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anaspida
:''For the reptiles, see Anapsida; for the crustaceans, see Anaspididae''
Anaspida ("without shield") is an extinct group of primitive jawless vertebrates that lived primarily during the Silurian period, and went extinct soon after the start of the Devonian. They were once classically regarded as the ancestors of lampreys. Anaspids were small marine agnathans that lacked heavy bony shield and paired fins, but have a striking highly hypocercal tail. They first appeared in the early Silurian, and flourished until the early Devonian, where they then disappear from the fossil record.
==Anatomy==
Compared to other prehistoric agnathan groups, such as the Heterostraci and Osteostraci, anaspids did not possess a bony shield or armor, hence their name. The anaspid head is instead covered in an array of small, weakly mineralized scales, with a row of massive scutes running down the back, and at least confirmed among the birkeniids, the body was covered in rows of tile-like scales made of aspidine. Anaspids all had prominent, laterally placed eyes with no sclerotic ring, with the gills opened as a row of holes along either side of the animal, typically numbering anywhere from 6-15 pairs. The major synapomorphy for the anaspids is the large, tri-radiate spine behind the series of the gill openings.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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