Haikouichthys

Haikouichthys was one of the very first vertebrates ever to live. Its fossils, dating to the Early Cambrian period, were found in the Maotianshan Shales amongst other Chengjiang biota; its fossils preserved pharyngeal gill slits and a notochord, ancestral to all chordates, but the most striking discovery is a skull, lending it to be a stem-group vertebrate at the absolute least, but even if it was indeed, a basal crown-group vertebrate, it wasn't derived enough to be called a "fish". In Walking With Monsters, a school of Haikouichthys feasted upon the blood of a wounded Anomalocaris, swarming towards it in an unlikely, zerg rush-like manner.