A Jurassic eutherian mammal and divergence of marsupials and placentals

Journal name:
Nature
Volume:
476,
Pages:
442–445
Date published:
DOI:
doi:10.1038/nature10291
Received
Accepted
Published online

Placentals are the most abundant mammals that have diversified into every niche for vertebrates and dominated the world’s terrestrial biotas in the Cenozoic. A critical event in mammalian history is the divergence of eutherians, the clade inclusive of all living placentals, from the metatherian–marsupial clade1, 2, 3, 4, 5, 6, 7, 8. Here we report the discovery of a new eutherian of 160Myr from the Jurassic of China, which extends the first appearance of the eutherian–placental clade by about 35Myr from the previous record, reducing and resolving a discrepancy between the previous fossil record and the molecular estimate for the placental–marsupial divergence9, 10, 11, 12, 13. This mammal has scansorial forelimb features, and provides the ancestral condition for dental and other anatomical features of eutherians.

At a glance

Figures

  1. Holotype specimen of Juramaia sinensis, Beijing Museum of Natural History (BMNH) PM1343B.
    Figure 1: Holotype specimen of Juramaia sinensis, Beijing Museum of Natural History (BMNH) PM1343B.

    a, b, Specimen photograph and morphological identification. c, Restoration of the partly preserved skeleton and skull. d, Restoration of hand (ventral view; alignment of incomplete and scattered carpals is conjectural). Abbreviations: ac, acromion (scapula); ag, angular process (dentary); C, c, upper or lower canine; ca, carpals; cl, clavicle; cod, coronoid (dentary); cos, coracoid process (scapula); cv1–7, cervical vertebrae 1–7; dc, dentary condyle; ecc, ectepicondyle; enf, entepicondylar foramen; hh, humeral head; ht, humeral trochlea; I1–5, upper incisors 1–5; Ju, jugal; M, m, upper or lower molar; manus, hand; mc1–5, metacarpals 1–5; oc, occipital condyles; ol, olecranon process; P1–5, upper premolars 1–5; ph, phalanges; r1–13; thoracic ribs 1–13; ra, radius; sn, semilunar notch (ulna); sp., scapular spine; tv1–13, thoracic vertebrae 1–13; ul, ulna.

  2. Dental and mandibular features of Juramaia sinensis (BMNH PM1343B).
    Figure 2: Dental and mandibular features of Juramaia sinensis (BMNH PM1343B).

    ad, Right upper M2 in mesial, occlusal, labial and distal views (composite restoration from both the right and the left sides). e, Stereo photographs of right premolars and molars. f, Stereo photographs of left premolars and molars. g, Right P3–M3 in occlusal view. h, Left upper dentition restoration in labial view. i, Left lower dentition (restoration) and mandible. Grey-shaded areas represent reconstruction from incomplete bone or tooth structure or mould outline in matrix. Abbreviations: ag, angular process; cod, coronoid process of dentary; dc; dentary condyle; M, m, upper and lower molars; mf, mental foramen; P, p, upper and lower premolars; dP3, deciduous P3 in situ. Terminology of tribosphenic molar follows Fig. 11.1 of ref. 1.

  3. Time-calibrated phylogeny of the eutherian Juramaia among other boreosphenidan mammals, and comparative morphology of some key molar features.
    Figure 3: Time-calibrated phylogeny of the eutherian Juramaia among other boreosphenidan mammals, and comparative morphology of some key molar features.

    a, Basal eutherian and metatherian phylogeny from parsimony analysis of data set of ref. 24 (446 characters of 103 cynodont–mammaliaform clades; based on the strict consensus of 172 equally parsimonious trees (each with treelength 2,243; consistency index 0.373, retention index 0.803) from 1,000 PAUP heuristic runs, without any topology constraints and with all multi-state characters unordered, multi-state taxa interpreted as polymorphism). Placement of Juramaia in eutherians is significantly different (*P<0.050) from suboptimal hypotheses of Juramaia as either a boreosphenidan or a metatherian by Templeton tests. This topology is corroborated by a separate analysis on a different and complementary data set by refs 4, 5 (389 informative characters of 71 eutherian taxa and outgroups), by the strict consensus of 41 equally parsimonious trees, from 1,000 heuristic runs, without topology constraints and 33 multi-state characters ordered, multi-state taxa as polymorphism. Placement of Juramaia among basal eutherians is consistent with topologies from constrained search under molecular scaffolding of extant taxa in the main data set of ref. 24 and the complementary data set of refs 4, 5 (details in Supplementary Information). b, The increased en-echelon postvallum shearing of upper molars in the earliest eutherians17, in contrast to metatherians18 that lack a strongly developed postvallum shearing by metacingulum, except for the Late Cretaceous Pediomys1. Nodes (1) Cladotheria, (2) Boreosphenida1, 2, (3) crown Theria, (4) Eutheria (including Placentalia), and (5) Metatheria (including Marsupialia).

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Author information

Affiliations

  1. Carnegie Museum of Natural History, Pittsburgh, Pennsylvania 15213, USA

    • Zhe-Xi Luo
  2. Chinese Academy of Geological Sciences, Beijing 100037, China

    • Chong-Xi Yuan &
    • Qiang Ji
  3. Beijing Museum of Natural History, Beijing 100050, China

    • Qing-Jin Meng

Contributions

Z.-X.L. and Q.J. designed the research plan. All authors participated in morphological studies. Z.-X.L. and C.-X.Y. performed phylogenetic analyses. Z.-X.L. wrote the paper with discussion from all authors.

Competing financial interests

The authors declare no competing financial interests.

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