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Dual origin of tribosphenic mammals

Abstract

Marsupials, placentals and their close therian relatives possess complex (tribosphenic) molars that are capable of versatile occlusal functions. This functional complex is widely thought to be a key to the early diversification and evolutionary success of extant therians and their close relatives (tribosphenidans). Long thought to have arisen on northern continents, tribosphenic mammals have recently been reported from southern landmasses. The great age and advanced morphology of these new mammals has led to the alternative suggestion of a Gondwanan origin for the group. Implicit in both biogeographic hypotheses is the assumption that tribosphenic molars evolved only once in mammalian evolutionary history. Phylogenetic and morphometric analyses including these newly discovered taxa suggest a different interpretation: that mammals with tribosphenic molars are not monophyletic. Tribosphenic molars evolved independently in two ancient (holotherian) mammalian groups with different geographic distributions during the Jurassic/Early Cretaceous: an australosphenidan clade endemic to Gondwanan landmasses, survived by extant monotremes; and a boreosphenidan clade of Laurasian continents, including extant marsupials, placentals and their relatives.

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Figure 1: Phylogenetic relationships of main taxa of therian tribosphenic mammals.
Figure 2: Comparison of molars of australosphenidans, pretribosphenids (‘eupantotheres’: Henkelotherium and Peramus) and boreosphenidans, in lingual view.
Figure 3: Lower molar shape in early mammals.

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Acknowledgements

We thank T. H. Rich, P. Vickers-Rich and M. Archer for providing comparative casts; and K. C. Beard, A. W. Crompton, M. R. Dawson, J. J. Flynn, J. J. Hurum, M. C. McKenna, M. J. Novacek, R. Presley, T. H. Rich, D. Sigogneau-Russell, J. R. Wible and A. R. Wyss for relevant discussion, and M. Klingler for assistance in illustration. Research was supported by National Science Foundation (USA) and National Geographic Society (Z.-X. L. and R.L.C), Carnegie Museum (Z.-X.L.) and Institute of Paleobiology, PAN (Z.K.-J.).

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Correspondence to Richard L. Cifelli.

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Luo, ZX., Cifelli, R. & Kielan-Jaworowska, Z. Dual origin of tribosphenic mammals. Nature 409, 53–57 (2001). https://doi.org/10.1038/35051023

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