Ontogeny is of paramount importance to understand evolutionary relationships of organisms. However, in the fossil record, early developmental stages are rarely preserved because of their unmineralized skeleton. Here, we describe the ontogeny of the Devonian jawless fish, Euphanerops longaevus, based on observations from more than 3500 specimens. The ontogeny of Euphanerops is compared with the development of the median fins of the extant jawless fish, the sea lamprey (Petromyzon marinus). From a subsample of 216 specimens of Euphanerops, we define three ontogenetic stages: larvae (2–38 mm total length (TL), mostly composed of “Scaumenella mesacanthi”), juveniles (28–98 mm TL), and adults (≥90 mm TL) based on the degree of skeletal development, chemical and microscopic compositions. Larvae display three cranial plates, a simple branchial apparatus, notochordal elements, and caudal fin supports. In juveniles, we document the development of paired anteroventral, anal, and median dorsal fins. Given how little information is generally available on ostracoderm ontogeny, the growth series of Euphanerops provides an unparalleled opportunity to understand the development of early vertebrate characters.

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