Cephalaspis

Cephalaspis (head shield) was a genus in the class Osteostraci of Palaeozoic armoured jawless fishes, or ostracoderms. It was a medium sized detritivorous fish that lived in freshwater streams and estuaries of the early Devonian.

Anatomy and morphology
Like its immediate osteostracan ancestors, Cephalaspis was heavily armored, presumedly to defend against predatory placoderms and eurypterids, as well as to serve as a source of calcium for metabolic functions in calcium-poor freshwater environments. It had sensory patches along the rim and center of its head shield, which were used to sense for worms and other burrowing organisms in the mud.

Diet
Because its mouth was situated directly beneath its head, Cephalaspis was thought of as being a bottom-feeder, akin to a heavily armoured catfish or sturgeon. It moved its plow-like head from side to side, Cephalaspis easily stirring sand and dust into the water, along with revealing the hiding places of its prey, digging up worms or crustaceans hidden in the mud and algae, as well as sifting through detritus (inferred from its lack of jaws and inability to bite).