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Highly specialized mammalian skulls from the Late Cretaceous of South America

Abstract

Dryolestoids are an extinct mammalian group belonging to the lineage leading to modern marsupials and placentals1,2. Dryolestoids are known by teeth and jaws from the Jurassic period of North America and Europe2,3, but they thrived in South America up to the end of the Mesozoic era and survived to the beginnings of the Cenozoic2,4,5,6,7. Isolated teeth and jaws from the latest Cretaceous of South America provide mounting evidence that, at least in western Gondwana, dryolestoids developed into strongly endemic groups by the Late Cretaceous4,5,6,7,8,9. However, the lack of pre-Late Cretaceous dryolestoid remains made study of their origin and early diversification intractable. Here we describe the first mammalian remains from the early Late Cretaceous of South America, including two partial skulls and jaws of a derived dryolestoid showing dental and cranial features unknown among any other group of Mesozoic mammals, such as single-rooted molars preceded by double-rooted premolars, combined with a very long muzzle, exceedingly long canines and evidence of highly specialized masticatory musculature. On one hand, the new mammal shares derived features of dryolestoids1,2,3 with forms from the Jurassic of Laurasia, whereas on the other hand, it is very specialized and highlights the endemic, diverse dryolestoid fauna from the Cretaceous of South America. Our specimens include only the second mammalian skull known for the Cretaceous of Gondwana, bridging a previous 60-million-year gap in the fossil record, and document the whole cranial morphology of a dryolestoid, revealing an unsuspected morphological and ecological diversity for non-tribosphenic mammals.

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Figure 1: Cronopio dentiacutus MPCA 454 type skull and MPCA 453 right lower jaw.
Figure 2: Cronopio dentiacutus MPCA 454 type skull and MPCA 453 skull and right lower jaw.
Figure 3: Cronopio dentiacutus MPCA 454 holotype (upper teeth).
Figure 4: Reconstruction of the skull and jaws of Cronopio dentiacutus based on MPCA 453 and 454.
Figure 5: Simplified phylogenetic tree of the major taxa included in the analysis.

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Acknowledgements

We thank M. Salinas, P. A. Gallina and P. J. Makovicky for finding the best specimens; the Avelás and Pincheira families for logistical support; C. Muñoz and R. Barbieri from the MPCA for access to collections under their care; J. A. González and A. Davidson for illustration and technical assistance. Field work permits and loans were facilitated by M. Solorza. C. Corbitt and J. R. Wible read an earlier version of the manuscript. Field work and research was supported by the Antorchas Foundation, American Museum of Natural History and NSF grants DEB 0946430, DEB 1068089 and ATOL 0629959 (to G.W.R.), The Jurassic Foundation (to S.A.) and NASA and Field Museum Womens’ Board (to P. J. Makovicky). This is L.C.G.’s R-46 contribution to the IDEAN.

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G.W.R. wrote the manuscript with contributions from all authors, edited the figures, scored the matrix and performed the phylogenetic analysis; S.A. edited the manuscript and figures; L.C.G. edited the manuscript, figures, matrix and performed the phylogenetic analysis.

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Correspondence to Guillermo W. Rougier.

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The authors declare no competing financial interests.

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The file contains Supplementary Text (Parts 1 and 2), Supplementary Figures 1-4 with legends and additional references. (PDF 1362 kb)

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Rougier, G., Apesteguía, S. & Gaetano, L. Highly specialized mammalian skulls from the Late Cretaceous of South America. Nature 479, 98–102 (2011). https://doi.org/10.1038/nature10591

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