Galeaspida (from

Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...

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

fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...

. 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 24.6 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 shortest period of the Paleoz ...

and

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, wh ...

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. It is the world's List of countries and dependencies by population, most populous country, with a Population of China, population exceeding 1.4 billion, slig ...

Tibet Tibet (; ''Böd''; ) is a region in East Asia, covering much of the Tibetan Plateau and spanning about . It is the traditional homeland of the Tibetan people. Also resident on the plateau are some other ethnic groups such as Monpa people, ...

and

Vietnam Vietnam or Viet Nam ( vi, Việt Nam, ), officially the Socialist Republic of Vietnam,., group="n" is a country in Southeast Asia, at the eastern edge of mainland Southeast Asia, with an area of and population of 96 million, making ...

. Superficially, their morphology appears more similar to that of

Heterostraci Heterostraci ( Ancient Greek, ἕτερος+ὄστρακον "those itha different shell" i is pl. of -us is an extinct subclass of pteraspidomorph jawless vertebrate that lived primarily in marine and estuary environments. Heterostraci ...

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, t ...

, 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, or brain-pan 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 calvaria or skul ...

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 * Dorsal c ...

surface of the head shield, which was connected to the

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

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 (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its struct ...

duct of

hagfish Hagfish, of the class Myxini (also known as Hyperotreti) and order Myxiniformes , are eel-shaped, slime-producing marine fish (occasionally called slime eels). They are the only known living animals that have a skull but no vertebral column, ...

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 () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...

which have the largest number of

gill A gill () is a respiratory organ that many 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 respiration on land provided they ar ...

s, as some species of the

order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...

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 distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as se ...

. The mouth and gill openings are situated on the

ventral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. 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, which comprises the genera '' Sinogaleaspis'', '' Meishanaspis'', and '' Anjianspis'', and the

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

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 The Pituriaspida ('Pituri-shield' or 'hallucinogen-shield') are a small group of extinct armored jawless fishes with tremendous nose-like rostrums, which lived in the marine, deltaic environments of Middle Devonian Australia (about 390 Ma). The ...

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 In the geologic timescale, the Telychian is the age of the Llandovery Epoch of the Silurian Period of the Paleozoic Era of the Phanerozoic Eon. The Telychian Age was between 438.5 ± 1.2 million years ago (Ma) and 433.4 ± 0.8 Ma. The Telych ...

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, Englan ...

, 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 The Pragian is one of three faunal stages in the Early Devonian Epoch. It lasted from 410.8 ± 2.8 million years ago to 407.6 ± 2.8 million years ago. It was preceded by the Lochkovian Stage and followed by the Emsian Stage. The most importan ...

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 407.6 ± 2.6 million years ago to 393.3 ± 1.2 million years ago. It was preceded by the Pragian Stage and followed by the Eifelian Stage. It is named after the ...

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 Ningxia (,; , ; alternately romanized as Ninghsia), officially the Ningxia Hui Autonomous Region (NHAR), is an autonomous region in the northwest of the People's Republic of China. Formerly a province, Ningxia was incorporated into Gansu in 1 ...

Taxa

* Family Hanyangaspididae Pan & Liu 1975 **'' Hanyangaspis'' Pan & Liu 1975 ** '' Nanjiangaspis'' Wang et al. 2002 ** '' Kalpinolepis'' Wang, Wang & Zhu 1996 ** '' Konoceras'' Pan 1992 **'' Latirostra'''' spis'' Wang, Xia & Chen 1980 * Family Xiushuiaspididae Pan & Liu 1975 **'' Changxingaspis'' Wan 1991 ** '' Microphymaspis'' Wang et al. 2002 ** '' Xiushuiaspis'' Pan & Wang 1983 * Family Dayongaspididae Pan & Zen 1985 **'' Dayongaspis'' Pan & Zen 1985 ** '' Platycarpaspis'' Wang et al. 2002 * Order Eugaleaspidiformes Liu 1965 ** ''

Yongdongaspis ''Yongdongaspis'', colloquially known as the Binhai Yongdong, is an extinct genus of galeaspid within the monotypic family Yongdongaspididae. The type species is ''Y. littoralis'' and it was found in the Silurian-aged Huixingshao Formation of C ...

'' Chen et al., 2022 ** Family Shuyuidae Shan et al. 2020 *** '' Meishanaspis'' Wang 1991 *** ''

Shuyu ''Shuyu'' is an extinct genus of early jawless vertebrate from the Early or Middle Silurian period (late Telychian to early Wenlock stages). It is the basalmost known eugaleaspidiform galeaspid and it lived in what is now northwestern Zhejian ...

'' Gai et al. 2011 ** Family Tridenaspididae Liu 1986 *** '' Pterogonaspis'' Zhu 1992 *** '' Tridenaspis'' Liu 1986 ** Family Sinogaleaspididae Pan & Wang 1980 *** '' Anjiaspis'' Gai & Zhu 2005 *** '' Sinogaleaspis'' Pan & Wang 1980 *** '' Rumporostralis'' Shan et al. 2020 ** Family Eugaleaspididae Liu 1980 *** ?'' Liuaspis'' Whitley 1976 non Borchsenius 1960 *** ''

Dunyu ''Dunyu'' (meaning "shield fish" in Hanyu Pinyin) is an extinct genus of eugaleaspidiform galeaspid known from the Silurian of Yunnan, southwestern China. Two species are known: ''D longiforus'', the type species and ''D. xiushanensis'', which w ...

'' Zhu et al. 2012 *** '' Eugaleaspis'' Liu 1965 /big>''Galeaspis''_Liu_1965_non_Ivshin_ex_Borukaev_1955.html" ;"title="Galeaspis.html" ;"title="/big>''Galeaspis">/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 * Super order Polybranchiaspidida Liu 1965 ** Order Polybranchiaspidiformes Janvier 1996 ***Family Gumuaspididae Gai et al. 2018 ****'' Gumuaspis'' Wang & Wang 1992 ****'' Platylomaspis'' Gai et al. 2018 ****'' Nanningaspis'' Gai et al. 2018 ****'' Laxaspis'' Liu 1975 *** 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 ****'' Duyunolepis'' Pan & Wang 1982 **** '' Pseudoduyunolepis'' **** '' Paraduyunolepis'' Pan & Wang 1978 **** '' Neoduyunolepis'' Pan & Wang 1978 **** '' Lopadaspis'' Wang et al. 2002 ***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 ****'' Sanchaspis'' 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 ****'' Rhegmaspis'' Gai et al. 2015 *** Family Huananaspidae Liu 1973 ****'' Huanaspis'' Liu 1973 (sister-taxon of Macrothyraspinae) ****'' Asiaspis'' Pan ex Pan, Wang & Liu 1975 ****'' Nanpanaspis'' 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