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A new eutriconodont mammal and evolutionary development in early mammals

Abstract

Detachment of the three tiny middle ear bones from the reptilian mandible is an important innovation of modern mammals. Here we describe a Mesozoic eutriconodont nested within crown mammals that clearly illustrates this transition: the middle ear bones are connected to the mandible via an ossified Meckel’s cartilage. The connected ear and jaw structure is similar to the embryonic pattern in modern monotremes (egg-laying mammals) and placental mammals, but is a paedomorphic feature retained in the adult, unlike in monotreme and placental adults. This suggests that reversal to (or retention of) this premammalian ancestral condition is correlated with different developmental timing (heterochrony) in eutriconodonts. This new eutriconodont adds to the evidence of homoplasy of vertebral characters in the thoraco-lumbar transition and unfused lumbar ribs among early mammals. This is similar to the effect of homeobox gene patterning of vertebrae in modern mammals, making it plausible to extrapolate the effects of Hox gene patterning to account for homoplastic evolution of vertebral characters in early mammals.

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Figure 1: New mammal Yanoconodon allini.
Figure 2: Relationship of Yanoconodon allini.
Figure 3: Comparison of mandible and middle ear of Yanoconodon.
Figure 4: Homoplastic evolution of lumbar ribs among Mesozoic mammals and patterning of vertebral and rib development by Hox10 gene in extant mammals.

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Acknowledgements

We thank A. Tabrum for preparing this fossil; X.-N. Yang, Y.-K. Shi, J.-R. Liu, Q. Yang, J.-G. Sha, H.-C. Zhang for support; Q. Ji and J. Wible for access to comparative collections; R. Cifelli, Z. Kielan-Jaworowska, T. Martin, T. Rowe, J. Wible and G. Wilson for discussions; M. Dawson and J. Wible for improving the manuscript; and M. Klingler for assistance with the figures. This work was supported by the National Natural Science Foundation of China (P.C., G.L. and Z.-X.L.), the National Science Foundation and National Geographic Society (Z.-X.L.), the Ministry of Science and Technology of China (the 973 Project under C.-S. Wang) and the State Key Laboratory of Palaeobiology and Stratigraphy of NIGPAS (G. L.).

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Luo, ZX., Chen, P., Li, G. et al. A new eutriconodont mammal and evolutionary development in early mammals. Nature 446, 288–293 (2007). https://doi.org/10.1038/nature05627

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