The name often used in older textbooks for the fossil, armoured, jawless, agnathan fish of the Ordovician to early Carboniferous Periods. Probably the name indicates only a grade of development; some ostracoderms are close to the line of ancestry of the living Agnatha (lamprey and hagfish), others to the origin of the jawed vertebrates. The informal version of the name survives. See also Cephalaspidomorphi; Pteraspidomorphi.
Subjects: Zoology and Animal Sciences — Earth Sciences and Geography.