Dev Dyn 2014 May 01; doi: 10.1002/dvdy.24236.

Development, Metamorphosis, Morphology and Diversity: The Evolution of Chordate muscles and the Origin of Vertebrates.

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Recent findings that urochordates are the closest sister-group of vertebrates have dramatically changed our understanding of chordate evolution and vertebrate origins. To continue to deepen understanding of chordate evolution and diversity, in particular the morphological and taxonomical diversity of the vertebrate clade, one must explore the origin, development and comparative anatomy of not only hard tissues, but also soft tissues such as muscles. Building on a recent overview of the discovery of a cardiopharyngeal field in urochordates and the profound implications for reconstructing the origin and early evolution of vertebrates, in this paper we focus on the broader comparative and developmental anatomy of chordate cephalic muscles and their relation to life history, developmental, and to morphological and taxonomical diversity. We combine our recent findings on cephalochordates, urochordates, and vertebrates with a literature review and suggest that developmental changes related to metamorphosis and/or heterochrony (e.g. peramorphosis) played a crucial role in the early evolution of chordates and vertebrates. Recent studies reviewed here supported deBeer's "law of diversity" that peramorphic animals (e.g., ascidians, lampreys) are taxonomically and morphologically less diverse than non-peramorphic animals (e.g., gnathostomes), probably because their "too specialized" development and adult anatomy constrain further developmental and evolutionary innovations. This article is protected by copyright. All rights reserved.

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