Australaglaspis stonyensis
Late Cambrian (Idamean)
Siltstone East of jetty at Stone point, Montagu, Tasmania, Australia
20mm on 52mm slab
Arthropod, Aglaspidid.

Aglaspidids have the general shape of a trilobite but they belonged to a different group of marine arthropods (Order Aglaspidida).  The group survived into the Early Ordovician but died out by the end of the period.

It has been proposed that aglaspidids are related to the ancestors of horseshoe crabs and arachnids.