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Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary


Estimates of the time of origin for placental mammals from DNA studies span nearly the duration of the Cretaceous period (145 to 65 million years ago), with a maximum of 129 million years ago1 and a minimum of 78 million years ago2. Palaeontologists too are divided on the timing. Some3,4,5 support a deep Cretaceous origin by allying certain middle Cretaceous fossils (97–90 million years old) from Uzbekistan with modern placental lineages, whereas others6,7 support the origin of crown group Placentalia near the close of the Cretaceous. This controversy has yet to be addressed by a comprehensive phylogenetic analysis that includes all well-known Cretaceous fossils and a wide sample of morphology among Tertiary and recent placentals6. Here we report the discovery of a new well-preserved mammal from the Late Cretaceous of Mongolia and a broad-scale phylogenetic analysis. Our results exclude Cretaceous fossils from Placentalia, place the origin of Placentalia near the Cretaceous/Tertiary (K/T) boundary in Laurasia rather than much earlier within the Cretaceous in the Southern Hemisphere8,9, and place afrotherians and xenarthrans in a nested rather than a basal position8,9 within Placentalia.

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Figure 1: Maelestes gobiensis gen. et sp. nov. (holotype, PSS-MAE 607) skull and mandible in comparison to other Djadokhta eutherians.
Figure 2: Maelestes gobiensis gen. et sp. nov. (holotype, PSS-MAE 607) left upper and lower penultimate and ultimate premolars (P4, P5) and molars (M1, M2, M3).
Figure 3: Relationships of Maelestes to other eutherians.

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We thank A. Davidson for specimen preparation; P. Bowden for illustration; N. B. Simmons, J. J. Flynn, R. L. Cifelli, J. G. Mead and H. L. Kafka for specimens; and J. D. Archibald, K. C. Beard, J. I. Bloch, R. L. Cifelli, M. R. Dawson, T. J. Gaudin, J. A. Hopson, I. Horovitz, Z. Kielan-Jaworowska, Z.-X. Luo, G. Metais, M. A. O’Leary, K. D. Rose and S. P. Zack for discussions. This work was supported by the NSF (J.R.W., G.W.R, M.J.N. and R.J.A.), the Carnegie Museum of Natural History, and the American Museum of Natural History.

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Wible, J., Rougier, G., Novacek, M. et al. Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary. Nature 447, 1003–1006 (2007).

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