The Anaspida are stem gnathostomes, and are classically regarded as the ancestors of lampreys. Anaspids were small marine agnathans that lacked scales and paired fins. They first appeared sometime during the early Silurian, and flourished until the Late Devonian Extinction Event, during the late Devonian, where most species, save for lampreys, went extinct due to the environmental upheaval during that time. The gills opened as a row of holes along either side of the animal that typically numbered anywhere from from 6-15 pairs.