1 Nature 1997 Vol: 390(6656):137-142. DOI: 10.1038/36505

A new symmetrodont mammal from China and its implications for mammalian evolution

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.

Mentions
Figures
Figure 1: Zhangheotherium quinquecuspidens (IVPP V7466, holotype).Stereophotographs (a) and outline (b) of the skeleton in ventral view of the dorsoventrally compressed specimen (broken lines indicate the morphology preserved in impressions that can be examined on the silicon rubber mould of the impressions). Vertical scale bar in b represents 1 cm. Abbreviations: ac, acromion of scapula; c1–c7, cervical vertebrae 1 to 7; ca1, canine; ca2–ca4, caudal vertebrae 2 to 4 (caudal vertebrae are incomplete); cd, coracoid process of scapula; cl, clavicle; cm, calcaneum; co? coronoid fossa?; cp 1–9, carpals 1 to 9; csp, crista parotica of petrosal; ctp, caudal tympanic recess of petrosal; dn, dentary; ep, epipubis; er, epitympanic recess; fc, fenestra cochlearis (round window); fe, femur; ff?, facial nerve foramen?; fi, fibula; frs, foramen for ramus superior; fst, fossa for stapedial muscle; fv, fenestra vestibuli (oval window); g, glenoid of the scapula; gj, groove for jugal (on the squamosal); gl, glenoid fossa of squamosal; hu, humerus; ic, interclavicle; if, infraspinous fossa; il, ilium; is, ischium; i1–i3, incisors 1 to 3; ju, jugal; L1–L6, lumbar vertebrae 1 to 6 (impressions only); lm, lambdoidal crest; mas, mastoid exposure of petrosal; mc, metacoracoid; mf, mandibular foramen; mg, meckelian groove; mp1–mp5, metacarpals I–V; mt1–mt5, metacarpals I–V; mst, manubrium of sternum; mx, maxillary; oc, occipital condyle; of, obturator foramen of pectoral girdle; on, odontoid notch for dens of atlas; pc, procoracoid; pcd, condylar process of dentary; pcl, preclavicle (a homologue of procoracoid, sensuref. 50); pcr, coronoid process of dentary; pf, pterygoid fossa; pgc, postglenoid crest; pgd, postglenoid depression; pmx, premaxillary; pp, paroccipital process of petrosal; ppe, paroccipital process of exoccipital (incomplete); pr, promontorium of petrosal; prs, prootic sinus canal; ptc, post-temporal canal; pts, post-tympanic sinus; p1 and p2, premolars 1 and 2; ra, radius; r1–r13, thoracic ribs 1 to 13 (posterior thoracic ribs preserved only in impressions); sc, scapula; sl?, fossa for splenial? on the dentary; sm, stylomastoid notch; sp, spine of scapula; sq, squamosal; ss, supraspinous fossa of scapula; stb1–stb6, sternebra 1 to sternebra 6; stl, embryonic sternal bands; sym, mandibular symphysis; s1–s4, sacral vertebrae 1 to 4 (represented mostly by impression); th, attachment site for tympanohyal; ti, tibia; ts, lateral tarsal spur of ankle; t1–t13, thoracic vertebrae 1 to 13; ul, ulna; x, xiphoid process of sternum. Figure 2: Dentition and mandible of Zhangheotherium quinquecuspidens.a, Upper molars M3–5; 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, 11, 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, 11, 12. Cust B' 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 a-b legend. Figure 3: Reconstruction of the partial basicranium of Zhangheotherium quiquecuspidens (IVPP V7466).Right side, ventral view. Composite reconstruction (a) and outline with labels (b). Hatched pattern indicates the areas of damage or matrix. Arrows in a indicate the constricted neck of the squamosal. For abbreviations, see a-b legend. Figure 4: Comparison of the sternal apparatus and pectoral girdle of Zhangheotherium and living mammals.a, b, Embryonic (a) and adult (b) stages of the sternum and pectoral girdle in marsupials (modified from ref. 50). c–e, The adult sternal and girdle structures of Ornithorhynchus (modified from ref. 49) (c), Zhangheotherium (d) and Didelphis (CMNH c45, and several uncatalogued specimens at Carnegie Museum) (e). The chondral element of the interclavicle and the medial part(s) of the embryonic coraco-scapular plate are considered to be incorporated into the sternal manubrium in adult marsupials49, 50. Zhangheotherium has retained a separate interclavicle, a primitive character of non-therian mammals, but a more derived character of a mobile clavicle–interclavicle joint which is present in multituberculates and in a modified form in living therians. For abbreviations, see a-b legend. Figure 5: Phylogenetic relationships of Zhangheotherium quinquecuspidens.Asterisk: given this phylogeny, the coiled membranous labyrinth without corresponding coiling of the bony labyrinth in monotremes28, 33 is considered to be a convergence to the coiled cochlea of derived therians. See Methods for details of nodes A–G.
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References
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    • . . . Jianshangou Valley (approximately 41° 41' 01" N, 120° 59' 30" E), about 32 km east of Chaoyang City, Liaoning Province, northeastern China1. . . .
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    • . . . Vertebrate faunal correlation1, 4, 5 and previous radiometric dates4 suggest that the Jianshangou Beds are either of the latest Jurassic age, or near the Jurassic–Cretaceous transition6 . . .
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    • . . . The Jianshangou Beds, consisting primarily of shales, are the lowest lacustrine intercalation in the neutro-basic volcanic beds of the Yixian Formation1, 2, 3. . . .
    • . . . The Jianshangou Beds have yielded diverse fossil fish4, the birds Confuciusornis, Liaoningornis5, 6 and Protarch-aeopteryx7, the theropod Sinosauropteryx8, and diverse gastropods, bivalves, ostracods, conchostracans and insects2. . . .
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    • . . . The Jianshangou Beds, consisting primarily of shales, are the lowest lacustrine intercalation in the neutro-basic volcanic beds of the Yixian Formation1, 2, 3. . . .
    • . . . An Early Cretaceous age was also suggested by invertebrate faunal correlation3, and supported by a recent radiometric date9 . . .
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    • . . . The Jianshangou Beds have yielded diverse fossil fish4, the birds Confuciusornis, Liaoningornis5, 6 and Protarch-aeopteryx7, the theropod Sinosauropteryx8, and diverse gastropods, bivalves, ostracods, conchostracans and insects2. . . .
    • . . . Vertebrate faunal correlation1, 4, 5 and previous radiometric dates4 suggest that the Jianshangou Beds are either of the latest Jurassic age, or near the Jurassic–Cretaceous transition6 . . .
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    • . . . The Jianshangou Beds have yielded diverse fossil fish4, the birds Confuciusornis, Liaoningornis5, 6 and Protarch-aeopteryx7, the theropod Sinosauropteryx8, and diverse gastropods, bivalves, ostracods, conchostracans and insects2. . . .
    • . . . Vertebrate faunal correlation1, 4, 5 and previous radiometric dates4 suggest that the Jianshangou Beds are either of the latest Jurassic age, or near the Jurassic–Cretaceous transition6 . . .
  6. Hou, L.-H., Martin, L. D., Zhou, Z.-H. & Feduccia, A. Early adaptive radiation of birds: evidence from fossils from Northeastern China. Science 274, 1164-1165 (1996) , .
    • . . . The Jianshangou Beds have yielded diverse fossil fish4, the birds Confuciusornis, Liaoningornis5, 6 and Protarch-aeopteryx7, the theropod Sinosauropteryx8, and diverse gastropods, bivalves, ostracods, conchostracans and insects2. . . .
    • . . . Vertebrate faunal correlation1, 4, 5 and previous radiometric dates4 suggest that the Jianshangou Beds are either of the latest Jurassic age, or near the Jurassic–Cretaceous transition6 . . .
  7. Ji, Q. & Ji, S.-A. Protarchaeopteryx, a new genus of Archaeopteridae of China. Chin. Geol. 1997 (3), 38-41 (1997). (In Chinese.) , .
    • . . . The Jianshangou Beds have yielded diverse fossil fish4, the birds Confuciusornis, Liaoningornis5, 6 and Protarch-aeopteryx7, the theropod Sinosauropteryx8, and diverse gastropods, bivalves, ostracods, conchostracans and insects2. . . .
  8. Ji, Q. & Ji, S.-A. Discovery of the earliest bird fossils in China and the origin of birds. Chin. Geol. 1996 (10), 30-33 (1996). (In Chinese.) , .
    • . . . The Jianshangou Beds have yielded diverse fossil fish4, the birds Confuciusornis, Liaoningornis5, 6 and Protarch-aeopteryx7, the theropod Sinosauropteryx8, and diverse gastropods, bivalves, ostracods, conchostracans and insects2. . . .
  9. Smith, P. E. et al. Date and rates in ancient lakes: 40Ar-39Ar Evidence for an early Cretaceous age for the Jehol group, Northeast China. Can. J. Earth Sci. 32, 1426-1431 (1995) , .
    • . . . An Early Cretaceous age was also suggested by invertebrate faunal correlation3, and supported by a recent radiometric date9 . . .
  10. Simpson, G. G. A Catalogue of the Mesozoic Mammalia in the Geological Department of the British Museum (Oxford Univ. Press, London, (1928)) , .
    • . . . A spalacotheriid symmetrodont (Fig. 2a-c) with the dental formula 3125/3126; differing from all other known symmetrodonts in having a hypertrophied cusp B' between the main cusp A and the stylocone (cusp B) (Fig. 2a-c); distinguishable from other spalacotheriids10, 11, 12 in the conical shape of the main cusps and the lack of lingual and labial cingulids on the lower molars11; distinctive from Spalacotherium11 in having more robust and rounded main cusps that lack connecting cristae; unique among early mammals13, 14, 15, 16, 17, 18 in having fused sternebrae, and a posteriorly expanded xiphoid process. . . .
    • . . . Spalacotherium and Peralestes were established on some dissociated lower and upper teeth, respectively, from the same fauna in the Late Jurassic of England10, 11, 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, 11, 12 . . .
    • . . . Zhangheotherium resembles other known spalacotheriid symmetrodonts10, 11, 12, 19, 20 in that the central cusp (a) and the two accessory cuspules (b, d) of the lower molar form an acute triangle . . .
    • . . . The associated upper and lower teeth of Zhangheotherium could occlude into the embrasures of the opposing tooth row, a pattern unique to spalacotheriids among archaic therians10 . . .
    • . . . Zhangheotherium is unique among spalacotheriids in having short and weak cingula in the upper molars, and a relatively low trigonid and two large accessory cuspules (cusps d and e) on the lower molars10, 11, 12, 19, 20 . . .
    • . . . Upper molars have a wide labial shelf and their cusp pattern resembles that of Peralestes, a Late Jurassic symmetrodont from England10, 11, 12 . . .
    • . . . The topographic relationships of these mandibular features (Fig. 2a-c) are very similar to those of more derived therian mammals ('eupantotheres')10, 16, but differs from that of the earliest known therian Kuehneotherium (Late Triassic), and from those of the non-therian mammals Morganucodon21 and Sinoconodon22 . . .
  11. Clemens, W. A. Late Jurassic mammalian fossils in the Sedgwick Museum, Cambridge. Palaeontology 6, 373-377 (1963) , .
    • . . . A spalacotheriid symmetrodont (Fig. 2a-c) with the dental formula 3125/3126; differing from all other known symmetrodonts in having a hypertrophied cusp B' between the main cusp A and the stylocone (cusp B) (Fig. 2a-c); distinguishable from other spalacotheriids10, 11, 12 in the conical shape of the main cusps and the lack of lingual and labial cingulids on the lower molars11; distinctive from Spalacotherium11 in having more robust and rounded main cusps that lack connecting cristae; unique among early mammals13, 14, 15, 16, 17, 18 in having fused sternebrae, and a posteriorly expanded xiphoid process. . . .
    • . . . Spalacotherium and Peralestes were established on some dissociated lower and upper teeth, respectively, from the same fauna in the Late Jurassic of England10, 11, 12 . . .
    • . . . Zhangheotherium resembles other known spalacotheriid symmetrodonts10, 11, 12, 19, 20 in that the central cusp (a) and the two accessory cuspules (b, d) of the lower molar form an acute triangle . . .
    • . . . Zhangheotherium is unique among spalacotheriids in having short and weak cingula in the upper molars, and a relatively low trigonid and two large accessory cuspules (cusps d and e) on the lower molars10, 11, 12, 19, 20 . . .
  12. Cassiliano, M. L. & Clemens, W. A. J in Mesozoic Mammals: the First Two-thirds of Mammalian History (eds Lillegraven, J. A., Kielan-Jaworowska, Z. & Clemens, W. A.) 150-161 (Univ. California Press, Berkeley, (1979)) , .
    • . . . A spalacotheriid symmetrodont (Fig. 2a-c) with the dental formula 3125/3126; differing from all other known symmetrodonts in having a hypertrophied cusp B' between the main cusp A and the stylocone (cusp B) (Fig. 2a-c); distinguishable from other spalacotheriids10, 11, 12 in the conical shape of the main cusps and the lack of lingual and labial cingulids on the lower molars11; distinctive from Spalacotherium11 in having more robust and rounded main cusps that lack connecting cristae; unique among early mammals13, 14, 15, 16, 17, 18 in having fused sternebrae, and a posteriorly expanded xiphoid process. . . .
    • . . . Spalacotherium and Peralestes were established on some dissociated lower and upper teeth, respectively, from the same fauna in the Late Jurassic of England10, 11, 12 . . .
    • . . . Zhangheotherium resembles other known spalacotheriid symmetrodonts10, 11, 12, 19, 20 in that the central cusp (a) and the two accessory cuspules (b, d) of the lower molar form an acute triangle . . .
    • . . . Zhangheotherium is unique among spalacotheriids in having short and weak cingula in the upper molars, and a relatively low trigonid and two large accessory cuspules (cusps d and e) on the lower molars10, 11, 12, 19, 20 . . .
    • . . . Owing to the lack of better evidence, the affinities of this lineage to the more derived therians have been based solely on dental evidence12, 43, 44, 45, 46 . . .
    • . . . Zhangheotherium has provided more extensive basicranial and postcranial evidence to corroborate the traditional hypothesis12, 43, 44, 45, 46 that symmetrodonts represent a part of the basal therian radiation . . .
  13. Jenkins, F. A. J & Parrington, F. R. Postcranial skeleton of the Triassic mammals Eozostrodon, Megazostrodon, and Erythrotherium. Phil. Trans. R. Soc. Lond. B 273, 387-431 (1976) , .
    • . . . A spalacotheriid symmetrodont (Fig. 2a-c) with the dental formula 3125/3126; differing from all other known symmetrodonts in having a hypertrophied cusp B' between the main cusp A and the stylocone (cusp B) (Fig. 2a-c); distinguishable from other spalacotheriids10, 11, 12 in the conical shape of the main cusps and the lack of lingual and labial cingulids on the lower molars11; distinctive from Spalacotherium11 in having more robust and rounded main cusps that lack connecting cristae; unique among early mammals13, 14, 15, 16, 17, 18 in having fused sternebrae, and a posteriorly expanded xiphoid process. . . .
    • . . . The postaxial cervical ribs remain unfused in adults, as in morganucodontids13 and multituberculates38 . . .
    • . . . The humerus also has a weakly developed ulnar condyle, resembling those of non-therian mammals13, 14, 38, 41 . . .
  14. Krause, D. W. & Jenkins, F. A. J The postcranial skeleton of North American multituberculates. Bull. Mus. Comp. Zool. 150, 199-246 (1983) , .
    • . . . A spalacotheriid symmetrodont (Fig. 2a-c) with the dental formula 3125/3126; differing from all other known symmetrodonts in having a hypertrophied cusp B' between the main cusp A and the stylocone (cusp B) (Fig. 2a-c); distinguishable from other spalacotheriids10, 11, 12 in the conical shape of the main cusps and the lack of lingual and labial cingulids on the lower molars11; distinctive from Spalacotherium11 in having more robust and rounded main cusps that lack connecting cristae; unique among early mammals13, 14, 15, 16, 17, 18 in having fused sternebrae, and a posteriorly expanded xiphoid process. . . .
    • . . . Distally, the humerus has an incipient trochlea for the ulna, a therian apomorphy16, 17, 41 that is absent in multituberculates14, 18, 42 . . .
    • . . . The humerus also has a weakly developed ulnar condyle, resembling those of non-therian mammals13, 14, 38, 41 . . .
    • . . . As in multituberculates14, 38, the spherical femoral head is set off from the shaft by a well-defined neck, and the greater trochanter is directed dorsally . . .
    • . . . This differs from the fully functioning (but more primitive) ulnar condyle of multituberculates14, 18, 38, 42 . . .
  15. Jenkins, F. A. J & Schaff, C. R. The Early Cretaceous mammal Gobiconodon (Mammalia Triconodonta) from the Cloverly Formation in Montana. J. Vert. Paleontol. 6, 1-24 (1988) , .
    • . . . A spalacotheriid symmetrodont (Fig. 2a-c) with the dental formula 3125/3126; differing from all other known symmetrodonts in having a hypertrophied cusp B' between the main cusp A and the stylocone (cusp B) (Fig. 2a-c); distinguishable from other spalacotheriids10, 11, 12 in the conical shape of the main cusps and the lack of lingual and labial cingulids on the lower molars11; distinctive from Spalacotherium11 in having more robust and rounded main cusps that lack connecting cristae; unique among early mammals13, 14, 15, 16, 17, 18 in having fused sternebrae, and a posteriorly expanded xiphoid process. . . .
    • . . . As in living monotremes and Gobiconodon15, Zhangheotherium has an external pedal spur . . .
  16. Krebs, B. Das Skelett von Henkelotherium guimarotae gen. et sp. nov. (Eupantotheria Mammalia) aus dem Oberen Jura von Portugal. Berlin. Geowiss. Abh. A 133, 1-110 (1991) , .
    • . . . A spalacotheriid symmetrodont (Fig. 2a-c) with the dental formula 3125/3126; differing from all other known symmetrodonts in having a hypertrophied cusp B' between the main cusp A and the stylocone (cusp B) (Fig. 2a-c); distinguishable from other spalacotheriids10, 11, 12 in the conical shape of the main cusps and the lack of lingual and labial cingulids on the lower molars11; distinctive from Spalacotherium11 in having more robust and rounded main cusps that lack connecting cristae; unique among early mammals13, 14, 15, 16, 17, 18 in having fused sternebrae, and a posteriorly expanded xiphoid process. . . .
    • . . . The topographic relationships of these mandibular features (Fig. 2a-c) are very similar to those of more derived therian mammals ('eupantotheres')10, 16, but differs from that of the earliest known therian Kuehneotherium (Late Triassic), and from those of the non-therian mammals Morganucodon21 and Sinoconodon22 . . .
    • . . . A slightly rugose area along the anterior meckelian groove suggests the presence of the splenial, and a rough area near the base of the coronoid process is probably for a poorly developed coronoid, as occurs in the more derived therian Henkelotherium from the Upper Jurassic of Portugal16 . . .
    • . . . Torsion of the proximal end of humerus relative tothe distal end is about 30 degrees, about the same as in Henkelotherium16, and close to the 40 degree torsion in Vincelestes17 . . .
    • . . . Distally, the humerus has an incipient trochlea for the ulna, a therian apomorphy16, 17, 41 that is absent in multituberculates14, 18, 42 . . .
    • . . . There are few relatively complete fossils of archaic therians16, 17 . . .
    • . . . It is more primitive than Henkelotherium16 and Vincelestes17 in retaining the interclavicle in its pectoral girdle . . .
    • . . . It also has a primitive dentition in which the lower molars have no distinct talonid, and the upper and lower molars occlude in the embrasures of the opposing tooth rows (Fig. 2a-c), in contrast to more derived therians in which the lingual part of the upper molar occludes with the talonid of the lower molar16, 45. . . .
    • . . . The trochlea is also incipient in the Late Jurassic Henkelotherium16, but more developed in the Early Cretaceous Vincelestes17 . . .
    • . . . The parasagittal forelimb posture of living therians was not present in such archaic therians as Zhangheotherium, Henkelotherium16 and Vincelestes17, and it is best considered to have arisen later in therian evolution. . . .
  17. Rougier, G. W. Vincelestes neuquenianus Bonaparte (Mammalia, Theria), un primitivo mammifero del Cretacico Inferior de la Cuenca Neuqina. Thesis, Univ. Nacional de Buenos Aires (1993) , .
    • . . . A spalacotheriid symmetrodont (Fig. 2a-c) with the dental formula 3125/3126; differing from all other known symmetrodonts in having a hypertrophied cusp B' between the main cusp A and the stylocone (cusp B) (Fig. 2a-c); distinguishable from other spalacotheriids10, 11, 12 in the conical shape of the main cusps and the lack of lingual and labial cingulids on the lower molars11; distinctive from Spalacotherium11 in having more robust and rounded main cusps that lack connecting cristae; unique among early mammals13, 14, 15, 16, 17, 18 in having fused sternebrae, and a posteriorly expanded xiphoid process. . . .
    • . . . Zhangheotherium is similar in the latter two features to derived therians17, 23, 24, but is different from non-therian mammals which have a narrow cranial moiety of the squamosal but no squamosal wall for the epitympanic recess25, 26 . . .
    • . . . The postglenoid area of the squamosal has a posterolateral depression that resembles the broad external auditory meatus of Vincelestes17, the marsupial Pucadelphys24 and placentals, in contrast to non-therian mammals in which the squamosal has no postglenoid region or the external auditory meatus21, 22, 27, 28, 29. . . .
    • . . . Zhangheotherium is very different from derived therian mammals, which have oval-shaped and more bulbous promontoria with cochleae coiled for at least 270 degrees17, 23, 33, 36. . . .
    • . . . Torsion of the proximal end of humerus relative tothe distal end is about 30 degrees, about the same as in Henkelotherium16, and close to the 40 degree torsion in Vincelestes17 . . .
    • . . . Distally, the humerus has an incipient trochlea for the ulna, a therian apomorphy16, 17, 41 that is absent in multituberculates14, 18, 42 . . .
    • . . . There are few relatively complete fossils of archaic therians16, 17 . . .
    • . . . It is more primitive than Henkelotherium16 and Vincelestes17 in retaining the interclavicle in its pectoral girdle . . .
    • . . . However, torsion of the humerus and the large lesser tubercle relative to the greater tubercle in Zhangheotherium, Henkelotherium and Vincelestes17 suggest that the humeri of these archaic therians were more abducted than those of most living therians . . .
    • . . . The parasagittal forelimb posture of living therians was not present in such archaic therians as Zhangheotherium, Henkelotherium16 and Vincelestes17, and it is best considered to have arisen later in therian evolution. . . .
    • . . . Because the finger-like promontorium is closely correlated to an uncoiled cochlea in diverse early mammals30, 31, 33, 34, 35, its presence in Zhangheotherium suggests that the coiled cochlea and the inflated promontorium of more derived therians17, 33, 36 are not present in symmetrodonts . . .
    • . . . Within the framework of mammalian phylogeny supported by many independent characters (Fig. 5), the coiled cochlea of therians would best be considered to have evolved later in such derived mammals as Vincelestes17, marsupials and placentals, independently of the partially coiled cochlea of monotremes (Fig. 5). . . .
  18. Sereno, P. & McKenna, M. C. Cretaceous multituberculate skeleton and the early evolution of the mammalian shoulder girdle. Nature 377, 144-147 (1995) , .
    • . . . A spalacotheriid symmetrodont (Fig. 2a-c) with the dental formula 3125/3126; differing from all other known symmetrodonts in having a hypertrophied cusp B' between the main cusp A and the stylocone (cusp B) (Fig. 2a-c); distinguishable from other spalacotheriids10, 11, 12 in the conical shape of the main cusps and the lack of lingual and labial cingulids on the lower molars11; distinctive from Spalacotherium11 in having more robust and rounded main cusps that lack connecting cristae; unique among early mammals13, 14, 15, 16, 17, 18 in having fused sternebrae, and a posteriorly expanded xiphoid process. . . .
    • . . . Zhangheotherium and multituberculates share strong similarities in the articulations of the interclavicle to the clavicle and to the sternum18 . . .
    • . . . Distally, the humerus has an incipient trochlea for the ulna, a therian apomorphy16, 17, 41 that is absent in multituberculates14, 18, 42 . . .
    • . . . Therians have been hypothesized to be the sister taxon of either multituberculates18, 26, 27 or monotremes on the basis of some derived molar characters44 . . .
    • . . . The sister-taxon relationship between therians and multituberculates18, 26, 27 is strongly supported by the evidence from Zhangheotherium . . .
    • . . . Such a forelimb posture has been hypothesized to have evolved only once in the common ancestry of multituberculates, living therians and their extinct relatives18, although this view has been contested42 . . .
    • . . . Several osteological characters are considered to be crucial to the parasagittal posture of the forelimb of therians18, 38, 39, 41: a mobile joint between the clavicle and sternal apparatus (including the interclavicle); a greater tubercle much wider than the lesser tubercle; a narrow intertubercular groove42; a humeral trochlea that constrains the movement of the ulna41; and possibly the lack of torsion of the humerus18 (but see ref. 42) . . .
    • . . . This differs from the fully functioning (but more primitive) ulnar condyle of multituberculates14, 18, 38, 42 . . .
  19. Fox, R. C. Upper molar structure in the Late Cretaceous symmetrodont Symmetrodontoides Fox, and a classification of the Symmetrodonta (Mammalia). J. Paleontol. 59, 21-26 (1985) , .
    • . . . Zhangheotherium resembles other known spalacotheriid symmetrodonts10, 11, 12, 19, 20 in that the central cusp (a) and the two accessory cuspules (b, d) of the lower molar form an acute triangle . . .
    • . . . Zhangheotherium is unique among spalacotheriids in having short and weak cingula in the upper molars, and a relatively low trigonid and two large accessory cuspules (cusps d and e) on the lower molars10, 11, 12, 19, 20 . . .
  20. Cifelli, R. Cretaceous mammals of southern Utah. III. Therian mammals from the Turonian (Early Late Cretaceous). J. Vert. Paleontol. 10, 332-345 (1990) , .
    • . . . Zhangheotherium resembles other known spalacotheriid symmetrodonts10, 11, 12, 19, 20 in that the central cusp (a) and the two accessory cuspules (b, d) of the lower molar form an acute triangle . . .
    • . . . This suggests that the teeth were used more for crushing and puncturing than shearing20. . . .
    • . . . Zhangheotherium is unique among spalacotheriids in having short and weak cingula in the upper molars, and a relatively low trigonid and two large accessory cuspules (cusps d and e) on the lower molars10, 11, 12, 19, 20 . . .
  21. Kermack, K. A., Mussett, F. & Rigney, H. W. The skull of Morganucodon. Zool. J. Linn. Soc. 71, 1-158 (1981) , .
    • . . . The topographic relationships of these mandibular features (Fig. 2a-c) are very similar to those of more derived therian mammals ('eupantotheres')10, 16, but differs from that of the earliest known therian Kuehneotherium (Late Triassic), and from those of the non-therian mammals Morganucodon21 and Sinoconodon22 . . .
    • . . . The postglenoid area of the squamosal has a posterolateral depression that resembles the broad external auditory meatus of Vincelestes17, the marsupial Pucadelphys24 and placentals, in contrast to non-therian mammals in which the squamosal has no postglenoid region or the external auditory meatus21, 22, 27, 28, 29. . . .
    • . . . Its structure is very similar to those of Sinoconodon30, morganucodontids21, 22, 31, triconodonts22, 26 and multituberculates32, 33, 34 . . .
    • . . . The petrosal has a prominent paroccipital process that is similar to those of morganucodontids21, 22, triconodonts22, 26, multituberculates and ornithorhynchids37 . . .
  22. Crompton, A. W. & Luo, Z. in Mammal Phylogeny (eds Szalay, F. S., Novacek, M. J. & McKenna, M. C.) 30-44 (Springer, New York, (1993)) , .
    • . . . The topographic relationships of these mandibular features (Fig. 2a-c) are very similar to those of more derived therian mammals ('eupantotheres')10, 16, but differs from that of the earliest known therian Kuehneotherium (Late Triassic), and from those of the non-therian mammals Morganucodon21 and Sinoconodon22 . . .
    • . . . The postglenoid area of the squamosal has a posterolateral depression that resembles the broad external auditory meatus of Vincelestes17, the marsupial Pucadelphys24 and placentals, in contrast to non-therian mammals in which the squamosal has no postglenoid region or the external auditory meatus21, 22, 27, 28, 29. . . .
    • . . . Its structure is very similar to those of Sinoconodon30, morganucodontids21, 22, 31, triconodonts22, 26 and multituberculates32, 33, 34 . . .
    • . . . The petrosal has a prominent paroccipital process that is similar to those of morganucodontids21, 22, triconodonts22, 26, multituberculates and ornithorhynchids37 . . .
  23. Rougier, G. W., Wible, J. R. & Hopson, J. A. Reconstruction of the cranial vessels in the Early Cretaceous mammal Vincelestes neuquenianus: implications for the evolution of the mammalian cranial vascular system. J. Vert. Paleontol. 12, 188-216 (1992) , .
    • . . . Zhangheotherium is similar in the latter two features to derived therians17, 23, 24, but is different from non-therian mammals which have a narrow cranial moiety of the squamosal but no squamosal wall for the epitympanic recess25, 26 . . .
    • . . . Zhangheotherium is very different from derived therian mammals, which have oval-shaped and more bulbous promontoria with cochleae coiled for at least 270 degrees17, 23, 33, 36. . . .
  24. Marshall, L. G. & Muizon, C. d in Pucadelphys andinus (Marsupialia, Mammalia) from the early Paleocene of Bolivia (ed. Muizon, C. de) Mém. Mus. Natl Hist. Nat. Paris 165, 21-90 (1995) , .
    • . . . Zhangheotherium is similar in the latter two features to derived therians17, 23, 24, but is different from non-therian mammals which have a narrow cranial moiety of the squamosal but no squamosal wall for the epitympanic recess25, 26 . . .
    • . . . The postglenoid area of the squamosal has a posterolateral depression that resembles the broad external auditory meatus of Vincelestes17, the marsupial Pucadelphys24 and placentals, in contrast to non-therian mammals in which the squamosal has no postglenoid region or the external auditory meatus21, 22, 27, 28, 29. . . .
  25. Luo, Z. & Crompton, A. W. Transformations of the quadrate (incus) through the transition from non-mammalian cynodonts to mammals. J. Vert. Paleontol. 14, 341-374 (1994) , .
    • . . . Zhangheotherium is similar in the latter two features to derived therians17, 23, 24, but is different from non-therian mammals which have a narrow cranial moiety of the squamosal but no squamosal wall for the epitympanic recess25, 26 . . .
  26. Rougier, G. W., Wible, J. R. & Hopson, J. A. Basicranial anatomy of Priacodon fruitaensis (Triconodontidae, Mammalia) from the Late Jurassic of Colorado, and a reappraisal of mammaliaform interrelationships. Am. Mus. Novit. 3183, 1-28 (1996) , .
    • . . . Zhangheotherium is similar in the latter two features to derived therians17, 23, 24, but is different from non-therian mammals which have a narrow cranial moiety of the squamosal but no squamosal wall for the epitympanic recess25, 26 . . .
    • . . . Its structure is very similar to those of Sinoconodon30, morganucodontids21, 22, 31, triconodonts22, 26 and multituberculates32, 33, 34 . . .
    • . . . The petrosal has a prominent paroccipital process that is similar to those of morganucodontids21, 22, triconodonts22, 26, multituberculates and ornithorhynchids37 . . .
    • . . . Therians have been hypothesized to be the sister taxon of either multituberculates18, 26, 27 or monotremes on the basis of some derived molar characters44 . . .
    • . . . The sister-taxon relationship between therians and multituberculates18, 26, 27 is strongly supported by the evidence from Zhangheotherium . . .
  27. Rowe, T. 1988. Definition, diagnosis, and origin of Mammalia. J. Vert. Paleontol. 8, 241-264 (1988) , .
    • . . . The postglenoid area of the squamosal has a posterolateral depression that resembles the broad external auditory meatus of Vincelestes17, the marsupial Pucadelphys24 and placentals, in contrast to non-therian mammals in which the squamosal has no postglenoid region or the external auditory meatus21, 22, 27, 28, 29. . . .
    • . . . Its astragalar process contacted the fibula, a plesiomorphy for therians27 . . .
    • . . . The relationships of therians to extinct non-therian mammalian lineages have received much attention, as hypotheses of therian relationships are central to the understanding of early mammalian evolution27, 43 . . .
    • . . . Therians have been hypothesized to be the sister taxon of either multituberculates18, 26, 27 or monotremes on the basis of some derived molar characters44 . . .
    • . . . It has been argued that dental characters are as homoplasic as non-dental characters27, 43, 47, 48, and the reliability of dental characters for inferring the relationships of major lineages of mammals has been questioned . . .
    • . . . The sister-taxon relationship between therians and multituberculates18, 26, 27 is strongly supported by the evidence from Zhangheotherium . . .
    • . . . Node A (Mammalia of ref. 43 or Mammaliaformes of ref. 27): prezygapophysis absent on axis; shallow patellar groove present on femur; notches for quadrate and quadratojugal absent in squamosal; promontorium present, unilateral occlusion of lower jaw; rotation of lower jaw during occlusion; differentiation of postcanine crowns into premolars and molars . . .
  28. Zeller, U. Die Entwicklung und Morphologie des Schädels von Ornithorhynchus anatinus (Mammalia: Prototheria: Monotremata). Abh. Senckenb. Naturf. Gesel. 545, 1-188 (1989) , .
    • . . . The postglenoid area of the squamosal has a posterolateral depression that resembles the broad external auditory meatus of Vincelestes17, the marsupial Pucadelphys24 and placentals, in contrast to non-therian mammals in which the squamosal has no postglenoid region or the external auditory meatus21, 22, 27, 28, 29. . . .
    • . . . Asterisk: given this phylogeny, the coiled membranous labyrinth without corresponding coiling of the bony labyrinth in monotremes28, 33 is considered to be a convergence to the coiled cochlea of derived therians . . .
    • . . . In contrast, the cochleae of monotremes lack such internal bony structures28, 33. . . .
  29. Wible, J. R. Origin of Mammalia: the craniodental evidence reexamined. J. Vert. Paleontol. 11, 1-28 (1991) , .
    • . . . The postglenoid area of the squamosal has a posterolateral depression that resembles the broad external auditory meatus of Vincelestes17, the marsupial Pucadelphys24 and placentals, in contrast to non-therian mammals in which the squamosal has no postglenoid region or the external auditory meatus21, 22, 27, 28, 29. . . .
  30. Luo, Z., Crompton, A. W. & Lucas, S. G. Evolutionary origins of the mammalian promontorium and cochlea. J. Vert. Paleontol. 15, 113-121 (1995) , .
    • . . . Its structure is very similar to those of Sinoconodon30, morganucodontids21, 22, 31, triconodonts22, 26 and multituberculates32, 33, 34 . . .
    • . . . As documented for a wide range of early mammals30, 31, 33, 34, 35, a cylindrical and finger-like promontorium is closely correlated with either a straight or slightly curved (but uncoiled) cochlea . . .
    • . . . Because the finger-like promontorium is closely correlated to an uncoiled cochlea in diverse early mammals30, 31, 33, 34, 35, its presence in Zhangheotherium suggests that the coiled cochlea and the inflated promontorium of more derived therians17, 33, 36 are not present in symmetrodonts . . .
  31. Graybeal, A., Rosowski, J., Ketten, D. R. & Crompton, A. W. Inner ear structure in Morganucodon, an early Jurassic mammal. Zool. J. Linn. Soc. 96, 107-117 (1989) , .
    • . . . Its structure is very similar to those of Sinoconodon30, morganucodontids21, 22, 31, triconodonts22, 26 and multituberculates32, 33, 34 . . .
    • . . . As documented for a wide range of early mammals30, 31, 33, 34, 35, a cylindrical and finger-like promontorium is closely correlated with either a straight or slightly curved (but uncoiled) cochlea . . .
    • . . . Because the finger-like promontorium is closely correlated to an uncoiled cochlea in diverse early mammals30, 31, 33, 34, 35, its presence in Zhangheotherium suggests that the coiled cochlea and the inflated promontorium of more derived therians17, 33, 36 are not present in symmetrodonts . . .
  32. Kielan-Jaworowska, Z., Presley, R. & Poplin, C. The cranial vascular system in taeniolabidoid multituberculate mammals. Phil. Trans. R. Soc. Lond. B 313, 525-602 (1986) , .
    • . . . Its structure is very similar to those of Sinoconodon30, morganucodontids21, 22, 31, triconodonts22, 26 and multituberculates32, 33, 34 . . .
  33. Luo, Z. & Ketten, D. R. CT scanning and computerized reconstructions of the inner ear structure of multituberculate mammals. J. Vert. Paleontol. 11, 220-228 (1991) , .
    • . . . Its structure is very similar to those of Sinoconodon30, morganucodontids21, 22, 31, triconodonts22, 26 and multituberculates32, 33, 34 . . .
    • . . . As documented for a wide range of early mammals30, 31, 33, 34, 35, a cylindrical and finger-like promontorium is closely correlated with either a straight or slightly curved (but uncoiled) cochlea . . .
    • . . . Zhangheotherium is very different from derived therian mammals, which have oval-shaped and more bulbous promontoria with cochleae coiled for at least 270 degrees17, 23, 33, 36. . . .
    • . . . Asterisk: given this phylogeny, the coiled membranous labyrinth without corresponding coiling of the bony labyrinth in monotremes28, 33 is considered to be a convergence to the coiled cochlea of derived therians . . .
    • . . . In early marsupials and placentals33, 36, the promontorium that houses the cochlea is inflated and bulbous as a result of the cochlear coiling . . .
    • . . . In contrast, the cochleae of monotremes lack such internal bony structures28, 33. . . .
    • . . . Because the finger-like promontorium is closely correlated to an uncoiled cochlea in diverse early mammals30, 31, 33, 34, 35, its presence in Zhangheotherium suggests that the coiled cochlea and the inflated promontorium of more derived therians17, 33, 36 are not present in symmetrodonts . . .
  34. Meng, J. & Wyss, A. Monotreme affinities and low-frequency hearing suggested by multituberculate ear. Nature 377, 141-144 (1995) , .
    • . . . Its structure is very similar to those of Sinoconodon30, morganucodontids21, 22, 31, triconodonts22, 26 and multituberculates32, 33, 34 . . .
    • . . . As documented for a wide range of early mammals30, 31, 33, 34, 35, a cylindrical and finger-like promontorium is closely correlated with either a straight or slightly curved (but uncoiled) cochlea . . .
    • . . . Basicranial studies34, 37 have suggested that monotremes and multituberculates were sister taxa to the exclusion of therians . . .
    • . . . Because the finger-like promontorium is closely correlated to an uncoiled cochlea in diverse early mammals30, 31, 33, 34, 35, its presence in Zhangheotherium suggests that the coiled cochlea and the inflated promontorium of more derived therians17, 33, 36 are not present in symmetrodonts . . .
  35. Lillegraven, J. A. & Krusat, G. Cranio-mandibular anatomy of Haldanodon exspectatus (Docondontia; Mammalia) from the Late Jurassic of Portugal and its implications to the evolution of mammalian characters. Contrib. Geol. 28, 39-138 (1991) , .
    • . . . As documented for a wide range of early mammals30, 31, 33, 34, 35, a cylindrical and finger-like promontorium is closely correlated with either a straight or slightly curved (but uncoiled) cochlea . . .
    • . . . Because the finger-like promontorium is closely correlated to an uncoiled cochlea in diverse early mammals30, 31, 33, 34, 35, its presence in Zhangheotherium suggests that the coiled cochlea and the inflated promontorium of more derived therians17, 33, 36 are not present in symmetrodonts . . .
  36. Meng, J. & Fox, R. C. Therian petrosals from the Oldman and Milk River Formations (Late Cretaceous), Alberta, Canada. J. Vert. Paleontol. 15, 122-130 (1995) , .
    • . . . Zhangheotherium is very different from derived therian mammals, which have oval-shaped and more bulbous promontoria with cochleae coiled for at least 270 degrees17, 23, 33, 36. . . .
    • . . . In early marsupials and placentals33, 36, the promontorium that houses the cochlea is inflated and bulbous as a result of the cochlear coiling . . .
    • . . . Within the cochlear canal, the osseous spiral laminae of the bony labyrinth are well developed to support the membranous labyrinth36 . . .
    • . . . Because the finger-like promontorium is closely correlated to an uncoiled cochlea in diverse early mammals30, 31, 33, 34, 35, its presence in Zhangheotherium suggests that the coiled cochlea and the inflated promontorium of more derived therians17, 33, 36 are not present in symmetrodonts . . .
  37. Wible, J. R. & Hopson, J. A. in Mammal Phylogeny Vol. 1(eds Szalay, F. S., Novacek, M. J. & McKenna, M. C.) 45-62 (Springer, New York, (1993)) , .
    • . . . The petrosal has a prominent paroccipital process that is similar to those of morganucodontids21, 22, triconodonts22, 26, multituberculates and ornithorhynchids37 . . .
    • . . . Basicranial studies34, 37 have suggested that monotremes and multituberculates were sister taxa to the exclusion of therians . . .
  38. Kielan-Jaworowska, Z. & Gambaryan, P. P. Postcranial anatomy and habits of Asian multituberculate mammals. Fossils & Strata 36, 1-92 (1994) , .
    • . . . The postaxial cervical ribs remain unfused in adults, as in morganucodontids13 and multituberculates38 . . .
    • . . . The humerus also has a weakly developed ulnar condyle, resembling those of non-therian mammals13, 14, 38, 41 . . .
    • . . . Although the ischiopubic plate is not well preserved in IVPP V7466, its impressions indicate a shallow pelvis, differing from the deep pelvis of multituberculates38 . . .
    • . . . As in multituberculates14, 38, the spherical femoral head is set off from the shaft by a well-defined neck, and the greater trochanter is directed dorsally . . .
    • . . . Several osteological characters are considered to be crucial to the parasagittal posture of the forelimb of therians18, 38, 39, 41: a mobile joint between the clavicle and sternal apparatus (including the interclavicle); a greater tubercle much wider than the lesser tubercle; a narrow intertubercular groove42; a humeral trochlea that constrains the movement of the ulna41; and possibly the lack of torsion of the humerus18 (but see ref. 42) . . .
    • . . . This differs from the fully functioning (but more primitive) ulnar condyle of multituberculates14, 18, 38, 42 . . .
  39. Jenkins, F. A. J The movement of the shoulder in claviculate and aclaviculate mammals. J. Morphol. 144, 71-84 (1974) , .
    • . . . Zhangheotherium has mobile articulations of the interclavicle, clavicle and scapula that allow the clavicle to move and act as a pivotal strut39 for a wider rotation of the scapula, as observed during the locomotion of the opossum40. . . .
    • . . . In contrast to the sprawling posture of living monotremes, most living therian mammals have a more parasagittal posture (sensu ref. 42), with the elbows positioned close to the thorax39, 40, 41 . . .
    • . . . Several osteological characters are considered to be crucial to the parasagittal posture of the forelimb of therians18, 38, 39, 41: a mobile joint between the clavicle and sternal apparatus (including the interclavicle); a greater tubercle much wider than the lesser tubercle; a narrow intertubercular groove42; a humeral trochlea that constrains the movement of the ulna41; and possibly the lack of torsion of the humerus18 (but see ref. 42) . . .
    • . . . This allows the clavicle to function as a pivot for the shoulder joint, and allows a greater range of rotation of the scapula during locomotion39, 40 than is possible in monotremes . . .
  40. Jenkins, F. A. J & Weijs, W. A. The functional anatomy of the shoulder in the Virginia opossum (Didelphis virginiana). J. Zool. 188, 379-410 (1979) , .
    • . . . Zhangheotherium has mobile articulations of the interclavicle, clavicle and scapula that allow the clavicle to move and act as a pivotal strut39 for a wider rotation of the scapula, as observed during the locomotion of the opossum40. . . .
    • . . . In contrast to the sprawling posture of living monotremes, most living therian mammals have a more parasagittal posture (sensu ref. 42), with the elbows positioned close to the thorax39, 40, 41 . . .
    • . . . This allows the clavicle to function as a pivot for the shoulder joint, and allows a greater range of rotation of the scapula during locomotion39, 40 than is possible in monotremes . . .
  41. Jenkins, F. A. J The functional anatomy and evolution of the mammalian humero-ulnar joint. Am. J. Anat. 137, 281-298 (1973) , .
    • . . . Distally, the humerus has an incipient trochlea for the ulna, a therian apomorphy16, 17, 41 that is absent in multituberculates14, 18, 42 . . .
    • . . . The humerus also has a weakly developed ulnar condyle, resembling those of non-therian mammals13, 14, 38, 41 . . .
    • . . . In contrast to the sprawling posture of living monotremes, most living therian mammals have a more parasagittal posture (sensu ref. 42), with the elbows positioned close to the thorax39, 40, 41 . . .
    • . . . Several osteological characters are considered to be crucial to the parasagittal posture of the forelimb of therians18, 38, 39, 41: a mobile joint between the clavicle and sternal apparatus (including the interclavicle); a greater tubercle much wider than the lesser tubercle; a narrow intertubercular groove42; a humeral trochlea that constrains the movement of the ulna41; and possibly the lack of torsion of the humerus18 (but see ref. 42) . . .
  42. Gambaryan, P. P. & Kielan-Jaworowska, Z. Sprawling versus parasagittal stance in multituberculate mammals. Acta Palaeontol. Pol. 42, 13-44 (1997) , .
    • . . . Distally, the humerus has an incipient trochlea for the ulna, a therian apomorphy16, 17, 41 that is absent in multituberculates14, 18, 42 . . .
    • . . . In contrast to the sprawling posture of living monotremes, most living therian mammals have a more parasagittal posture (sensu ref. 42), with the elbows positioned close to the thorax39, 40, 41 . . .
    • . . . Such a forelimb posture has been hypothesized to have evolved only once in the common ancestry of multituberculates, living therians and their extinct relatives18, although this view has been contested42 . . .
    • . . . Several osteological characters are considered to be crucial to the parasagittal posture of the forelimb of therians18, 38, 39, 41: a mobile joint between the clavicle and sternal apparatus (including the interclavicle); a greater tubercle much wider than the lesser tubercle; a narrow intertubercular groove42; a humeral trochlea that constrains the movement of the ulna41; and possibly the lack of torsion of the humerus18 (but see ref. 42) . . .
    • . . . This differs from the fully functioning (but more primitive) ulnar condyle of multituberculates14, 18, 38, 42 . . .
  43. Hopson, J. A. in Major Features of Vertebrate Evolution (eds Prothero, D. R. & Schoch, R. M.) 190-219 (Paleontological Society Short Courses, Knoxville, TN, (1994)) , .
    • . . . The relationships of therians to extinct non-therian mammalian lineages have received much attention, as hypotheses of therian relationships are central to the understanding of early mammalian evolution27, 43 . . .
    • . . . Owing to the lack of better evidence, the affinities of this lineage to the more derived therians have been based solely on dental evidence12, 43, 44, 45, 46 . . .
    • . . . It has been argued that dental characters are as homoplasic as non-dental characters27, 43, 47, 48, and the reliability of dental characters for inferring the relationships of major lineages of mammals has been questioned . . .
    • . . . Node A (Mammalia of ref. 43 or Mammaliaformes of ref. 27): prezygapophysis absent on axis; shallow patellar groove present on femur; notches for quadrate and quadratojugal absent in squamosal; promontorium present, unilateral occlusion of lower jaw; rotation of lower jaw during occlusion; differentiation of postcanine crowns into premolars and molars . . .
  44. Kielan-Jaworowska, Z., Crompton, A. W. & Jenkins, F. A. J The origin of egg-lying mammals. Nature 326, 871-873 (1987) , .
    • . . . Therians have been hypothesized to be the sister taxon of either multituberculates18, 26, 27 or monotremes on the basis of some derived molar characters44 . . .
    • . . . Owing to the lack of better evidence, the affinities of this lineage to the more derived therians have been based solely on dental evidence12, 43, 44, 45, 46 . . .
  45. Crompton, A. W. in Early Mammals (eds Kermack, D. M. & Kermack, K. A.) 65-87 (Academic, London, (1971)) , .
    • . . . 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 . . .
    • . . . Owing to the lack of better evidence, the affinities of this lineage to the more derived therians have been based solely on dental evidence12, 43, 44, 45, 46 . . .
    • . . . It also has a primitive dentition in which the lower molars have no distinct talonid, and the upper and lower molars occlude in the embrasures of the opposing tooth rows (Fig. 2a-c), in contrast to more derived therians in which the lingual part of the upper molar occludes with the talonid of the lower molar16, 45. . . .
  46. Crompton, A. W. & Jenkins, F. A. J in Mesozoic Mammals: the First Two-thirds of Mammalian History (eds Lillegraven, J. A., Kielan-Jaworowska, Z. & Clemens, W. A.) 59-72 (Univ. California Press, Berkeley, (1979)) , .
    • . . . Owing to the lack of better evidence, the affinities of this lineage to the more derived therians have been based solely on dental evidence12, 43, 44, 45, 46 . . .
  47. Kemp, T. S. The relationships of mammals. Zool. J. Linn. Soc. 77, 353-384 (1983) , .
    • . . . It has been argued that dental characters are as homoplasic as non-dental characters27, 43, 47, 48, and the reliability of dental characters for inferring the relationships of major lineages of mammals has been questioned . . .
  48. Rowe, T. in Mammal Phylogeny Vol. 1(eds Szalay, F. S., Novacek, M. J. & McKenna, M. C.) 129-145 (Springer, New York, (1993)) , .
    • . . . It has been argued that dental characters are as homoplasic as non-dental characters27, 43, 47, 48, and the reliability of dental characters for inferring the relationships of major lineages of mammals has been questioned . . .
  49. Klima, M. Die Frühentwicklung des Schültergürtels und des Brustbeins bei den Monotremen (Mammalia: Prototheria). Adv. Anat. Embryol. Cell Biol. 47, 1-80 (1973) , .
    • . . . a, b, Embryonic (a) and adult (b) stages of the sternum and pectoral girdle in marsupials (modified from ref. 50). c–e, The adult sternal and girdle structures of Ornithorhynchus (modified from ref. 49) (c), Zhangheotherium (d) and Didelphis (CMNH c45, and several uncatalogued specimens at Carnegie Museum) (e) . . .
    • . . . The chondral element of the interclavicle and the medial part(s) of the embryonic coraco-scapular plate are considered to be incorporated into the sternal manubrium in adult marsupials49, 50 . . .
    • . . . A large and reptile-like interclavicle is present in adult monotremes49 . . .
  50. Klima, M. Early development of the shoulder girdle and sternum in marsupials (Mammalia: Metatheria). Adv. Anat. Embryol. Cell Biol. 109, 1-91 (1987) , .
    • . . . Abbreviations: ac, acromion of scapula; c1–c7, cervical vertebrae 1 to 7; ca1, canine; ca2–ca4, caudal vertebrae 2 to 4 (caudal vertebrae are incomplete); cd, coracoid process of scapula; cl, clavicle; cm, calcaneum; co? coronoid fossa?; cp 1–9, carpals 1 to 9; csp, crista parotica of petrosal; ctp, caudal tympanic recess of petrosal; dn, dentary; ep, epipubis; er, epitympanic recess; fc, fenestra cochlearis (round window); fe, femur; ff?, facial nerve foramen?; fi, fibula; frs, foramen for ramus superior; fst, fossa for stapedial muscle; fv, fenestra vestibuli (oval window); g, glenoid of the scapula; gj, groove for jugal (on the squamosal); gl, glenoid fossa of squamosal; hu, humerus; ic, interclavicle; if, infraspinous fossa; il, ilium; is, ischium; i1–i3, incisors 1 to 3; ju, jugal; L1–L6, lumbar vertebrae 1 to 6 (impressions only); lm, lambdoidal crest; mas, mastoid exposure of petrosal; mc, metacoracoid; mf, mandibular foramen; mg, meckelian groove; mp1–mp5, metacarpals I–V; mt1–mt5, metacarpals I–V; mst, manubrium of sternum; mx, maxillary; oc, occipital condyle; of, obturator foramen of pectoral girdle; on, odontoid notch for dens of atlas; pc, procoracoid; pcd, condylar process of dentary; pcl, preclavicle (a homologue of procoracoid, sensu ref. 50); pcr, coronoid process of dentary; pf, pterygoid fossa; pgc, postglenoid crest; pgd, postglenoid depression; pmx, premaxillary; pp, paroccipital process of petrosal; ppe, paroccipital process of exoccipital (incomplete); pr, promontorium of petrosal; prs, prootic sinus canal; ptc, post-temporal canal; pts, post-tympanic sinus; p1 and p2, premolars 1 a nd 2; ra, radius; r1–r13, thoracic ribs 1 to 13 (posterior thoracic ribs preserved only in impressions); sc, scapula; sl?, fossa for splenial? on the dentary; sm, stylomastoid notch; sp, spine of scapula; sq, squamosal; ss, supraspinous fossa of scapula; stb1–stb6, sternebra 1 to sternebra 6; stl, embryonic sternal bands; sym, mandibular symphysis; s1–s4, sacral vertebrae 1 to 4 (represented mostly by impression); th, attachment site for tympanohyal; ti, tibia; ts, lateral tarsal spur of ankle; t1–t13, thoracic vertebrae 1 to 13; ul, ulna; x, xiphoid process of sternum. . . .
    • . . . a, b, Embryonic (a) and adult (b) stages of the sternum and pectoral girdle in marsupials (modified from ref. 50). c–e, The adult sternal and girdle structures of Ornithorhynchus (modified from ref. 49) (c), Zhangheotherium (d) and Didelphis (CMNH c45, and several uncatalogued specimens at Carnegie Museum) (e) . . .
    • . . . The chondral element of the interclavicle and the medial part(s) of the embryonic coraco-scapular plate are considered to be incorporated into the sternal manubrium in adult marsupials49, 50 . . .
    • . . . In living marsupials (Fig. 4a-e), it first appears embryonically as a separate ossification(s), but later becomes incorporated, at least in part, within the sternal manubrium in the adult50 . . .
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