A new interpretation of Pikaia reveals the origins of the chordate body plan.

Mussini, Giovanni; Smith, M Paul; Vinther, Jakob; Rahman, Imran A; Murdock, Duncan J E; Harper, David A T; Dunn, Frances S

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, Yunnanozoon, and the iconic Pikaia. 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 origins and revealing a "lost chapter" in the history of the phylum. [Abstract copyright: Crown Copyright © 2024. Published by Elsevier Inc. All rights reserved.]

Citation

Mussini, G., Smith, M. P., Vinther, J., Rahman, I. A., Murdock, D. J. E., Harper, D. A. T., & Dunn, F. S. (2024). A new interpretation of Pikaia reveals the origins of the chordate body plan. Current Biology, 34(13), 2980-2989. https://doi.org/10.1016/j.cub.2024.05.026

• Journal Article Type: Article
• Acceptance Date: May 14, 2024
• Online Publication Date: Jun 5, 2024
• Publication Date: Jun 5, 2024
• Deposit Date: Jul 3, 2024
• Publicly Available Date: Jul 3, 2024
• Journal: Current Biology
• Print ISSN: 0960-9822
• Electronic ISSN: 1879-0445
• Publisher: Cell Press
• Peer Reviewed: Peer Reviewed
• Volume: 34
• Issue: 13
• Article Number: S0960-9822(24)00669-9
• Pages: 2980-2989
• DOI: https://doi.org/10.1016/j.cub.2024.05.026
• Keywords: vertebrate origins, amphioxus, chordate nervous systems, Pikaia, deuterostome origins, chordate evolution, Burgess Shale, vetulicolians, Yunnanozoon
• Public URL: https://durham-repository.worktribe.com/output/2512760