Transformation and diversification in early mammal evolution


Evolution of the earliest mammals shows successive episodes of diversification. Lineage-splitting in Mesozoic mammals is coupled with many independent evolutionary experiments and ecological specializations. Classic scenarios of mammalian morphological evolution tend to posit an orderly acquisition of key evolutionary innovations leading to adaptive diversification, but newly discovered fossils show that evolution of such key characters as the middle ear and the tribosphenic teeth is far more labile among Mesozoic mammals. Successive diversifications of Mesozoic mammal groups multiplied the opportunities for many dead-end lineages to iteratively evolve developmental homoplasies and convergent ecological specializations, parallel to those in modern mammal groups.

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Figure 1: Phylogeny and diversification of Mesozoic and major extant mammal groups.
Figure 2: Diverse evolutionary experiments of Mesozoic mammals and their ecological convergence to modern mammal ecomorphotypes.
Figure 3: Evolution of the mammalian cranio-mandibular joint and the definitive mammalian middle ear through the cynodont–mammal transition.
Figure 4: Convergent and iterative evolution of protocones and pseudo-protocones in Mesozoic mammals.


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I benefited from years of stimulating discussion about early mammal evolution with R. Cifelli, T. Martin, J. Wible, Z. Kielan-Jaworowska, T. Rowe, H. Sues, M. Dawson, K. C. Beard, G. Wilson, G. Rougier, J. Bonaparte, W. Maier, P.-J. Chen and Q. Ji, and discussion on diversification pattern with D. Erwin and M. Benton. Many helped my research: A. Tabrum, X.-N. Yang, Q. Yang, P.-J. Chen, Z.-M. Dong, K.-Q. Gao. I thank Q. Ji and J. R. Wible for access to comparative collections; M. R. Dawson, T. Martin and J. R. Wible for improving the manuscript; M. Klingler for assistance with graphics. Support was from the National Science Foundation (USA), National Natural Science Foundation of China, National Geographic Society and the Carnegie Museum.

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Correspondence to Zhe-Xi Luo.

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Luo, Z. Transformation and diversification in early mammal evolution. Nature 450, 1011–1019 (2007).

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