Galeaspida
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| Galeaspida Fossil range: Mid Silurian to Early Devonian |
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Galeaspida ("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 the Silurian and Devonian times (430 to 370 million years ago) of what is now China and Vietnam. Their morphology is superficially similar to that of the Heterostraci more than the Osteostraci, 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 the braincase 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 and gill chamber. It may have served both the olfaction and the intake of the respiratory water similar to the nasopharyngeal duct of hagfishes. 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 side of the head, which is flat and suggests that they were bottom-dwellers.
There are about seventy species of Galeaspids divided into Eugaleaspidiformes and Polybranchiaspidida.
[edit] 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 0-8018-5438-5
[edit] See also
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