Galeaspida

Galeaspida (from Latin, 'Helmet shields') is an extinct taxon of jawless marine and freshwater fish. The name is derived from galea, the Latin word for helmet, and refers to their massive bone shield on the head. 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. Superficially, their morphology appears more similar to that of Heterostraci than Osteostraci, there being currently no evidence that the galeaspids had paired fins. 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. Earlier than this, Galeaspida were already in fact regarded as being more closely related to Osteostraci, based on the closer similarity of the morphology of the braincase.

Morphology

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.Fossil jawless fish from China foreshadows early jawed vertebrate anatomy The galeaspids are characterized by a large median dorsal opening in the anterior part of the headshield that serves as both a common nostril and the main water intake device. 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.

Taxonomy

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 Analyses by Zhang and colleagues in 2026 obtained the following phylogenetic tree by strict consensus of 6 maximum parsimony trees:

Fossil record 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.

Taxa

Genus Hyperaspis Pan, 1992 (homonym) Genus Pseudoduyunaspis Wang, Wang & Zhu, 1996 Genus Diploholcaspis Wang et al. 2009 Genus Jiaoyu Liu et al., 2019 Genus Microphymaspis Wang et al., 2002 Family Hanyangaspididae Pan & Liu, 1975 Genus Pan & Liu, 1975 Genus Nanjiangaspis Wang et al., 2002 Genus Kalpinolepis Wang, Wang & Zhu, 1996 Genus Konoceraspis Pan, 1992 Genus Hongshanaspis Yilong et al., 2021 Genus Latirostraspis Wang, Xia & Chen, 1980 Family Deanaspidae Lin et al., 2025 Genus Deanichthys Zhang et al., 2026 [Deanaspis Hughes, Ingham & Addison, 1975] Family Xiushuiaspididae Pan & Liu, 1975 Genus Changxingaspis Wan, 1991 Genus Xiushuiaspis Pan & Wang, 1983 Genus Xiyuichthys (Liu et al., 2019) Family Dayongaspidae Pan & Zen, 1985 Genus Dayongaspis Pan & Zen, 1985 Genus Platycaraspis Wang et al., 2002 Genus Xihaiaspis Zhang et al., 2026 Order Eugaleaspidiformes Liu, 1980 Genus Anjiaspis Gai & Zhu, 2005 Genus Yunnanogaleaspis Pan & Wang, 1980 Genus Nochelaspis Zhu, 1992 Family Tujiaaspidae Chen et al., 2024 Genus Miaojiaaspis Chen et al., 2024 Genus Tujiaaspis Gai et al., 2022 Family Shuyuidae Shan et al., 2020 Genus Meishanaspis Wang, 1991 Genus Shuyu Gai et al., 2011 Genus Qingshuiaspis Shan et al, 2022 Genus Liu et al., 2021 Family Sinogaleaspidae Pan & Wang, 1980 Genus Sinogaleaspis Pan & Wang, 1980 Genus Shan et al., 2020 Family Yongdongaspidae Chen et al., 2022 Genus Yongdongaspis Chen et al., 2022 Family Tridensaspidae Liu, 1986 Genus Zhu, 1992 Genus Tridensaspis Liu, 1986 Genus Meng & Gai, 2021 Genus Bataspis Meng, Ferrón, Pisani, Gai & Donoghue, 2026 Family Eugaleaspididae Liu, 1980 Genus Dunyu Zhu et al., 2012 Genus Eugaleaspis Liu, 1980 [Galeaspis Liu 1965 non Ivshin ex Borukaev 1955] Genus Xitunaspis Sun et al., 2022 Superorder Polybranchiaspidida Liu, 1965 Family Geraspididae Pan & Chen, 1993 Genus Geraspis Pan & Chen, 1993 Genus Kwangnanaspis Cao, 1979 Family Zhaotongaspididae Wang & Zhu, 1994 Genus Zhaotongaspis Wang & Zhu, 1994 Genus Wenshanaspis Zhao, Zhu & Jia, 2002 Order Polybranchiaspidiformes Janvier, 1996 Family Gumuaspididae Gai et al., 2018 Genus Gumuaspis Wang & Wang, 1992 Genus Platylomaspis Gai et al., 2018 Genus Gai et al., 2018 Genus Pseudolaxaspis Gai et al., 2018 Family Asioaspidae Zhang et al., 2026 Genus Asioaspis Zhang et al., 2026 Family Pentathyraspididae Pan, 1992 Genus Pentathyraspis Pan, 1992 Genus Microhoplonaspis Pan, 1992 Family Duyunolepididae Pan & Wang, 1978a Genus Duyunolepis Pan & Wang, 1982 Genus Paraduyunaspis P’an et Wang, 1978a Genus Neoduyunaspis P’an et Wang, 1978a Genus Lopadaspis Wang et al., 2002 Genus Foxaspis Gai et al., 2023 Family Polybranchiaspididae Liu, 1965 Genus Polybranchiaspis Liu, 1965 Genus Cyclodiscaspis Liu, 1975 Genus Diandongaspis Liu, 1975 Genus Dongfangaspis Liu, 1975 Genus Laxaspis Liu, 1975 Genus Damaspis Wang & Wang, 1982 Genus Siyingia Wang & Wang, 1982 Genus Bannhuanaspis Janvier, Than & Phuon, 1993 Genus Clororbis Pan & Ji, 1993 Genus Altigibbaspis Liu, Gai & Zhu, 2017 Order Huananaspidiformes Janvier, 1975 Family Sanchaspididae Pan & Wang, 1981 Genus Sanchaspis Pan & Wanao, 1981 Genus Antiquisagittaspis Liu, 1985 Family Gantarostrataspididae Wang & Wang, 1992 Genus Gantarostrataspis Wang & Wang, 1992 Genus Wumengshanaspis Wang & Lan, 1984 Genus Gai et al., 2015 Genus Jiang et al., 2021 Family Sanqiaspididae Liu, 1975 Genus Sanqiaspis Liu, 1975 Family Huananaspidae Liu, 1973 Subfamily Nanpanaspinae Genus Asiaspis Pan ex Pan, Wang & Liu, 1975 Genus Stephaspis Gai & Zhu, 2007 Genus Liu, 1965 Subfamily Huanaspinae Genus Huanaspis Liu, 1973 Subfamily Macrothyraspinae Genus Macrothyraspis Pan, 1992 Genus Lungmenshanaspis Pan & Wang, 1975 Genus Qingmenaspis Pan & Wang, 1981 Genus Sinoszechuanaspis Pan & Wang, 1975

Galeaspida

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