The vast majority of Mesozoic and early Cenozoic metatherian mammals (extinct relatives of modern marsupials) are known only from partial jaws or isolated teeth, which give insight into their probable diets and phylogenetic relationships but little else. The few skulls known are generally crushed, incomplete or both1,2,3,4, and associated postcranial material is extremely rare. Here we report the discovery of an exceptionally large number of almost undistorted, nearly complete skulls and skeletons of a stem-metatherian, Pucadelphys andinus, in the early Palaeocene epoch5 of Tiupampa in Bolivia6,7,8. These give an unprecedented glimpse into early metatherian morphology, evolutionary relationships and, especially, ecology. The remains of 35 individuals have been collected, with 22 of these represented by nearly complete skulls and associated postcrania. These individuals were probably buried in a single catastrophic event, and so almost certainly belong to the same population9. The preservation of multiple adult, sub-adult and juvenile individuals in close proximity (<1 m2) is indicative of gregarious social behaviour or at least a high degree of social tolerance and frequent interaction. Such behaviour is unknown in living didelphids, which are highly solitary and have been regarded, perhaps wrongly, as the most generalized living marsupials. The Tiupampan P. andinus population also exhibits strong sexual dimorphism, which, in combination with gregariousness, suggests strong male–male competition and polygyny. Our study shows that social interactions occurred in metatherians as early as the basal Palaeocene and that solitary behaviour may not be plesiomorphic for Metatheria as a whole.
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Field expeditions were supported by the National Geographic Society (grants 2908-84) in 1985 and by the ‘Institut Français d’Études Andines’ in 1997. The specimens collected are the property of the ‘Museo de Historia Natural Alcide d’Orbigny’ in Cochabamba, Bolivia. Additional financial support was provided by Research Project MO/36/020 of the Belgian Federal Science Policy Office (S.L.), and NSF grant DEB-0743039 (R.M.D.B., in collaboration with R. Voss at the American Museum of Natural History). Photographs are by C. Lemzaouda and P. Loubry, scanning electron microscope photographs are by J. Cillis and the reconstruction in Fig. 1 is by S. Fernandez. We thank E. Westwig for access to the AMNH collections, and O. Lambert, A. Pradel, and T. Soderquist for discussions.
The authors declare no competing financial interests.
This file contains Supplementary Information including the referred material with catalogue numbers, the methods of measurements and the methods and results of the statistic analyses performed in this study. It also includes Supplementary Tables 1-3 and additional references. (PDF 3323 kb)
This file contains 17 Supplementary Tables which show measurement datasets and calculations. (XLS 378 kb)
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Ladevèze, S., de Muizon, C., Beck, R. et al. Earliest evidence of mammalian social behaviour in the basal Tertiary of Bolivia. Nature 474, 83–86 (2011). https://doi.org/10.1038/nature09987
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