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The Galeaspida, or galeaspids, are a highly diversified group of fossil, armored, jawless vertebrates, which lived in Silurian and Devonian times (430 to 370 million years ago). The head of galeaspids is a massive endo- and exoskeletal shield, which is generally oval or horseshoe-shaped, but can be produced into large rostral and lateral processes. The mouth and gill openings are situated on the ventral side of the head, which is flat and suggests that they were bottom-dwellers (as are the Osteostraci) The body is covered with minute scales, and there is no other fin than the caudal fin. The most peculiar feature of galeaspids is a large, median dorsal opening in the headshield, which communicates ventrally with the pharynx and gill chamber. It is assumed to have served both the olfaction and the intake of the respiratory water, like the nasopharyngeal duct of hagfishes (see Hyperotreti). Galeaspids are the vertebrates which have the largest number of gills, as some species had up to 45 gill openings.
Galeaspids are known exclusively in the Silurian and Devonian rocks of China (including Tibet) and northern Vietnam, and are likely to have been endemic to these areas. They lived in shallow water, deltaic and lagoonal marine environments, and are most diverse in the Lower Devonian (-400 million years ago).