- Galeaspida
Taxobox
name = Galeaspida
fossil_range = fossil range|430|400MidSilurian to EarlyDevonian
regnum =Animal ia
phylum =Chordata
subphylum =Vertebrata
classis = GaleaspidaGaleaspida ("Helmet shields") is an extinct taxon of jawless marine and freshwater
fish . Their name is derived from a latin word for helmet, "galea", and refers to their massive bone shield on the head. Galeaspida lived in shallow, fresh water and marine environments during theSilurian andDevonian times (430 to 370 million years ago) of what is nowChina andVietnam . Their morphology is superficially similar to that of theHeterostraci more than theOsteostraci , as there is currently no evidence that the galeaspids had paired fins. However, the Galeaspida are regarded as being more closely related to the Osteostraci than Heterostraci as the morphology of thebraincase is more similar to that of Osteostraci than Heterostraci.Galeaspida had a large opening on the dorsal surface of the head shield, which was connected to the
pharynx andgill chamber . It may have served both theolfaction and the intake of the respiratory water similar to thenasopharyngeal duct ofhagfish es. Galeaspids are also thevertebrates which have the largest number ofgill s, as some species of the orderPolybranchiaspidida (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 thecaudal fin . The mouth and gill openings are situated on theventral side of the head, which is flat and suggests that they were bottom-dwellers.There are about seventy species of Galeaspids divided into
Eugaleaspidiformes andPolybranchiaspidida .References
*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 0-8018-5438-5ee also
*
Osteostraci
*Heterostraci External links
* [http://tolweb.org/tree?group=Galeaspida Galeaspida - tolweb.org]
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