Galeaspida lived in shallow, fresh water and marine environments during the Silurian and Devonian times (430 to 370 million years ago) in what is now Southern China, Tibet and Vietnam.

[citation needed] A galeaspid Tujiaaspis vividus from the Silurian period of China was described in 2022 as having a precursor condition to the form of paired fins seen in Osteostraci and gnathostomes.

[citation needed] The defining characteristic of all galeaspids was a large opening on the dorsal surface of the head shield, which was connected to the pharynx and gill chamber, and a scalloped pattern of the sensory-lines.

The oldest known galeaspids, such as those of the genera Hanyangaspis and Dayongaspis, first appear near the start of the Telychian age, of the latter half of the Llandovery epoch of the Silurian, about 436 million years ago.

The last galeaspid is an as yet undescribed species and genus from the Fammenian epoch of the Late Devonian, found in association with the tetrapod Sinostega and the antiarch placoderm Remigolepis, in strata from the Northern Chinese province of Ningxia.