Large median dorsal scute lost. First canal leading from the inner ear to lateral fields branches close to the orbit. Most of them have a characteristic horseshoe-shaped head, which consists of a massive endoskeletal skull, covered with a shield of dermal bone. The mouth and gill openings are, like in the Galeaspida, situated on the ventral side of the head.
Osteostracans also have large, pad-shaped paired fins.
Most osteostracans are about 20 to 40 cm. The largest species is about one metre in length. The zenaspidid osteostracan Zenaspis pagei, from the Lower Devonian of Scotland. Most osteostracans lived in marginal, marine environments, some of them being regarded as fresh water. Their horseshoe-shaped headshield suggests a benthic mode of life, but it is possible that some of them, such as the Boreaspididae or the Kiaeraspidida were more active swimmers.