Everything about Cephalaspidomorphi totally explained
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.
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.
Lampreys
Many reference works still regard Cephalaspidomorphi as a
Linnean class whose sole living representaties are the
lampreys.
Further Information
Get more info on 'Cephalaspidomorphi'.
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