Galeaspid anatomy and the origin of vertebrate paired appendages

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 sistergroup 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.

Automated summary

Paired fins are a significant innovation that evolved in jawed vertebrates after diverging from jawless vertebrates. Extinct jawless armoured stem gnathostomes had various paired body-wall extensions, while osteostracans (sister to jawed vertebrates) are believed to have had the first true paired appendages in a pectoral position. The study shows that galeaspids (sister group to both osteostracans and jawed vertebrates) had three unpaired dorsal fins, a symmetrical hypochordal tail, and a pair of continuous ventrolateral fins. The ventrolateral fins are similar to paired fin flaps in other stem gnathostomes, supporting the fin-fold hypothesis for the origin of vertebrate paired appendages.