13" Partial Giant Radiodont (Aegirocassis), Xiphosurida & Trilobite

This is a partial giant Radiodont arthropod (Aegirocassis benmoulai) associated with a primitive horseshoe crab fossil and Asaphid trilobite. It comes from the Lower Ordovician Fezouata Formation of Morocco. The partial radiodont, which measures 13" long, includes its filter feeding appendage and most of its head. Just above the head is a well preserved Xiphosurid arthropod, a type of very primitive horseshoe crab. There is also a ventrally-preserved 6" long Asaphellus trilobite present on the shale.

Aegirocassis could reach lengths of up to 2m long making it the largest known animal at the time. It likely evolved from the early predatory radiodonts of the Cambrian.

Giant sea creature hints at early arthropod evolution