Anomalocaridids are a group of very early marine animals known primarily from fossils found in Cambrian deposits in China, USA, Canada, Poland and Australia. They were long thought to be restricted to this range, but the discovery of the Devonian Schinderhannes extended their record by some hundred million years — their non-mineralised nature means they are absent from the intermediate fossil record. Anomalocarids are the largest Cambrian animals known — some Chinese forms may have reached 2 m in length — and most of them were probably active carnivores.
Five genera of anomalocarids are known: Anomalocaris, Laggania, Schinderhannes, Amplectobelua and Hurdia. A variety of other related animals including Parapeytoia, Pambdelurion and Kerygmachela are sometimes classified as anomalocarids, but probably belong to different clades. Anomalocaridids are quite common throughout the Cambrian, and are known from the earliest Burgess shale-type fauna, in the early Cambrian of Poland, predating the first appearance of trilobites.
Compared with Anomalocaris species, Laggania species lacked tail structures and had a considerably larger head with the eyes placed behind instead of in front of the mouth, which would have been disadvantagous for active hunting. Because of these characteristics, some scientists have described Laggania as a cruising, plankton feeder. Amplectobelua species, in contrast to Anomalocaris, were smaller and had a much wider body front with eyes placed lateral to the mouth.
The anomalocarids appear to be closely related to the opabinidids, and they fall somewhere in the arthropod stem. The discovery of a Devonian anomalocarid suggests that the group is a paraphyletic group, containing the arthropods.