Protopteraspidids were once thought to represent a taxon of basal pteraspidids (hence "proto") but recent evaluations demonstrate that Protopteraspididae is a paraphyletic group of various transitional forms representing a gradual transition between the more advanced Pteraspoidei (comprising Pteraspididae, Gigantaspis and Protaspididae),[1] and the anchipteraspidids and the Psammosteids.

Protopteraspids are found in Lower Devonian marine strata of the Western United States, Northern Canada, England, France, Belgium, Svalbard, Norway, and Podolia, Ukraine.

[2] This genus is known from several species from Lower Devonian England, France and Belgium This is a genus of aberrant pteraspidids with lower lips elongated into long, dagger-like organs of unknown function.

"Trygonaspis" is a nomen nudum given to a well-preserved complete armor that strongly resembles Protopteraspis in form, but with a long, recurved dorsal spine, and orbital plates that have bookshelf-like extensions.

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