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agnathan


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Any jawless craniate animal. Agnathans were formerly classified in the subphylum (or superclass) Agnatha, with the living representatives – lampreys and hagfishes – constituting the class Cyclostomata. However, it is now accepted that the lampreys are more closely related to the jawed vertebrates than to the hagfishes, and the agnathans are now placed in two distinct clades of the subphylum Craniata, the hagfishes forming the class Myxini (Hyperotreti), and the lampreys the Hyperoartia. The closest living relatives of the latter are the jawed vertebrates (Gnathostomata), which with the lampreys constitute the superclass Vertebrata. Fossil agnathans, covered in an armour of bony plates, are the oldest known fossil vertebrates. They have been dated from the Silurian and Devonian periods, 440–345 million years ago. Fossil chordates resembling hagfishes have been found dating from much earlier times, notably Myllokunmingia of the Cambrian Chengjiang fossils.

Subjects: Biological Sciences.


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