Groenlandaspis sp. is an extinct placoderm of Late Devonian period (360 ma).  It is a member of the order Arthrodira (gk. jointed neck) and it had a distinctive thoracic shield with a pyramid-like peak that would have helped it plane through the water column.

Although most of its bony plates were fused together and rigid, the nuchal gap would have allowed for some movement between the thorax and head.  This anatomical feature allowed many Arthrodiri to develop large bites but Groenlandaspis is not thought to have been a predator as its mouth parts were quite small.  However it would have been an adept swimmer and was probably a far-ranging seabed scavenger as its remains have been found right around the world, most notably in Greenland and Australia.

Groenlandaspis anatomy from Paleozoo Evolutionary Models by Bruce Currie

Initially the armour plating of the placoderms was probably used as a form of defence against other predators, such as the large arthropods that dominated the Silurian seas, but once the Arthrodire's 'jointed neck' became combined with an articulated jaw, placoderms quickly became formidable predators in their own right.

With thanks to Dr Alex Ritchie for assistance with anatomical detail.