Wuttagoonaspis fletcheri (Ritchie 1973; Middle Devonian) was originally considered 'an unusual arthrodire' from Australia. Here it nests basal to the heterostracan clade. The mouth and nares must be ventral because they are not visible dorsally.

Kalanaspis delectabilis (Tinn et al. 2021, Early Silurian) is the earliest known osteostracan and splits the time gap between moe derived Early Cambrian Haikouichthys and more primitive Early Devonian Hemicyclaspis. This fish was losing armor found in predecessor taxa. The dorsal naris had split in two and maybe four. The nares were originally misidentified as dorsal orbits.

Hemicyclaspis murchisoni (Sepkoski 2002; Early Devonian, 420mya; 30cm) is a traditional member of the Osteostraci. The gill chamber is below the eyes. The gills and mouth open on the ventral side. The eyes are close together on top of the skull. Birkenia was ancestral.

Zenaspis (above) is a related taxon. Bone colors are best guesses.