Galeaspid anatomy and the origin of vertebrate paired appendages

• Published in: Nature (London) Vol. 609; no. 7929; pp. 959 - 963
• Main Authors: Gai, Zhikun, Li, Qiang, Ferrón, Humberto G., Keating, Joseph N., Wang, Junqing, Donoghue, Philip C. J., Zhu, Min
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 29.09.2022; Nature Publishing Group
• Subjects: 631/181/2806; 631/181/414; 631/601/1332; Animal Fins - anatomy & histology; Animals; Appendages; Biological Evolution; China; Extinction; Fins; Fossils; Humanities and Social Sciences; Jaw - anatomy & histology; multidisciplinary; Phylogeny; Science; Science (multidisciplinary); Silurian; Small mammals; Steering; Stems; Vertebrates; Vertebrates - anatomy & histology; China; Hunan China
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/s41586-022-04897-6

Paired fins are a major innovation 1 , 2 that evolved in the jawed vertebrate lineage after divergence from living jawless vertebrates 3 . Extinct jawless armoured stem gnathostomes show a diversity of paired body-wall extensions, ranging from skeletal processes to simple flaps 4 . By contrast, osteostracans (a sister group to jawed vertebrates) are interpreted to have the first true paired appendages in a pectoral position, with pelvic appendages evolving later in association with jaws 5 . Here we show, on the basis of articulated remains of Tujiaaspis vividus from the Silurian period of China, that galeaspids (a sister group to both osteostracans and jawed vertebrates) possessed three unpaired dorsal fins, an approximately symmetrical hypochordal tail and a pair of continuous, branchial-to-caudal ventrolateral fins. The ventrolateral fins are similar to paired fin flaps in other stem gnathostomes, and specifically to the ventrolateral ridges of cephalaspid osteostracans that also possess differentiated pectoral fins. The ventrolateral fins are compatible with aspects of the fin-fold hypothesis for the origin of vertebrate paired appendages 6 – 10 . Galeaspids have a precursor condition to osteostracans and jawed vertebrates in which paired fins arose initially as continuous pectoral–pelvic lateral fins that our computed fluid-dynamics experiments show passively generated lift. Only later in the stem lineage to osteostracans and jawed vertebrates did pectoral fins differentiate anteriorly. This later differentiation was followed by restriction of the remaining field of fin competence to a pelvic position, facilitating active propulsion and steering. Articulated remains of Tujiaaspis vividus reveal that galeaspids—extinct jawless vertebrates—had precursors to paired pectoral fins that consisted of paired, continuous pectoral–pelvic lateral fins that passively generated lift.