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A New Spalacotheriid Symmetrodont from the Early Cretaceous of Northeastern China

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A New Spalacotheriid Symmetrodont from the Early Cretaceous of Northeastern China PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024 Number 3475, 20 pp., 5 ®gures, 1 table May 11, 2005 A New Spalacotheriid Symmetrodont from the Early Cretaceous of Northeastern China YAO-MING HU,1 RICHARD C. FOX,2 YUAN-QING WANG,3 AND CHUAN-KUI LI4 ABSTRACT Symmetrodonts are Mesozoic mammals having lower molars with nearly symmetrical tri- gonids but lacking talonids. They appear to be stem members of the mammalian clade that led to extant tribosphenic mammals, but the fossil record of symmetrodonts is poor. Here we report a new genus and species of an acute-angled spalacotheriid symmetrodont, Heishanlestes changi, n.gen. and n.sp., represented by well-preserved lower jaws with teeth from the Early Cretaceous of northeastern China. The new mammal has four tightly spaced premolars and three morphological groups of lower molars, in which the ®rst molar has an obtuse trigonid angle and the last two molars have a large neomorphic cusp in the center of the trigonid, a feature not seen in other mammals. Heishanlestes appears to be a specialized member of the spalacotheriid subfamily, Spalacolestinae, which is otherwise only known from North America. The animal probably used the premolars to crush its prey before shearing it with the molars. 1 Division of Paleontology, American Museum of Natural History; Institute of Vertebrate Paleontology and Paleo- anthropology, Chinese Academy of Sciences, PO Box 643, Beijing 100044, China; Biology Program, Graduate School and City College of New York, City University of New York ([email protected]). 2 Laboratory for Vertebrate Paleontology, Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E9 ([email protected]). 3 Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, PO Box 643, Beijing 100044, China ([email protected]). 4 Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, PO Box 643, Beijing 100044, China ([email protected]). Copyright q American Museum of Natural History 2005 ISSN 0003-0082 2 AMERICAN MUSEUM NOVITATES NO. 3475 INTRODUCTION exposed and available for study. Spalacothe- rium, from the Purbeck beds of England, is Symmetrodonts are Mesozoic mammals represented by several upper and lower jaws having three principal cusps on upper and with teeth (Owen, 1854; Simpson, 1928; lower molars arranged in a reversed triangle Clemens, 1963a, 1963b; Ensom and Sigog- pattern (Cifelli and Madsen, 1999). The ®rst neau-Russell, 2000), but this material has known symmetrodont mammal, Spalacothe- suffered damage and much of the detail of rium, was described by Richard Owen in the dentition is no longer visible (Clemens, 1854. Two other symmetrodont genera, Tin- 1963b; Parrington, 1973). Numerous speci- odon and Peralestes, were also found in the mens of spalacotheriid symmetrodonts have 19th century (see Simpson, 1925a, 1928, been recovered from the Cretaceous of Utah 1929) and were ®rst regarded as specialized (Cifelli and Madsen, 1999, and references triconodonts (see Simpson, 1925a, 1925b, therein). The majority of these specimens are 1928; Cassiliano and Clemens, 1979). Simp- isolated teeth, a few are fragmentary lower son (1925a) recognized the unique signi®- jaws with teeth, and only one specimen, cance of the symmetrodont molar pattern in which has yet to be described (Cifelli et al., the evolution of the mammalian dentition 2000), preserves all of the cheek teeth in situ. and accordingly established the order Sym- Symmetrodonts have been accorded an im- metrodonta, including two families, Amphi- portant position in the evolution toward tri- dontidae and Spalacotheriidae (Simpson, bosphenic mammals (Cifelli and Madsen, 1925b). Crompton and Jenkins (1968) added 1999), but their poor preservation has ham- the family Kuehneotheriidae Kermack, Ker- pered the study of their morphology, func- mack & Mussett, 1968, to Symmetrodonta tion, and phylogeny. Here we describe a new based on the occlusal pattern shared between genus and species of spalacotheriid symme- Kuehneotherium and symmetrodonts. Most trodont from the Early Cretaceous of north- students have accepted this assignment (Cas- eastern China, with the lower jaw and all of siliano and Clemens, 1979; Datta, 1981; the lower cheek teeth well preserved. This Yadagiri, 1984; Fox, 1985; Kielan-Jawo- new material will help clarify morphological rowska, 1992; Sigogneau-Russell and En- details of the mandible and lower dentition som, 1998; Ensom and Sigogneau-Russell, of spalacotheriids, and adds new evidence to- 2000; Averianov, 2002), but a few excluded ward clarifying the position of symmetro- Kuehneotheriidae from the order based on donts in mammalian phylogeny. results of phylogenetic analyses (McKenna, 1975; Prothero, 1981; McKenna and Bell, MATERIALS AND METHODS 1997). In total, about 30 genera of Mesozoic mammals have been assigned to the order at During the ®eld seasons of 1994±1995 one time or another, but more than half of three specimens of symmetrodonts were col- these genera have since been found to be lected from coal-bearing sediments in three closer to other mammals than to symmetro- small coal mines in Badaohao Town, Hei- donts (see McKenna and Bell, 1997, and Av- shan County, Liaoning Province, China erianov, 2002, for latest versions of cladistic (Wang et al., 1995). The fossil-bearing layers and noncladistic classi®cation, respectively), are associated with the coal seams of the while some others are now regarded as junior Shahai Formation, which consists of varie- synonyms or nomina dubia. gated conglomerates, coal measures, and Most symmetrodonts are represented by grayish black and green shales. Other mam- isolated teeth or jaw fragments with teeth. mals collected from the same coal mines in- Only two genera, Zhangheotherium and clude triconodonts, multituberculates, an ae- Maotherium, both from the Jehol paleobiota gialodontid cladothere, and eutherians (Wang of northeastern China (Hu et al., 1997, 1998; et al., 1995, 2001). The formation has also Ji, 2002; Rougier et al., 2003) have the com- yielded the remains of bivalves, gastropods, plete skeleton preserved, but the dentitions ostracods, conchostracans, ®shes, lizards, of even these specimens are only partially and dinosaurs (teeth and egg shells). It over- 2005 HU ET AL.: A NEW SYMMETRODONT 3 lies the Jiufotang Formation, which pre- TABLE 1 serves components of Jehol paleobiota. The Measurements of Heishanlestes changi (in mm) estimated age of the Shahai Formation is Ap- Height Height tian (Early Cretaceous) (Wang et al., 2001). Length Width (lingual) (labial) All specimens were collected in the ®eld IVPP V 7480 by splitting the fossiliferous matrix with hammers and chisels, and they were further p1 N 0.77 Ð Ð p2 0.80 0.75 0.52 0.58 prepared in the laboratory using steel needles p3 0.73 0.76 0.64 0.71 under the binocular microscope (Drescher, p4 1.48 0.96 0.83 1.01 2000; Krebs, 2000). Stereoscopic photo- m1 1.62 1.13 0.86 1.22 graphs of the specimens were taken using a m2 1.21 1.33 0.76 1.42 RT SPOT digital camera mounted on a Ni- m3 1.15 1.32 0.84 1.71 kon SMZ-U stereo-microscope. The SEM m4 1.15 1.30 0.72 1.63 images were taken at 2.0 kV with specimens m5 1.01 1.09 0.58 0.97 m6 0.85 0.89 0.56 0.69 uncoated. All images were further processed using Adobe Photoshop software, Version Lower jaw 6.0.1, on a MacIntosh computer. Linear mea- Length (presevered portion) 17.37 surements (table 1) were taken with a Micro- Height below p4 1.54 1.37 Height below m3 2.31 1.67 code II digital measuring microscope. Angles Height below m6 2.70 2.13 of teeth were measured in crown view im- Coronoid process 3.48 (height) 3.25 (length) ages using the Measure Tool in Adobe Pho- Condyle 1.63 (width) 0.73 (length) toshop. IVPP V 7481 The de®nition of Mammalia adopted by a Luo et al. (2002: 5) is followed in this paper: ci 0.71 0.58 Ð Ð p1 0.92a 0.78 0.67 0.69 The Mammalia include the last ``common p2 0.91 0.76 0.80 0.79 ancestor of Sinoconodon, living monotremes, p3 0.92 0.75 0.78 0.81 and living therians, plus all its descendants.'' p4 1.58 1.02 0.87 1.20 Symmetrodonta in this paper is regarded as m1 1.69 1.32 0.84 1.59 an order of Mesozoic mammals with molars Lower jaw height below p4 1.67 1.53 characterized by an imperfectly symmetrical, IVPP V 7482 triangular arrangement of three principal p4 1.59 0.97 0.92 1.06 cusps and without talonid on lower teeth m1 1.75 1.10 0.93 1.18 (Simpson, 1925a, 1925b; Cassiliano and Lower jaw height below p4 1.72 1.66 Clemens, 1979). We use the terms ``premo- a Length of visible part. lar'' and ``molar'' for the postcanine teeth in the new symmetrodont based on the crown morphology of these teeth and that in other SYSTEMATIC PALEONTOLOGY symmetrodonts, but we have no information CLASS MAMMALIA LINNAEUS, 1758 about their replacement pattern. Abbrevia- tions for lower teeth are: ci, canine; p1, p2, ORDER SYMMETRODONTA SIMPSON, 1925 p3, p4, ®rst to fourth premolars; m1, m2, m3, FAMILY SPALACOTHERIIDAE MARSH, 1887 m4, m5, m6, ®rst to sixth molars. Molar ter- SUBFAMILY SPALACOLESTINAE CIFELLI AND minology follows that of Cifelli and Madsen MADSEN, 1999 (1999). Institutional abbreviations: IVPP, Institute Heishanlestes, new genus for Vertebrate Paleontology and Paleoanthro- TYPE SPECIES: Heishanlestes changi, n.sp. pology, Chinese Academy of Sciences, Bei- DIAGNOSIS: As for the type and only spe- jing, China; UALVP, Laboratory for Verte- cies. brate Paleontology, Department of Biological ETYMOLOGY: ``Heishan'' is the name of the Sciences, University of Alberta, Edmonton, county in northeastern China in which the Alberta, Canada.
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