1. Section of Mammals, Carnegie Museum of Natural History, Pittsburgh, Pennsylvania 15206, USA
2. Department of Anatomical Sciences and Neurobiology, School of Medicine, University of Louisville, Louisville, Kentucky 40292, USA
3. Division of Paleontology, American Museum of Natural History, New York, New York 10024, USA
4. Museum of Zoology, University of Cambridge, Cambridge CB2 3EJ, UK

Correspondence to: J. R. Wible¹ Correspondence and requests for materials should be addressed to J.R.W. (Email: wiblej@carnegiemnh.org).

Abstract

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. Some^{3, 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 others^{6, 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 Hemisphere^{8, 9}, and place afrotherians and xenarthrans in a nested rather than a basal position^{8, 9} within Placentalia.

1. Section of Mammals, Carnegie Museum of Natural History, Pittsburgh, Pennsylvania 15206, USA
2. Department of Anatomical Sciences and Neurobiology, School of Medicine, University of Louisville, Louisville, Kentucky 40292, USA
3. Division of Paleontology, American Museum of Natural History, New York, New York 10024, USA
4. Museum of Zoology, University of Cambridge, Cambridge CB2 3EJ, UK

Correspondence to: J. R. Wible¹ Correspondence and requests for materials should be addressed to J.R.W. (Email: wiblej@carnegiemnh.org).

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