Aegirocassis benmoulai
Ordovician
Fezouata Formation
Mecissi-Zagora Region, Draa Valley, Morocco.

139mm anomalocaridid appendage on 174mm slab.

Aegirocassis is a genus of anomalocaridid known only from the Early Ordovician of Morocco.  It was one of the largest of all arthropods that have ever lived, reaching about 6 1/2 feet long and was almost certainly the largest animal of its time.  Like the largest animal of our time, the blue whale, it has been determined to have been a filter-feeder that descended from predatory ancestors.

Have a look at this amazing 3D model of Aegirocassis courtesy of Google and darwinmuseum.ru. Click on the arrows at the bottom right corner to see it full screen!