安徽潜山新发现的钝脚类头骨化石

A SKULL OF ALTILAMBDA (MAMMALIA, PANTODONTA) FROM THE PALEOCENE OF QIANSHAN, ANHUI

本文记述了发现于安徽潜山盆地中古新统望虎墩组上段的一件较为完整的钝脚类头骨化石,将其归入 Altilanbda 属中,并建立—新种——余井高脊兽(A. yujingensis sp. nov.).

In January of 1989, a relatively well-preserved pantodont skull, described in this paper, was collected by the junior authors from the Middle Paleocene Wanghudun Formation near a small village in Yujing, Qianshan County, Anhui Province. This fossil represents a new form of pastoralodontid pantodonts. Class Mammalia Linnaeus, 1758 Order Pantodonta Cope, 1873 FamiLy Pastoralodontidae Chow et Qi, 1978 Genus Altilambda Chow et Wang, 1978 Type species Altilambda pactus Chow et Wang, 1978. Additional Diagnosis The suture of premaxillo-maxilla and nasal straight except at the posterior end; cheek teeth less hypsodont, their crowns low; M^1 and M^2 transversely elongated and rectanglar; paracones, metacones and protocones conical with flat outer surfaces; cristas well-developed; stylocone-and meta-stylocone-like structures present on both the vshaped ectolophs of p^3, p^4 and the W-shaped ectolophs of M^1, M^2; metacone of M^3 reduced but prominent. Distribution Qianshan, Anhui and Nanxiong, Guangdong, Middle and Late Paleocene. Altilambda yujingensis sp. nov. (Pl. I) Holotype The Cultural Relics Administrative Office of Qianshan County, Anhui Province, specimen no. QS055, a well-preserved skull, with essentially complete upper dentition, except occipital region and the posterior parts of zygomatic arches. Locality and Stratigraphical Horizon About 150m southeast to a small village named Chenjiachuanmenkou, Yujing, Qianshan, Anhui (approximately 30°41′ N, 116°37′ E), from the upper member of Wanghudun Formaton, Middle Paleocene. Diagnosis Comparatively large among Altilambda species with a skull robust (consulting the additional diagnostic features of the genus for other characteristics). Etymology The species is named after Yujing where the specimen was collected. Description Occipital part of the skull broken. Length of preserved part is 124mm, and the total length of a complete skull is estimated to be about 140mm. The width of the skull is about 93.8mm. Tbe skull looks rather robust. Dorsal view: Outline of the skull is roughly rhomboid. Nasal, damaged at the anterior end, is elongated anteroposteriorly and extends posteriorly to the same transverse line as anterior border of orbit. It is in contact with premaxilla at the anterior part, not contracted in the middle and expands slightly at the posterior end. Its surface is flat. The nasal incision is above the canine. The suture of nasal and premaxillo-maxilla is straight except at its posterior end. Frontal is short and does not obviousl insert forwards between two nasals. Frontal crests are prominent, at an angle of 45° to the sagittal plane, and submerge rapidly backwards into a sagittal crest which is not preserved. Postorbital process is not obvious. Parietal is large with small areas of depressions on its posterior surface. Lateral view: Premaxilla extends forwards to the anterior end of the skull. Well-developed ascending ramus of premaxilla extends posterodorsally to a point above P^2, inserting between nasal and maxilla. Premaxillo-maxilla suture is S-shaped. Infraorbital foramen is comparatively large and above p^3 at the same level as lower border of orbit. Orbit is small and margined by frontal, lacrimal and malar. Anterior border of orbit is above M^1. Lacrimal is between frontal and maxilla, and exposes in the face without lacrimal tubercle. Zygomatic process of maxilla extends backwards to the transverse line of the middle of M^2. Malar is anteriorly separated into two branches: upper one is longer and in contact with lacrimal but lower one shorter. Zygomatic process of temporal is quite long, and extends forwards to the same transverse line as posterior margin of M^3. Ventral view: Palate is narrow and elongated anteroposteriorly, about 24mm wide. Posterior edge of palatine fissure is at the line of posterior edge of canine. Palatine processes of maxillae are large and form about two-thirds of palate. Anterior end of palatine is at the line of the middle of M^1. Two pairs of palatine formina are at the transverse lines of the middle of M^1 and between M^1 and M^2. Posterior nares are at the line of posterior edges of M^3 with a posterior nasal spine, 5mm long, in the middle, and a pterygial process anterolateral to each side of posterior nares. Alar plate is well-developed with a laterally extending ectopterygoid process on it. Upper teeth: Incisors are very small. I^2 is larger than I^1. I^3 is not preserved and estimated to be larger than I^2 according to the size of its alveolus. Canine is larger than any of incisors, but not tusklike and still comparatively small. The cheek teeth are crowded without diastema. Their interior edges are generally at a straight line. P^1 has one root and its crown is compressed laterally. The outer side of its crown is broken. P^2 has two roots in an anteroposterior line. Its outline looks like a semicircle. There is a small cusp on the postcrista of main cone. p^3 is more transverse than P^2. Its V-shaped ectoloph is widely opened and asymmetric with stylocone- and meta-stylocone-like structures on it. Protocone is low and posterointerior to ectocone and looks like a hypocone shelf of a molar. P^4 is larger than P^3. Its V-shaped ectoloph, also bearing stylocone- and meta-stylocone-like structures, is more symmetric and less opened than that of P^3. The preprotocrista extends forwards little beyond ectocone, and is not in connection with an obvious precingulum. Protocone is low and interior to ectocone. Molars are transversely elongated with comparatively low crowns. The outline of M^1 and M^2 is like a rectangle. M^1 is much worn. Its paracone is nearly worn away. Metacone is more labially located than paracone. Ectoloph is a relatively symmetrical 'W' in shape. Parastyle, mesostyle and metastyle are well-developed, especially the mesostyle which is in connection with a quite well-developed ectocingulum to form a complicated structure. Stylar shelf is rather wide, about half the width of the tooth. Ectoflexus is slightly concave inwards and is anterior to mesostyle. Paracrista and metacrista are in connection with parastyle and metasyle respectively. Stylocone- and meta-stylocone-like structures are respectively present in the middle of paracrista and metacrista. Precentrocrista and postcentrocrista are in connection with mesostyle. Preprotocrista and postprotocrista, respectively in connection with prominent paraconule and metaconule, form a V-shaped structure. Both postparaconule crista and premetaconule crista are short or not obvious. Preparaconule crista and postmetaconule crista are very long and extend respectively to anteroexterior and posteroexterior corners to become paracingulum and metacingulum. Internal precingulum is obvious. Internal postcingulum forms a wide hypocone shelf with a prominent hypocone on it. M^2 is larger than M^1 and is the largest one among the cheek teeth. Its crown is less worn than M^1's. Protocone is larger and lower than both paracone and metacone which are nearly equal in size. All the three cones are conical in shape with flat outer surfaces. Its crown structure is quite similar to M^1's, except ectocingulum, stylocone-and meta-stylocone-like structures are less developed. M^3 is like an oval in outline. Its ectoloph is U-shaped. Mesostyle, metastyle, metacrista and stylocone-like structure are all absent. Metacone is reduced and located more labially. Paraconule and metaconule are less prominent. Internal postcingulum do not form a hypocone shelf and hypocone is not present. The measurement of the teeth of this specimen is listed in Table 1. Comparison and Discussion Another specimen of a skull with occlused lower jaws, found from another site in the same basin provides enough evidences for us to refer the here described specimen into the genus Altilambda Chow et Wang, 1978 (see another paper for details). So far, four species of Altilambda have been described: A. pactus Chow et Wang, 1978; A. tenuis Chow et Wang, 1978; A. minor Tong, 1982 and A. sp. Both A. tenuis of Qianshan, Anhui and A. minor of Nanxiong, Guangdong are far inferior to the new specimen in body size. The size of A. pactus is larger than that of he other two named species of this genus, but it is still distinctly smaller than that of QSO55. So that, under the circumstance of no directly comparable specimens, the authors consider the specimen under consideration as a new species of Al- tilambda, named Altilambda yujingensis sp. nov. Chow et Wang (1978) allocated Altiiatmbda to the family pastoralodontidae, based mainly on the features of lower dentition and outline of DP^4. The characters, shared by A. yujingensis and Pastoralodon lacustris, show that Altilambda is more closely related to Pastoralodon than to members of the other families of Pantodonta, and support Chow et Wand's (1978) view on the systematic position of Altilambda. Pastoralodon could be easily distinguished from Altilambda by its markedly larger body size, contracted middle part of nasals, arch maxillo-nasal suture, typically hypsodont teeth with comparatively high crowns, square outline of M^1 and M^2, lacking of stylocone- and metastylocone-like structures on the ectolophs of P^3-M^2, and more reduced metacone in M^3, in addition there are also differences in the morphology of lower dentition. Another genus of Pastoralodontidae, Convallisodon, is evidently quite different from Altilambda in upper teeth. Acknowledgments The authors are grateful to Drs. Zhou Mingzhen (Minchen. Chow) and Li Chuan-kuei for their encouragement and guidances. We thank Dr. Qi Tao for permitting us access to the specimens of Nei Mongol under his care. Thanks are also due to Drs. Zhai Renjie, Tong Yongsheng, Wang Jingwen, Huang Xueshi, Wang Banyue and Zheng Jiajian for their critical reading of the manuscripts, and to Mr. Zhang Jie for taking the photographs.