Galeaspida (from

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

, 'Helmet shields') is an extinct taxon of jawless marine and freshwater

fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...

. 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 The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...

and

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

times (430 to 370 million years ago) in what is now Southern

China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...

Tibet Tibet (; ''Böd''; ), or Greater Tibet, is a region in the western part of East Asia, covering much of the Tibetan Plateau and spanning about . It is the homeland of the Tibetan people. Also resident on the plateau are other ethnic groups s ...

and

Vietnam Vietnam, officially the Socialist Republic of Vietnam (SRV), is a country at the eastern edge of mainland Southeast Asia, with an area of about and a population of over 100 million, making it the world's List of countries and depende ...

. Superficially, their

morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines *Morphology (archaeology), study of the shapes or forms of artifacts *Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies, ...

appears more similar to that of

Heterostraci Heterostraci (Ancient Greek, ἕτερος+ὄστρακον "those

ith The Ith () is a ridge in Germany's Central Uplands which is up to 439 m high. It lies about 40 km southwest of Hanover and, at 22 kilometers, is the longest line of crags in North Germany. Geography Location The Ith is i ...

a different shell" i is pl. of -us is an extinct subclass (biology), subclass of Pteraspidomorphi, pteraspidomorph, ostracoderm, jawless vertebrate that lived prima ...

than

Osteostraci The class Osteostraci (meaning "bony shells") is an extinct taxon of bony-armored jawless fish, termed " ostracoderms", that lived in what is now North America, Europe and Russia from the Middle Silurian to Late Devonian. Anatomically speaking, ...

, 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 In human anatomy, the neurocranium, also known as the braincase, brainpan, brain-pan, or brainbox, is the upper and back part of the skull, which forms a protective case around the brain. In the human skull, the neurocranium includes the calv ...

Morphology

The defining characteristic of all galeaspids was a large opening on the

dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage The fus ...

surface of the head shield, which was connected to the

pharynx The pharynx (: pharynges) is the part of the throat behind the human mouth, mouth and nasal cavity, and above the esophagus and trachea (the tubes going down to the stomach and the lungs respectively). It is found in vertebrates and invertebrates ...

and gill chamber, and a scalloped pattern of the sensory-lines. The opening appears to have served both the

olfaction The sense of smell, or olfaction, is the special sense through which smells (or odors) are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste. In humans, ...

and the intake of the respiratory water similar to the

nasopharyngeal The pharynx (: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the esophagus and trachea (the tubes going down to the stomach and the lungs respectively). It is found in vertebrates and invertebrates, though its ...

duct of

hagfish Hagfish, of the Class (biology), class Myxini (also known as Hyperotreti) and Order (biology), order Myxiniformes , are eel-shaped Agnatha, jawless fish (occasionally called slime eels). Hagfish are the only known living Animal, animals that h ...

es.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 Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

which have the largest number of

gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...

s, as some species of the

order Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood ...

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 Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported only ...

. The mouth and gill openings are situated on the

ventral Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

side of the head, which is flat or flattened and suggests that they were bottom-dwellers.

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

Eugaleaspidiformes Eugaleaspidiformes (from Latin, "Helmet shield shapes") is an extinct order of jawless marine and freshwater fish, which lived in East Asia East Asia is a geocultural region of Asia. It includes China, Japan, Mongolia, North Korea, South Ko ...

, which comprises the genera '' Sinogaleaspis'', '' Meishanaspis'', and '' Anjianspis'', and the

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

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

to form the class Monorhina.

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 In the geological timescale, the Ludlow Epoch (from 427.4 ± 0.5 million years ago to 423.0 ± 2.3 million years ago) occurred during the Silurian Period, after the end of the Homerian Age. It is named for the town of Ludlow in Shropshire, Engl ...

, 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

. 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 The Lochkovian is one of three faunal stages in the Early Devonian Epoch. It lasted from 419.2 ± 3.2 million years ago to 410.8 ± 2.8 million years ago. It marked the beginning of the Devonian Period, and was followed by the Pragian Stage. It is ...

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 The Emsian is one of three faunal stages in the Early Devonian Epoch. It lasted from 410.62 ±1.95 million years ago to 393.47 ±0.99 million years ago. It was preceded by the Pragian Stage and followed by the Eifelian Stage. It is named after ...

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 Placoderms (from Ancient Greek πλάξ 'plax'', ''plakos'''Plate (animal anatomy), plate' and δέρμα 'derma'''skin') are vertebrate animals of the class (biology), class Placodermi, an extinct group of prehistoric fish known from Pal ...

'' Remigolepis'', in strata from the Northern Chinese province of

Ningxia Ningxia, officially the Ningxia Hui Autonomous Region, is an autonomous region in Northwestern China. Formerly a province, Ningxia was incorporated into Gansu in 1954 but was later separated from Gansu in 1958 and reconstituted as an autonomous ...

Taxa

*'' Jiaoyu'' * Family Hanyangaspididae Pan & Liu 1975 **' Pan & Liu 1975 ** '' Nanjiangaspis'' Wang et al. 2002 ** '' Kalpinolepis'' Wang, Wang & Zhu 1996 ** '' Konoceras'' Pan 1992 ** '' Hongshanaspis'' **'' Latirostraspis'' Wang, Xia & Chen 1980 * Family Xiushuiaspididae Pan & Liu 1975 **'' Changxingaspis'' Wan 1991 ** '' Microphymaspis'' Wang et al. 2002 ** '' Xiushuiaspis'' Pan & Wang 1983 ** '' Xiyuaspis'' * Family Dayongaspididae Pan & Zen 1985 **'' Dayongaspis'' Pan & Zen 1985 ** ' Wang et al. 2002 * Order

Liu 1965 ** '' Yongdongaspis'' Chen et al., 2022 ** ' ** Family Shuyuidae Shan et al. 2020 *** '' Meishanaspis'' Wang 1991 *** '' Shuyu'' Gai et al. 2011 *** '' Qingshuiaspis'' *** ' ** Family Tridenaspididae Liu 1986 *** ' Zhu 1992 *** '' Tridenaspis'' Liu 1986 *** ' ** Family Sinogaleaspidae Pan & Wang 1980 *** '' Anjiaspis'' Gai & Zhu 2005 *** '' Sinogaleaspis'' Pan & Wang 1980 *** ' Shan et al. 2020 ** Family Eugaleaspididae Liu 1980 *** ?'' Liuaspis'' Whitley 1976 non Borchsenius 1960 *** '' Dunyu'' Zhu et al. 2012 *** '' Eugaleaspis'' Liu 1965 /big>''Galeaspis'' Liu 1965 non Ivshin ex Borukaev 1955">Galeaspis.html" ;"title="/big>''Galeaspis">/big>''Galeaspis'' Liu 1965 non Ivshin ex Borukaev 1955/small> *** ''Nochelaspis'' Zhu 1992 *** ''Pseudoduyunaspis'' Wang, Wang & Zhu 1996 *** ''Yunnanogaleaspis'' Pan & Wang 1980 *** ''Xitunaspis'' * Superorder Polybranchiaspidida Liu 1965 ** Order Polybranchiaspidiformes Janvier 1996 ***Family Gumuaspididae Gai et al. 2018 ****'' Gumuaspis'' Wang & Wang 1992 ****'' Platylomaspis'' Gai et al. 2018 ****' Gai et al. 2018 ****'' Laxaspis'' Liu 1975 ****'' Pseudolaxaspis'' *** Family Geraspididae Pan & Chen 1993 ****'' Geraspis'' Pan & Chen 1993 ****'' Kwangnanaspis'' Cao 1979 *** Family Pentathyraspididae Pan 1992 ****'' Pentathyraspis'' Pan 1992 ****'' Microhoplonaspis'' Pan 1992 *** Family Duyunolepididae Pan & Wang 1978a ****' Pan & Wang 1982 **** '' Pseudoduyunolepis'' **** '' Paraduyunolepis'' Pan & Wang 1978 **** '' Neoduyunolepis'' Pan & Wang 1978 **** '' Lopadaspis'' Wang et al. 2002 **** '' Foxaspis'' ***Family Hyperaspididae Pan 1992 ****'' Hyperaspis'' Pan 1992 ***Family Polybranchiaspididae Liu 1965 ****'' Altigibbaspis'' Liu, Gai & Zhu 2017 ****'' Polybranchiaspis'' Liu 1965 ****'' Bannhuanaspis'' Janvier, Than & Phuon 1993 ****'' Clororbis'' Pan & Ji 1993 ****'' Dongfangaspis'' Liu 1975 ****'' Siyingia'' Wang & Wang 1982 ****'' Diandongaspis'' Liu 1975 ****'' Damaspis'' Wang & Wang 1982 ****'' Cyclodiscaspis'' Liu 1975 **Order Huananaspidiformes Janvier 1975 *** Family Sanqiaspididae Liu 1975 ****'' Sanqiaspis'' Liu 1975 *** Family Zhaotongaspididae Wang & Zhu 1994 ****'' Zhaotongaspis'' Wang & Zhu 1994 ****'' Wenshanaspis'' Zhao, Zhu & Jia 2002 *** Family Sanchaspididae Pan & Wang 1981 ****' Pan & Wanao 1981 (not to be confused with ''Sanqiaspis'') ****'' Antiquisagittaspis'' Liu 1985 *** Family Gantarostrataspididae Wang & Wang 1992 ****'' Gantarostrataspis'' Wang & Wang 1992 ****'' Wumengshanaspis'' Wang & Lan 1984 ****' Gai et al. 2015 ****' *** Family Huananaspidae Liu 1973 ****'' Huanaspis'' Liu 1973 (sister-taxon of Macrothyraspinae) ****'' Asiaspis'' Pan ex Pan, Wang & Liu 1975 ****' Liu 1965 ****'' Stephaspis'' Gai & Zhu 2007 **** Subfamily Macrothyraspinae *****'' Macrothyraspis'' Pan 1992 *****'' Lungmenshanaspis'' Pan & Wang 1975 *****'' Qingmenaspis'' Pan & Wang 1981 *****'' Sinoszechuanaspis'' Pan & Wang 1975

References

* Pan Jiang, "New Galeaspids (Agnatha) From the Silurian and Devonian of China In English" 1992, * Janvier, Philippe. ''Early Vertebrates'' Oxford, New York: Oxford University Press, 1998. * Long, John A. ''The Rise of Fishes: 500 Million Years of Evolution'' Baltimore: The Johns Hopkins University Press, 1996. *Zhu Min, Gai Zhikun. "Phylogenetic relationships of Galeaspids (Agnatha)" 2007 :Higher Education Press and Springer-Verlag 2007 * *

External links