 4 Postcranial skeleton. tusely conical with two cuspules at the external base. PP is of new specimen is generally consistent with that of the rectangular in crown view. Its lingual row of cusps, inter- other known multituberculates[6ü8]. Our description will secting the lingual margin at a small angle with posterior focus on the morphology of its hands and feet (Plate ĉ cusps more labially situated, is higher than the labial row. (a); fig. 1). The cusps on the labial row are small. P5 is also rectangu-

1) The cusp formula for M2 of Eobaatar was originally described as 1:2:3[4,9], but its cusp morphology is almost identical to that of the new specimen described here. Since the small anterior cusp is distinguishable, we assign the cusp formula of M2 for both Eobaatar and Sinobaatar as 1:3:4.

934 Chinese Science Bulletin Vol. 47 No. 11 June 2002 Fig. 1. Hand and foot of Sinobaatar lingyuanensis Hu and Wang, 2002. (IVPP V12517). (a) Dorsal view of right hand; (b) Ventral view of right hand; (c) Dorsal view of left hand. 1, scaphoid; 2, lunate; 3, triquetrum; 4, pisiform; 5, centrale; 6, trapezium; 7, trapezoid; 8, capitate; 9, hamate; R, radius; U, ulna; IüV, Metacarpals ĉüč; (d) ventral view of right foot. 1, astragalus; 2, calcaneus; 3, cuboid; T, tibia; Ts, tibial condyle; F, fibia; Č, metatarsal Č; č, metatarsal č; a, calcaneocuboid joint; b, cuboid-metatarsal č joint.

Impressions on both slates clearly show the mor- sent or not. The distal side of the square lunate contacts phologies of all carpals. The proximal surfaces of both both capitate and hamate. The triquetrum is slightly larger scaphoid and lunate are convex. The scaphoid is trans- than the lunate. Its proximal side articulates with both the versely wide with a concave distal surface for the centrale. ulna and the rod-like pisiform, and its distal side only with The tubercle of scaphoid does not obviously project ven- the hamate. The centrale is triangular and articulates with trally. It mainly articulates with the trapezium, but also the capitate laterally. Its distal side contacts both the trape- contacts trapezoid. It is uncertain if the prepollex is pre- zoid and the metacarpal Ċ. The trapezium is the largest

Chinese Science Bulletin Vol. 47 No. 11 June 2002 935 NOTES element in the distal row of carpals, which is laterally V12517. However, some differences between these genera compressed with a tubercle at the ventral side. The trape- and Sinobaatar need to be mentioned here. The cusp for- 1-3 zoid is the smallest carpal. The distal side of the capitate mula of M1 of Loxaulax is 3Ή2. P of Monobaatar have articulates mainly with the metacarpal ċ, and also the same crown morphology as those of Eobaatar, but 4 2 contacts the metacarpal Č. The hamate is triangular with there are only two rows of cusps on P , as in V12517. I of a concave distal articular surface that articulates with the Parendotherium has a basal cuspule. Eobaataridae is currently referred to ‘Plagiau- metacarpals Č and č. All the carpals are less exposed lacida’[4,11]. It is different from other plagiaulacidans in at the ventral side than at the dorsal side, except the lunate having three, rather than four, lower premolars, rectangu- and the pisiform. The thumb is somewhat divergent from the other four fingers. lar P4 with slightly convex dorsal edge and about 9ü10 ridges and a basal cuspule on labial side. Arginbaatari- The tarsals are also well preserved. The cuboid facet [4,9] of the calcaneus is situated at the medioventral side of the dae was found from the same sites as the genotype of anterior end of the bone. It bevels to the anterior end of Eobaatar. Its systematic position remains uncertain, but the calcaneus at about 45° and articulates with the outer its large and sectorial P4 is quite different from that of part of the proximal end of the cuboid. The cuboid is gen- eobaatarids. All other multituberculates are presently erally rectangular in the dorsal view. The inner part of its placed into Cimolodonta. The upper premolars of mem- proximal end contacts the navicular. The distal end of the bers in this group are fewer than five, while the lowers astragalus articulates with the proximal surface of the than three. The number of both cusps on cheek teeth and navicular. The distal articular facets of both the calcaneus ridges on P4 is comparatively large. The dental features of and the astragalus appear to be somewhat larger than the cimolodonts make them clearly distinguishable from eo- corresponding facets on the cuboid and the navicular, re- baatarids and other multituberculates. spectively, which suggests movable joints. The proximal The above discussion indicates that the new multituberculate from Lingyuan is a member of end of metatarsal č articulates with the distal end of the Eobaataridae, and it is similar to Eobaatar in dental cuboid, without contact with the calcaneus. morphology. Fragmentary specimens of Eobaatar hamper 2 Comparison and discussion any detailed comparison with V12517. More complete Dental morphology of the Lingyuan specimen specimens of Eobaatar are needed to clarify how similar (V12517) is similar to that of Eobaatar from the Early V12517 is to Eobaatar. Based on known morphology, we Cretaceous of Mongolian[4,9]. The holotype of Eobaatar name the Lingyuan multituberculate a new genus and magnus, the type species, is a P4, and referred specimens species, Sinobaatar lingyuanensis. include isolated teeth and fragmentary jaws. The speci- The postcranial elements of multituberculates show mens of other two species, E. minor and E. hispanicus[10], that the morphologies of corresponding structures in dif- are also fragmentary. Compared with the available mate- ferent forms are generally identical, which is sharply in rial, V12517 is similar to Eobaatar in having the same contrast to the clearly evolutionary changes shown by dental formula, rectangular P4 with slightly convex dorsal their dentitions. It may suggest that the locomotor patterns edge and only one basal cuspule on the labial side, same in different multituberculates were not greatly changed crown structure of both M1 and M2 with the cusps tending during evolution. Their skeletal morphology is relatively 1 to coaclose with each other, and rectangular M . The generalized. Multituberculates can live on a variety of differences between V12517 and Eobaatar are as follows: substrata. Some groups show tendency to be limitedly P4 of V12517 is proportionally longer and has one more arboreal or terrestrial. This differentiation is somewhat serration and one more ridge than that of Eobaatar; indicated in the foot structure. The large tibial condyle, V12517 has a conical lower incisor with complete enamel and asymmetrical but widely moveable astragalotibial coverage, whereas that of Eobaatar is chisel-like with joint and calcaneoastragalar joint of both Ptilodus enamel restricted at ventral side; the upper cheek teeth of and ?Eucosmodon sp. from the North American Paleogene, V12517 have finer ornamentation than those of Eobaatar; 4 plus their long and prehensile tails, allow them to climb P of V12517 has two rows of cusps and its labial cusps [13] 4 down the trees . In contrast, Mongolian Late Cretaceous are small, while the labial cusps on PP of Eobaatar are Kryptobaatar and Chulsanbaatar may have lived on the relatively large with additional accessory cuspules; P5 of ground, as shown by their small tibial condyle, less V12517 has a row of cusps lingually to the main cusp row, moveable astragalotibial joint and joints between tarsals[8]. which has not been seen in Eobaatar. Other genera re- The tibial condyle and astragalotibial joint of Sinobaatar, ferred to Eobaataridae include the Early Cretaceous Loxaulax, Monobaatar, and Parendotherium[4]. These resembling to those of Ptilodus, indicate that Sinobaatar genera are all based on a few fragmentary specimens and may be an arboreal animal, like Ptilodus. Other important have similar morphological features to Eobaatar and features in Sinobaatar foot include that the metatarsal ċ parallels to the calcaneus, and that the metatarsal č ar-

936 Chinese Science Bulletin Vol. 47 No. 11 June 2002 ticulates with the cuboid but is not in contact with the comotion on the uneven substrate. calcaneus. This resembles the reconstructed feet for both The geological age of the Jehol Biota has been in Ptilodus and ?Eucosmodon sp.[6,7], but different from the dispute for a long time[16]. Radiometric dating often pro- feet reconstructed by Kielan-Jaworowska and Gambary- vided different results[17,18]. Because of lacking reliable an[8] for multituberculates. Kielan-Jaworowska and Gam- evidence, this issue was not substantially discussed when baryan argued that, in multituberculates, the metatarsal V Zhangheotherium was described[16,19]. The finding of Si- articulated with the distal end of calcaneus, and the longi- nobaatar provides an opportunity to explore this issue on tudinal axis of the metatarsal ċ intersected that of the the basis of mammalian fossils. The continuous record of calcaneus at about 30° outwards. However, the metatarsal multituberculates from the Late Jurassic to Paleogene č on the specimen (ZPAL MgM-I/96b), based on the clearly shows the evolutionary trends in the dental mor- fact which the foot morphology is reconstructed, does not phology. As mentioned above, from the primitive Late exceed the distal end of the cuboid[8]. Because of its early Jurassic ‘plagiaulacidans’ to the later cimolodonts, the age and articulated preservation of foot structure, it is number of premolars decreased, while the number of more reliable to consider the foot morphology of Sino- ridges on P4 increased with the reduction of labial cuspules. Meanwhile, the number of molar cusps was ac- baatar as the morphotype of multituberculate foot. Com- [3] pared with other major mammalian groups, such as ‘tri- cordingly increased . These changes may indicate the conodonts’, monotremes, and holotherians, the foot struc- evolution of multituberculates, as omnivorous animals, in ture of Sinobaatar is most similar to that of holotherians. a tendency to process food items more effectively. Eo- baatarids are in a middle stage during such an evolution- The holotype of Sinobaatar lingyuanensis is the ary history. Their corresponding dental features are inter- specimen that by far reveals most morphological informa- mediate between those of the primitive Late Jurassic tion of multituberculate hand. Other known multitubercu- ‘plagiaulacids’ and more advanced cimolodonts. As all the late specimens only preserved isolated carpals. Recently, known eobaatarids were from the Early Cretaceous, it the wrist structure of Kryptobaatar was briefly reported[14]. seems reasonable to propose that the age of Sinobaatar, It is similar to that of Sinobaatar, but differs from the lat- and therefore of the Jehol Biota, is Early Cretaceous. This ter in having the trapezoid separated from the scaphoid, proposal is further supported by the finding of Gobio- and the metacarpal Č are not in contact with the capitate. conodon in the basal part of the Yixian Formation[20], be- Among the major groups of mammaliaformes, the wrist cause Gobioconodon was only known from the deposits structure of ‘triconodonts’ remains unknown. Zhanghe- of Early Cretaceous[21ü23]. This conclusion is also coinci- otherium, a representative of the holotherians, preserved [24ü26] the wrist[15]. The wrist morphology of Sinobaatar has a dent with latest radiometric studies . number of similarities to those of monotremes and Acknowledgements Dr. Wang Xiaolin, Dr. Zhang Fucheng, and others Zhangheotherium. The similarities include the shallow involving in the Jehol Biota Project of the Institute of Vertebrate Paleon- tology and Paleoanthropology, Chinese Academy of Sciences helped articular surface of the distal radius, a low styloid process, collecting the specimen. Xie Shuhua prepared the specimen. Wang Zhao a large pisiform; the articular facets between the proximal and Zhang Lifen made casts. Dr. Zhang Fucheng and Dr. Meng Jin carpal row and the distal row being continuous and helped taking photographs. The authors are also grateful to Prof. Li roughly perpendicular to the direction of the forearm; the Chuankui, Prof. Chang Meemann, Dr. Meng Jin, and Dr. Zhou Zhonghe for their critical reading of the manuscript. This work was supported by trapezium larger than the trapezoid. Additional similarities the Major Basic Research Project of the Ministry of Science and Tech- of Sinobaatar to Zhangheotherium include that the sca- nology of China (Grant No. G2000077700), the National Natural Sci- phoid, the lunate and the centrale are separated, that the ence Foundation of China (Grant No. 49832002), and the Chinese Acad- thumb is divergent from other four fingers and can move emy of Sciences (Grant No. KZCX3-J-03). in a wide range, that the metacarpal č does not contact References the triquetrum, and that both the lunate and the capitate 1. Simpson, G. G., A catalogue of the Mesozoic Mammalia in the are rectangular in shape. In monotremes, the scaphoid, the Geological Department of the British Museum. London: Oxford lunate and the centrale are fused; five fingers are equally University Press, 1928, 1ü215. separate; the metacarpal č contacts the triquetrum; and 2. Simpson, G. G., American Mesozoic Mammals. Mem. 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