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A New Symmetrodont Mammal from China and Its Implications for Mammalian Evolution

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A New Symmetrodont Mammal from China and Its Implications for Mammalian Evolution articles A new symmetrodont mammal from China and its implications for mammalian evolution Yaoming Hu*, Yuanqing Wang*†, Zhexi Luo† & Chuankui Li* * Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, PO Box 643, Beijing 100044, China † Section of Vertebrate Paleontology, Carnegie Museum of Natural History, 4400 Forbes Avenue, Pittsburgh, Pennsylvania 15213, USA ........................................................................................................................................................................................................................................................ A new symmetrodont mammal has been discovered in the Mesozoic era (Late Jurassic or Early Cretaceous period) of Liaoning Province, China. Archaic therian mammals, including symmetrodonts, are extinct relatives of the living marsupial and placental therians. However, these archaic therians have been mostly documented by fragmentary fossils. This new fossil taxon, represented by a nearly complete postcranial skeleton and a partial skull with dentition, is the best-preserved symmetrodont mammal yet discovered. It provides a new insight into the relationships of the major lineages of mammals and the evolution of the mammalian skeleton. Our analysis suggests that this new taxon represents a part of the early therian radiation before the divergence of living marsupials and placentals; that therians and multituberculates are more closely related to each other than either group is to other mammalian lineages; that archaic therians lacked the more parasagittal posture of the forelimb of most living therian mammals; and that archaic therians, such as symmetrodonts, retained the primitive feature of a finger-like promontorium (possibly with a straight cochlea) of the non-therian mammals. The fully coiled cochlea evolved later in more derived therian mammals, and is therefore convergent to the partially coiled cochlea of monotremes. Systematic palaeontology que, five (Latin); cuspis, point (Latin); dens, tooth (Latin), for the Class Mammalia three main cusps plus two large accessory cuspules on the lower Subclass Theria molars. Order Symmetrodonta Locality. Jianshangou Valley (approximately 418 419 010 N, 1208 Family Spalacotheriidae 599 300 E), about 32 km east of Chaoyang City, Liaoning Province, Zhangheotherium quinquecuspidens gen. et sp. nov. northeastern China1. Holotype. IVPP V7466 (Institute of Vertebrate Paleontology and Horizon. The Jianshangou Beds, consisting primarily of shales, are Paleoanthropology, Chinese Academy of Sciences, Beijing, China), a the lowest lacustrine intercalation in the neutro-basic volcanic beds well-preserved skeleton (Fig. 1) consisting of a partial skull, and of the Yixian Formation1–3. most of the postcranial skeleton, including all cervical and thoracic Associated fauna. The Jianshangou Beds have yielded diverse fossil vertebrae, forelimbs and pectoral girdle, as well as the excellent fish4, the birds Confuciusornis, Liaoningornis5,6 and Protarch- impressions of the hindlimbs and pelvic girdle, and ribs. Lumbar aeopteryx7, the theropod Sinosauropteryx8, and diverse gastropods, and sacral vertebrae are represented by poor impressions. Some bivalves, ostracods, conchostracans and insects2. tarsals and most caudal vertebrae are missing (for measurements, Age. The age of the Yixian Formation is equivocal. Vertebrate faunal see Tables 1 and 2). correlation1,4,5 and previous radiometric dates4 suggest that the Etymology. Zhanghe, in honour of Zhang He, who collected and Jianshangou Beds are either of the latest Jurassic age, or near the donated the holotype specimen to the Institute of Vertebrate Jurassic–Cretaceous transition6. An Early Cretaceous age was also Paleontology and Paleoanthropology; therium, beast (Greek); quin- suggested by invertebrate faunal correlation3, and supported by a Table 2 Dental measurements of Zhangheotherium quinquecuspidens Table 1 Skeletal measurements of Zhangheotherium quinquecuspidens Tooth Length (mm) Height (mm) Bone Length (mm) ............................................................................................................................................................................. ............................................................................................................................................................................. Lower dentition Dentary 30.2 I1 1.2 Axis (C2) 5.0 I2 0.5 Cervical vertebrae 3–7 10.0 (total length) I3 0.5 Thoracic vertebra 8 2.9 C 0.8 Caudal vertebra 2 3.5 P1 1.2 Clavicle 11.6 P2 1.3 Scapula (posterior border to glenoid) 17.2 M1 1.5 1.0 Humerus 22.1 M2 1.6 (1.5) Ulna 21.5 M3 1.8 1.7 Radius 17.1 M4 1.7 1.8 Metacarpal III 6.5 M5 1.5 1.8 Ilium 20.1 M6 1.1 1.0 Ischium 8.5 ............................................................................................................................................................................. Epipubis 9.1 Upper dentition Femur 22.0 M2 1. 5 Tibia 23.5 M3 (1.6) 1.5 Fibula 23.5 M4 (1.6) 1.6 Calcaneum 6.6 M5 1.8 1.5 Matatarsal III 8.6 ............................................................................................................................................................................. ............................................................................................................................................................................. Parentheses indicate approximate estimates. Nature © Macmillan Publishers Ltd 1997 NATURE | VOL 390 | 13 NOVEMBER 1997 137 articles Figure 1 Zhangheotherium quinquecuspidens (IVPP V7466, holotype). Stereo- mandibular foramen; mg, meckelian groove; mp1–mp5, metacarpals I–V; mt1– photographs (a) and outline (b) of the skeleton in ventral view of the dorsoventrally mt5, metacarpals I–V; mst, manubrium of sternum; mx, maxillary; oc, occipital compressed specimen (broken lines indicate the morphology preserved in condyle; of, obturator foramen of pectoral girdle; on, odontoid notch for dens of impressions that can be examined on the silicon rubber mould of the atlas; pc, procoracoid; pcd, condylar process of dentary; pcl, preclavicle (a impressions). Vertical scale bar in b represents 1 cm. Abbreviations: ac, acromion homologue of procoracoid, sensu ref. 50); pcr, coronoid process of dentary; pf, of scapula; c1–c7, cervical vertebrae 1 to 7; ca1, canine; ca2–ca4, caudal vertebrae pterygoid fossa; pgc, postglenoid crest; pgd, postglenoid depression; pmx, 2 to 4 (caudal vertebrae are incomplete); cd, coracoid process of scapula; cl, premaxillary; pp, paroccipital process of petrosal; ppe, paroccipital process of clavicle; cm, calcaneum; co? coronoid fossa?; cp 1–9, carpals 1 to 9; csp, crista exoccipital (incomplete); pr, promontorium of petrosal; prs, prootic sinus canal; parotica of petrosal; ctp, caudal tympanic recess of petrosal; dn, dentary; ep, ptc, post-temporal canal; pts, post-tympanic sinus; p1 and p2, premolars 1 and 2; epipubis; er, epitympanic recess; fc, fenestra cochlearis (round window); fe, ra, radius; r1–r13, thoracic ribs 1 to 13 (posterior thoracic ribs preserved only in femur; ff?, facial nerve foramen?; fi, fibula; frs, foramen for ramus superior; fst, impressions); sc, scapula; sl?, fossa for splenial? on the dentary; sm, stylomas- fossa for stapedial muscle; fv, fenestra vestibuli (oval window); g, glenoid of the toid notch; sp, spine of scapula; sq, squamosal; ss, supraspinous fossa of scapula; gj, groove for jugal (on the squamosal); gl, glenoid fossa of squamosal; scapula; stb1–stb6, sternebra 1 to sternebra 6; stl, embryonic sternal bands; hu, humerus; ic, interclavicle; if, infraspinous fossa; il, ilium; is, ischium; i1–i3, sym, mandibular symphysis; s1–s4, sacral vertebrae 1 to 4 (represented mostly by incisors 1 to 3; ju, jugal; L1–L6, lumbar vertebrae 1 to 6 (impressions only); lm, impression); th, attachment site for tympanohyal; ti, tibia; ts, lateral tarsal spur of lambdoidal crest; mas, mastoid exposure of petrosal; mc, metacoracoid; mf, ankle; t1–t13, thoracic vertebrae 1 to 13; ul, ulna; x, xiphoid process of sternum. Figure 2 Dentition and mandible of Zhangheotherium quinquecuspidens. a, 3–5 Upper molars M ; b, lower molars M3–6 (left, labial views); c, mandible (right side, reconstruction in medial view). Zhangheotherium closely resembles Spal- acotherium in lower molars, and Peralestes in upper molars. Spalacotherium and Peralestes were established on some dissociated lower and upper teeth, respectively, from the same fauna in the Late Jurassic of England10–12. The associated upper and lower dentitions of Zhangheotherium suggest that Spalacotherium and Peralestes are the lower and upper teeth of the same species, and that Peralestes should be synonymized with Spalacotherium10–12. Cust B9 is too large and positioned too far lingually to be the stylocone (cusp B). It could be either a homologue to the much smaller intermediate cusp in the comparable position in other symmetrodonts, or a neomorphic cusp. We follow ref. 45 for the designation of cusps A, B, C, D and E on the upper molars, and cusps a, b, c, d and e on the lower molars. For abbreviations, see Fig.1 legend. Nature © Macmillan Publishers Ltd 1997 138 NATURE | VOL 390 | 13 NOVEMBER 1997 articles recent radiometric date9. According to our most recent field with either a straight or slightly curved (but uncoiled) cochlea. We investigation, this date9 should be regarded as an upper age limit therefore infer from the slender promontorium of Zhangheotherium
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