Galeaspida

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 opening appears to have served both the olfaction and the intake of the respiratory water similar to the nasopharyngeal duct of hagfishes.^[3] Galeaspids are also the vertebrates which have the largest number of gills, as some species of the order Polybranchiaspidida (literally "many gills shields") had up to 45 gill openings. The body is covered with minute scales arranged in oblique rows and there is no other fin besides the caudal fin.

The mouth and gill openings are situated on the ventral surface of the head, which is flat or flattened and suggests that they were bottom-dwellers. In the most primitive forms, such as the Silurian genus Hanyangaspis, the median dorsal inhalent opening is broad and situated anteriorly. In other galeaspids, it is more posterior in position and can be oval, rounded, heart-shaped or slit-shaped. In some Devonian galeaspids, such as the hunanaspidiforms Lungmenshanaspis and Sanchaspis, the headshield is produced laterally and anteriorly into slender processes. The eugaleaspidiforms, such as Eugaleaspis have a horseshoe-shaped headshield and a slit-shaped median dorsal opening, which imitates the aspect of the headshield of osteostracans.

There are around 76 + described species of galeaspids in at least 53 genera.

If the families Hanyangaspidae and Xiushuiaspidae can be ignored as basal galeaspids, the rest of Galeaspida can be sorted into two main groups: the first being the order Eugaleaspidiformes, which comprises the genera Sinogaleaspis, Meishanaspis, and Anjianspis, and the family Eugaleaspididae, and the second being the Supraorder Polybranchiaspidida, which comprises the order Polybranchiaspidiformes, which is the sister taxon of the family Zhaotongaspididae and the order Huananaspidiformes, and the family Geraspididae, which is the sister taxon of Polybranchiaspidiformes + Zhaotongaspididae + Huananaspidiformes.

Some experts demote Galeaspida to the rank of subclass, and unite it with Pituriaspida and Osteostraci to form the class Monorhina.

Phylogeny

An analysis by Zhang and colleagues in 2026 obtained the following phylogenetic tree by strict consensus of 6 maximum parsimony trees:^[4]

Agnatha

Ateleaspis Galeaspida Hongshanaspis Hanyangaspidae Hanyangaspis Deanaspidae Deanaspis Xiushuiaspidae Xiushuiaspis Changxingaspis Xiyuichthys Microphymaspis Dayongaspidae Xihaiaspis Dayongaspis Eugaleaspidiformes ⊞ ⊟ Tujiaaspidae Miaojiaaspis Tujiaaspis Shuyuidae Shuyu Meishanaspis Qingshuiaspis Jiangxialepis Anjiaspis Sinogaleaspidae Sinogaleaspis Rumporostralis Yongdongaspidae Yongdongaspis Yunnanogaleaspis Nochelaspis Tridensaspidae Falxcornus Tridensaspis Pterogonaspis Eugaleaspidae Eugaleaspis Xitunaspis Dunyu Polybranchiaspidida Geraspidae Geraspis Kwangnanaspis Polybranchiaspiformes ⊞ ⊟ Gumuaspidae Gumuaspis Platylomaspis Nanningaspis Asioaspidae Asioaspis Pentathyraspidae Pentathyraspis Microhoplonaspis Duyunolepidae Lopadaspis Foxaspis Duyunolepis Polybranchiaspidae Dongfangaspis Bannhuanaspis Polybranchiaspis Altigibbaspis Laxaspis Damaspis Cyclodiscaspis Zhaotongaspidae Zhaotongaspis Wenshanaspis Huananaspiformes ⊞ ⊟ Sanchaspidae Sanchaspis Antiquisagittaspis Gantarostrataspidae Qushiaspis Gantarostrataspis Wumengshanaspis Rhegmaspis Sanqiaspidae Sanqiaspis Huananaspidae Nanpanaspinae Asiaspis Stephaspis Nanpanaspis Huananaspinae Huananaspis Macrothyraspinae Lungmenshanaspis Qingmenaspis Sinoszechuanaspis Macrothyraspis

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. During the transition from the Llandovery to the Wenlock, the Eugaleaspids underwent a diversification event. By the time the Wenlock epoch transitioned into the Ludlow Epoch, all of the eugaleaspids, save for the Eugaleaspidae, were extinct. The Eugaleaspidae lived from the Wenlock, and were fairly long-lived, especially the genus Eugaleaspis. The last of the Eugaleaspididae disappeared by the end of the Pragian Epoch of the Lower Devonian.

The first genus of Geraspididae, the eponymous Geraspis, appears during the middle of the Telychian. The other genera of Polybranchiaspidida appear in the fossil record a little after the beginning of the Lochkovian Epoch, at the start of the Devonian. The vast majority of the supraorder's genera either date from the Pragian epoch, or have their ranges end there. By the time the Emsian epoch starts, only a few genera, such as Duyunolepis and Wumengshanaspis, survive, with most others already extinct. 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.

• Osteostraci
• Heterostraci