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.

1. Carnegie Museum of Natural History, Pittsburgh, Pennsylvania 15213, USA

Correspondence to: Zhe-Xi Luo¹ Correspondence and requests for materials should be addressed to Z.-X.L. (Email: luoz@carnegiemnh.org).

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