A new interpretation of Pikaia reveals the origins of the chordate body plan
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2024-06-11
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2024-03-07
Subject Terms
pikaia
chordate evolution
deuterostome origins
vertebrate origins
vetulicolians
Yunnanozoon
amphioxus
chordate nervous systems
Burgess Shale
chordate evolution
deuterostome origins
vertebrate origins
vetulicolians
Yunnanozoon
amphioxus
chordate nervous systems
Burgess Shale
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Abstract
Our understanding of the evolutionary origin of Chordata, one of the most disparate and ecologically significant animal phyla, is hindered by a lack of unambiguous stem-group relatives. Problematic Cambrian fossils that have been considered as candidate chordates include vetulicolians (1) Yunnanozoon (2) and the iconic Pikaia (3) However, their phylogenetic placement has remained poorly constrained, impeding reconstructions of character evolution along the chordate stem lineage. Here we reinterpret the morphology of Pikaia, providing evidence for a gut canal and, crucially, a dorsal nerve cord—a robust chordate synapomorphy. The identification of these structures underpins a new anatomical model of Pikaia that shows that this fossil was previously interpreted upside down.We reveal a myomere configuration intermediate between amphioxus and vertebrates and establish morphological links between Yunnanozoon, Pikaia, and uncontroversial chordates. In this light, we perform a new phylogenetic analysis, using a revised, comprehensive deuterostome dataset, and establish a chordate stem lineage. We resolve vetulicolians as a paraphyletic group comprising the earliest diverging stem chordates, subtending a grade of more derived stem-group chordates comprising Yunnanozoon and Pikaia. Our phylogenetic results reveal the stepwise acquisition of characters diagnostic of the chordate crown group. In addition, they chart a phase in early chordate evolution defined by the gradual integration of the pharyngeal region with a segmented axial musculature, supporting classical evolutionary-developmental hypotheses of chordate origins4 and revealing a ‘‘lost chapter’’ in the history of the phylum.
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Mussini et al., A new interpretation of Pikaia reveals the origins of the chordate body plan, Current Biology (2024), https://doi.org/10.1016/j.cub.2024.05.026
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Copyright © 2024 Published by Elsevier Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). The linked document is the published version of the article.
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0960-9822