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A Jurassic mammaliaform and the earliest mammalian evolutionary adaptations

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A Corrigendum to this article was published on 04 December 2013

Abstract

The earliest evolution of mammals and origins of mammalian features can be traced to the mammaliaforms of the Triassic and Jurassic periods that are extinct relatives to living mammals. Here we describe a new fossil from the Middle Jurassic that has a mandibular middle ear, a gradational transition of thoracolumbar vertebrae and primitive ankle features, but highly derived molars with a high crown and multiple roots that are partially fused. The upper molars have longitudinal cusp rows that occlude alternately with those of the lower molars. This specialization for masticating plants indicates that herbivory evolved among mammaliaforms, before the rise of crown mammals. The new species shares the distinctive dental features of the eleutherodontid clade, previously represented only by isolated teeth despite its extensive geographic distribution during the Jurassic. This eleutherodontid was terrestrial and had ambulatory gaits, analogous to extant terrestrial mammals such as armadillos or rock hyrax. Its fur corroborates that mammalian integument had originated well before the common ancestor of living mammals.

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Figure 1: New Jurassic mammaliaform Megaconus mammaliaformis.
Figure 2: Dental, mandibular and ear structures of Megaconus and comparative taxa.
Figure 3: Comparison of the hindlimb and pes of Megaconus and other mammaliaforms.
Figure 4: Cynodont–mammal transition and evolution of mammal-like postcanines with multicusped rows that occlude alternately between uppers and lowers, for omnivory–herbivory feeding adaptations.

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Acknowledgements

We thank G. Sun for proposing this research, and for his encouragement and support throughout; O. Dülfer for preparation of the fossil; P. Göddertz and K. Jäger for their CT scanning and virtual reconstructions; G. Oleschinski for SEM photography; and A. Isch for graphics support. We benefited from discussion with K. C. Beard, W. A. Clemens, M. R. Dawson, D. Y. Hu, W. v. Koenigswald, G. Sun and J.R. Wible. J. R. Wible and M. R. Dawson helped to improve the manuscript. Support was from the Key Lab for Paleobiological Evolution of Northeastern Asia, Ministry of Land Resources of China, and Shenyang Normal University and Paleontological Museum of Liaoning (C.Z. and S.W.), Deutsche Forschungsmeinschaft (DFG) (T.M.), Alexander von Humboldt-Foundation, National Science Foundation and the University of Chicago (Z.-X.L.). This is publication no. 60 from DFG Research Unit 771, Universität Bonn.

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Authors and Affiliations

Authors

Contributions

C.-F.Z. authenticated the fossil and provided geological data; T.M. organized preparation, CT scan and SEM photography; T.M. and Z.-X.L. performed phylogenetic analyses; S.W., T.M. and Z.-X.L. participated in fossil morphological work and contributed to interpreting the fossil; T.M. and Z.-X.L. developed the manuscript with contributions from all authors.

Corresponding author

Correspondence to Zhe-Xi Luo.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Information

This file contains Supplementary Information A-L (see Table of Contents for details). Please note that Supplementary Figures 1-10 and Supplementary Video 1 are in separate files. This file was replaced on 4 December 2013 to correct the caption for fig S2. (PDF 3844 kb)

Supplementary Figures

This file contains Supplementary Figures 1-10 (see Supplementary Information file for extended figure legends). This file was replaced on 4th December 2013 to correct figures s2 and s3. (PDF 28745 kb)

Virtual Reconstruction from CT scanning of the teeth of Megaconus mammaliaformis. Upper teeth P2-M3

Video of virtual reconstruction from the micro-CT scanning and the animation before and after the correction of the fracture through M1. Lower teeth m1-m2: video of virtual reconstruction from the micro CT scanning and the animation of the correction for the taphonomic compression of lower m1 and m2. See Supplementary Information file for Technical details on CT scanning parameters and virtual reconstruction software. (MP4 5855 kb)

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Zhou, CF., Wu, S., Martin, T. et al. A Jurassic mammaliaform and the earliest mammalian evolutionary adaptations. Nature 500, 163–167 (2013). https://doi.org/10.1038/nature12429

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