Protaspididae is an extinct family of pteraspidid heterostracan agnathans. Fossils of the various genera are found in early Devonian-aged marine strata. Protaspidids were once thought to represent a transitional form between the Pteraspididae and the Psammosteida, bearing the broad head shield shape of the latter, due to a more benthic (bottom-dwelling) existence, but recent phylogenical comparisons demonstrate that the protaspidids are actually highly derived pteraspidids, and that the anchipteraspidids, the most primitive of pteraspidids, are the sister-group of the Psammosteids. Genera Many genera are found in Lower Devonian marine deposits of the United States, especially of Utah and Wyoming. Others are found in Lower Devonian marine strata of Europe, especially of Svalbard, Norway, and Podolia, Ukraine. Protaspis This genus is known from several species from the Early Devonian-aged Beartooth Butte Formation strata of Wyoming and Utah. Europrotaspis, Cosmaspis, Lampraspis, Gigantaspis, Eucyclaspis, and Cyrtaspidithchys have been formerly placed within Protaspis as subgenera, or otherwise have been treated as species therein. Europrotaspis Species of this genus are very similar to those of Protaspis, but, are found in Early Devonian-aged strata of Western Europe and Podolia, Ukraine. Eucyclaspis Species of Eucyclaspis are found in marine strata of the Water Canyon Formation, in Lower Devonian Utah. This genus differs from Protaspis by having unique processes emanating behind the eye on the orbital plate. Cosmaspis Cosmaspis is restricted to marine environments of Early Devonian Wyoming, and possibly of Utah, as well, where it coexisted with species of Protaspis, Lampraspis, Eucyclaspis, Oreaspis and Cyrtaspidichthys. Lampraspis Lampraspis is known from Early Devonian-aged marine environments of Utah and Wyoming. It is closely related to Oreaspis and Psephaspis, and the three genera were, at one time, placed together in a subfamily, "Oreaspidinae." Cyrtaspidichthys Species of Cyrtaspidichthys, originally "Cyrtaspis," are very similar to those of Protaspis. Fossils of Cyrtaspidichthys are found in Early Devonian-aged marine strata of Beartooth Butte, Wyoming. Oreaspis Oreaspis differs from Protaspis in ornamentation, and that the postbranchial plate (the plate posterior to the exhalant opening of the animal's gills) forms a tiny, wing-like process. Species of Oreaspis are found in Early Devonian marine strata of Water Canyon, Utah, and Beartooth Butte of Wyoming. At one time, this genus, together with Lampraspis and Psephaspis, was placed within its own subfamily, "Oreaspidinae." Psephaspis Species of Psephaspis, which are restricted to Idaho and Utah, have broad, flat, rounded and heart-shaped dorsal shields, and ornamentation similar to those of psammosteids.