Galeaspida

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Galeaspida
Temporal range: Llandovery[1]–Early Devonian, 439–400 Ma
Galeaspida 1.JPG
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertebrata
Class: †Galeaspida
Liu, 1965
Orders

Liu, 1965
Janvier, 1975
Janvier, 1996

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. However, Galeaspida are in fact regarded as being more closely related to Osteostraci, based on the closer similarity of the morphology of the braincase.

Morphology[]

Their mouth and gill openings are situated on the ventral surface of the head (top right). In the most primitive forms, such as the Silurian genus (top), 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 (middle) and (bottom right), the headshield is produced laterally and anteriorly into slender processes. The , such as (bottom left) have a horseshoe-shaped headshield and a slit-shaped median dorsal opening, which imitates the aspect of the headshield of osteostracans.
Headshield of Nochelaspis
Headshield of

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 , 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.[2] Galeaspids are also the vertebrates which have the largest number of gills, as some species of the order (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 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 and can be ignored as basal galeaspids, the rest of Galeaspida can be sorted into two main groups: the first being the order , which comprises the genera , , and , and the family , and the second being the Supraorder , which comprises the order , which is the sister taxon of the family and the order , and the family , 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 .

Fossil Record[]

The oldest known galeaspids, such as those of the genera and , 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 . The last of the Eugaleaspididae disappeared by the end of the Pragian Epoch of the Lower Devonian.

The first genus of Geraspididae, the eponymous , 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 and , survive, with most others already extinct. The last galeaspid is an as yet undescribed species and genus from the 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[]

Restoration of , , and
  • Family Pan & Liu 1975
    • Pan & Liu 1975
    • Wang et al. 2002
    • Wang, Wang & Zhu 1996
    • Pan 1992
    • Wang, Xia & Chen 1980
  • Family Pan & Liu 1975
    • Wan 1991
    • Wang et al. 2002
    • Pan & Wang 1983
  • Family Pan & Zen 1985
    • Pan & Zen 1985
    • Wang et al. 2002
  • Order Liu 1965
    • Dunyu Zhu et al. 2012
    • Wang 1991 [Shuyu Gai et al. 2011]
    • Family Liu 1986
      • Liu 1986
    • Family Pan & Wang 1980
      • Pan & Wang 1980
    • Family Liu 1980
      • ? Gai & Zhu 2005
      • ? Whitley 1976 non Borchsenius 1960
      • Liu 1965 [Galeaspis Liu 1965 non Ivshin ex Borukaev 1955]
      • Nochelaspis Zhu 1992
      • Wang, Wang & Zhu 1996
      • Zhu 1992
      • Pan & Wang 1980
  • Super order Liu 1965
    • Family Wang & Zhu 1994
    • Order Janvier 1996
      • Family Gai et al. 2018
        • Wang & Wang 1992
        • Gai et al. 2018
        • Gai et al. 2018
        • Liu 1975
      • Family Pan & Chen 1993
        • Pan & Chen 1993
        • Cao 1979
      • Family Pan 1992
        • Pan 1992
        • Pan 1992
      • Family Pan & Wang 1978a
        • Pan & Wang 1982
        • Pan & Wang 1978
        • Pan & Wang 1978
        • Wang et al. 2002
      • Family Pan 1992
        • Pan 1992
      • Family Liu 1965
        • Liu, Gai & Zhu 2017
        • Liu 1965
        • Janvier, Than & Phuon1993
        • Pan & Ji 1993
        • Liu 1975
        • Wang & Wang 1982
        • Liu 1975
        • Wang & Wang 1982
        • Liu 1975
    • Order Janvier 1975
      • Family Liu 1975
        • Liu 1975
      • Family Wang & Zhu 1994
        • Wang & Zhu 1994
        • Zhao, Zhu & Jia 2002
      • Family Pan & Wang 1981
        • Pan & Wanao 1981 (not to be confused with Sanqiaspis)
        • Liu 1985
      • Family Wang & Wang 1992
        • Wang & Wang 1992
        • Gai et al. 2015
      • Family Liu 1973
        • Liu 1973 (sister-taxon of Macrothyraspinae)
        • Pan ex Pan, Wang & Liu 1975
        • Liu 1965
        • Gai & Zhu 2007
        • Subfamily
          • Pan 1992
          • Pan & Wang 1975
          • Pan & Wang 1981
          • Pan & Wang 1975

See also[]

References[]

  1. ^ Sansom, Robert S.; Randle, Emma; Donoghue, Philip C. J. (February 7, 2015). "Discriminating signal from noise in the fossil record of early vertebrates reveals cryptic evolutionary history". Proceedings of the Royal Society B. 282 (1800): 20142245. doi:10.1098/rspb.2014.2245. PMC 4298210. PMID 25520359.
  2. ^ 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.
  • Pan Jiang, "New Galeaspids (Agnatha) From the Silurian and Devonian of China In English" 1992, ISBN 7-116-01025-4
  • Janvier, Philippe. Early Vertebrates Oxford, New York: Oxford University Press, 1998. ISBN 0-19-854047-7
  • Long, John A. The Rise of Fishes: 500 Million Years of Evolution Baltimore: The Johns Hopkins University Press, 1996. ISBN 0-8018-5438-5
  • Zhu Min, Gai Zhikun. "Phylogenetic relationships of Galeaspids (Agnatha)" 2007 :Higher Education Press and Springer-Verlag 2007
  • Nelson, Joseph S.; Terry C. Grande; Mark V. H. Wilson (2016). Fishes of the World (5th ed.). John Wiley & Sons. ISBN 9781118342336.
  • "Fish classification 2017". mayatan.web.fc2.com. Retrieved 2018-12-27.

External links[]

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