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Convergent dental adaptations in pseudo-tribosphenic and tribosphenic mammals

Abstract

Tribosphenic molars of basal marsupials and placentals are a major adaptation, with the protocone (pestle) of the upper molar crushing and grinding in the talonid basin (mortar) on the lower molar1,2,3,4. The extinct pseudo-tribosphenic mammals have a reversed tribosphenic molar in which a pseudo-talonid is anterior to the trigonid, to receive the pseudo-protocone of the upper molar. The pseudo-protocone is analogous to the protocone, but the anteriorly placed pseudo-talonid is opposite to the posterior talonid basin of true tribosphenic mammals5,6,7. Here we describe a mammal of the Middle Jurassic period with highly derived pseudo-tribosphenic molars but predominantly primitive mandibular and skeletal features, and place it in a basal position in mammal phylogeny. Its shoulder girdle and limbs show fossorial features similar to those of mammaliaforms and monotremes, but different compared with those of the earliest-known Laurasian tribosphenic (boreosphenid) mammals. The find reveals a much greater range of dental evolution in Mesozoic mammals than in their extant descendants, and strengthens the hypothesis of homoplasy of ‘tribosphenic-like’ molars among mammals.

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Figure 1: Skeleton of the new Mesozoic mammal Pseudotribos robustus.
Figure 2: Dental and mandibular features of Pseudotribos.
Figure 3: Comparison of tribosphenic and pseudo-tribosphenic molars.
Figure 4: Evolution of interclavicle and anterior sternal structure in the cynodont–mammal transition.

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Acknowledgements

We thank A. R. Tabrum for fossil preparation, H.-L. You for his assistance, M. R. Dawson for improving the manuscript, M. A. Klingler for help with graphics, and J. R. Wible, G.-H. Cui and K.-Q. Gao for access to comparative materials. This work was supported by the 973 Project by Ministry of Science and Technology, Chinese Academy of Geological Sciences (Beijing) (Q.J.), the National Science Foundation (USA), the National Natural Science Foundation (China) and the National Geographic Society (Z.-X.L.).

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Correspondence to Zhe-Xi Luo.

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Luo, ZX., Ji, Q. & Yuan, CX. Convergent dental adaptations in pseudo-tribosphenic and tribosphenic mammals. Nature 450, 93–97 (2007). https://doi.org/10.1038/nature06221

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