Galeaspid anatomy and the origin of vertebrate paired appendages

Paired fins are a major innovation that evolved within the jawed vertebrate lineage after divergence from the living jawless vertebrates. Extinct jawless armored stem-gnathostomes exhibit a diversity of paired bodywall extensions, from skeletal processes to simple flaps. However, osteostracans (sister to jawed vertebrates) are interpreted to manifest the first true paired appendages in a pectoral position, with pelvic appendages evolving later in association with jaws. Here we show, based on articulated remains of Tujiaaspis vividus from the Silurian of China, that galeaspids (sister to osteostracans and jawed vertebrates) possessed three unpaired dorsal fins, an approximately symmetrical hypochordal tail and a pair of continuous, branchial to caudal ventro-lateral fins. The ventro-lateral fins compare to paired fin flaps in other stem-gnathostomes but specifically to the ventro-lateral ridges of cephalaspid osteostracans which also possess differentiated pectoral fins. The ventro-lateral fins as compatible with aspects of the fin-fold hypothesis for the origin of vertebrate paired appendages. Galeaspids manifest a precursor condition to osteostracans and jawed vertebrates where paired fins arose initially as continuous pectoral-pelvic lateral fins that our Computed Fluid Dynamic experiments demonstrate passively generate lift. Only later in the stem-lineage to osteostracans and jawed vertebrates did pectoral fins differentiate anteriorly. This was followed by restriction of the remaining field of fin competence to a pelvic position, facilitating active propulsion and steering.