Osteostracans are Paleozoic jawless vertebrates, likely the sister group to gnathostomes (jawed vertebrates). It was previously believed that the osteostracan dermoskeleton began ossification when the individual neared adult size, restricting further growth once a dermoskeletal covering was achieved. However, we describe here small, well-preserved articulated specimens of the osteostracan Superciliaspis gabrielsei. These specimens are the best-known juvenile osteostracans, and document dermoskeletal development at an evolutionary stage highly relevant to the origin of gnathostomes. Ossification began before the animal reached 20% of adult size. Bones grew by multiple means, including initiation of new bone units, marginal growth, and fusion into larger bones. In the head, paired elements were among the first to ossify. In the trunk dermoskeleton of juvenile heterostracans, bones developed first along the midline, but in Superciliaspis gabrielsei, a lateral to median polarity is seen, with ventrolateral ridge scales the first to appear.