Abstract
The skeleton of a eutherian (placental) mammal has been discovered from the Lower Cretaceous Yixian Formation of northeastern China. We estimate its age to be about 125 million years (Myr), extending the date of the oldest eutherian records with skull and skeleton by about 40–50 Myr. Our analyses place the new fossil at the root of the eutherian tree and among the four other known Early Cretaceous eutherians, and suggest an earlier and greater diversification of stem eutherians that occurred well before the molecular estimate for the diversification of extant placental superorders (104–64 Myr). The new eutherian has limb and foot features that are known only from scansorial (climbing) and arboreal (tree-living) extant mammals, in contrast to the terrestrial or cursorial (running) features of other Cretaceous eutherians. This suggests that the earliest eutherian lineages developed different locomotory adaptations, facilitating their spread to diverse niches in the Cretaceous.
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Acknowledgements
We thank K.-Q. Gao, M. Ellison, S.-A. Ji, M. A. Norell and D. Ren for collaborative field work; J. D. Archibald, R. L. Cifelli, Z. Kielan-Jaworowska, M. J. Novacek and G. W. Rougier for sharing ideas on early mammal research; K. C. Beard, M. Fischer, D. Gebo, M. Sánchez-Villagra and F. S. Szalay for discussions on limb anatomy and reconstructing locomotory patterns of fossil mammals; M. R. Dawson and A. Weil for improving the paper; A. Henrici for her skilful preparation; and M. Klingler for illustration of Fig. 1. We received funding from the Ministry of Land Resources of People's Republic of China and National Natural Science Foundation of China (Q.J.), National Science Foundation of USA and National Geographic Society (Z.-X.L.), and the Netting/O'Neil Funds of Carnegie Museum (Z.-X.L. and J.R.W.), and Brackenridge Fellowship of the University of Pittsburgh (J.A.G.).
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Ji, Q., Luo, ZX., Yuan, CX. et al. The earliest known eutherian mammal. Nature 416, 816–822 (2002). https://doi.org/10.1038/416816a
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DOI: https://doi.org/10.1038/416816a
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