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An Early Cretaceous eutherian and the placental–marsupial dichotomy

Abstract

Molecular estimates of the divergence of placental and marsupial mammals and their broader clades (Eutheria and Metatheria, respectively) fall primarily in the Jurassic period. Supporting these estimates, Juramaia—the oldest purported eutherian—is from the early Late Jurassic (160 million years ago) of northeastern China. Sinodelphys—the oldest purported metatherian—is from the same geographic area but is 35 million years younger, from the Jehol biota. Here we report a new Jehol eutherian, Ambolestes zhoui, with a nearly complete skeleton that preserves anatomical details that are unknown from contemporaneous mammals, including the ectotympanic and hyoid apparatus. This new fossil demonstrates that Sinodelphys is a eutherian, and that postcranial differences between Sinodelphys and the Jehol eutherian Eomaia—previously thought to indicate separate invasions of a scansorial niche by eutherians and metatherians—are instead variations among the early members of the placental lineage. The oldest known metatherians are now not from eastern Asia but are 110 million years old from western North America, which produces a 50-million-year ghost lineage for Metatheria.

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Fig. 1: Holotype specimen of A. zhoui STM33-5, Tianyu Museum of Nature, Shandong Province, China.
Fig. 2: Dental and upper and lower jaw features of A. zhoui STM33-5.
Fig. 3: Forefoot and hyoid apparatus of A. zhoui compared with those of other mammals.
Fig. 4: Relationships of A. zhoui to other mammals.
Fig. 5: Ectotympanic and malleus in A. zhoui and other mammals.

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Acknowledgements

We thank S. Xie for specimen preparation; P. Bowden for illustration; W. Gao for photography; Y. Hou and P. Yin for computed tomography scanning; D. Koyabu and V. Weisbecker for providing the computed tomography dataset of Monodelphis; and J. Meng, X. Xu, B. Jiang and Y. Huang for assistance and discussion. The study was supported by the National Natural Science Foundation of China (41688103, 41728003, 41372014, 41472023) and Chinese Academy of Sciences (XDPB0503). Support for S.B. was provided by the MEC International Joint Laboratory for Palaeobiology and Palaeoenvironment, Yunnan University. Support for J.R.W. is provided by the National Science Foundation Grant DEB 1654949 and Carnegie Museum of Natural History.

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Nature thanks R. Cifelli, D. Krause and G. Rougier for their contribution to the peer review of this work.

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Authors

Contributions

S.B. and J.R.W. conceived the study, undertook comparative and analytical work and wrote the paper; N.E.C. performed the ternary plot; and X.Z., S.Y. and X.W. contributed to fossil interpretation and provided feedback to the paper.

Corresponding authors

Correspondence to Shundong Bi, Xiaoli Wang or John R. Wible.

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The authors declare no competing interests.

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Extended data figures and tables

Extended Data Fig. 1 Line drawings of A. zhoui STM33-5.

a, Main slab. b, Counterpart slab.

Extended Data Fig. 2 Close-up views of craniodental features of A. zhoui STM33-5.

a, Partial skull and left dentary of the main slab in lateral view. b, Partial skull and right dentary of the counterpart slab in medial view. M and m, upper and lower molars, respectively; P and p, upper and lower premolars, respectively. ch, choanae; ec, ectotympanic; end, entoconid; fm, foramen magnum; hyld, hypoconulid; iof, infraorbital foramen; ju, jugal; mf, mental foramina; mfo, masseteric foramen; oc, occipital condyle; ppt, postpalatine torus; pr, promontorium of petrosal.

Extended Data Fig. 3 Left upper and lower teeth of A. zhoui STM33-5 as preserved on the main slab (A) of the specimen from 3D rendering (Mimics) of computed tomography scans.

a, Buccal view. b, Lingual view. c, Occlusal view. The lingual face of M1–M3, including the entoconid and hypoconulid, has been sheared off.

Extended Data Fig. 4 Comparison of left upper and lower jaws in lateral view of Ambolestes, Sinodelphys, Juramaia and Eomaia.

Sinodelphys was redrawn from a previous study7 and reversed from the original; Juramaia was redrawn from a previous study5; and Eomaia was redrawn from a previous study2 and reversed from the original. Scale bars, 2 mm.

Extended Data Fig. 5 Left upper dentition of Sinodelphys szalayi.

a, Dental formula redrawn from a previous study7. b, Dental formula proposed in this work on drawing of CM 79002 (a cast of the holotype). Note that the tooth identified as M1 in the previous study7 is not molariform, but is instead built on the same pattern as the tall, trenchant premolariform tooth that is mesial to it. On the M1 and M2 (as interpreted here), the paracone and metacone are hidden by the stylar shelf.

Extended Data Fig. 6 The strict consensus tree of four equally most-parsimonious trees.

The consensus tree length = 1,779, consistency index = 0.318 and retention index = 0.597. A simplified version of this consensus tree is presented in Fig. 4.

Extended Data Fig. 7 Analysis of limb elements of A. zhoui STM33-5 for locomotor behaviour.

a, Ternary plot showing intrinsic manual ray III proportions. b, Box plots of the intermembral index. The line that divides the box into two parts represents the median, the box shows the upper and lower quartiles, and the whiskers show extreme values for each group.

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Bi, S., Zheng, X., Wang, X. et al. An Early Cretaceous eutherian and the placental–marsupial dichotomy. Nature 558, 390–395 (2018). https://doi.org/10.1038/s41586-018-0210-3

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