Cephalaspidomorphi
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 Reconstruction of Cephalaspis sp.
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Osteostraci |
Cephalaspidomorphs are a taxon of jawless fishes named for the cephalaspids, a group of osteostracans. Most of the members of this group are extinct; however, it interests modern biologists because it may include the lampreys. If so, the lampreys extend the known range of the group from the Silurian and Devonian periods to the present day.
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[edit] Classification
In the 1920s, the biologists Kiaer and Stensiö first recognized the Cephalaspidomorphi as including the osteostracans, anaspids, and lampreys, because all three groups share a single dorsal "nostril", now known as a nasohypophysial opening.
Since then, opinions on the relations among jawless vertebrates have varied. Most workers have come to regard the agnatha as paraphyletic, having given rise to the jawed fishes. Because of shared features such as paired fins, the origins of the jawed vertebrates may lie within the Cephalaspidomorphi. Some biologists no longer use the name Cephalaspidomorphi because relations among osteostraci and anaspida are unclear, and the affinities of the lampreys are also contested. Others have restricted the cephalaspidomorphs to include only groups more clearly related to the Osteostraci, such as Galeaspida and Pituriaspida, that were largely unknown in the 1920s.
[edit] Lampreys
Many reference works still regard Cephalaspidomorphi as a Linnean class whose sole living representaties are the lampreys.
[edit] External links
[edit] References
Janvier, Philippe. Early Vertebrates Oxford, New York: Oxford University Press, 1998. ISBN 0-19-854047-7
Stensiö, E. A. (1927). The Devonian and Downtonian vertebrates of Spitsbergen. 1. Family Cephalaspidae. Skrifter om Svalbard og Ishavet, 12, 1-391.
