Anomalocaridids are a group of very early marine animals known from fossils found in Cambrian deposits in China, USA, Canada, Poland and Australia. 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.
Parapeytoia yunnanensis, one species of anomalocarid (many scientists debate whether or not Parapeytoia was a true anomalocarid, or rather more closely related to Yohoia or Haikoucaris), may even have had legs.
Compared with many of the other sea-dwelling creatures of its time, anomalocarids were extremely agile. The flaps along its body could probably be moved in a wave-like formation, allowing it to move at great speeds or to 'hover'. This motion could be compared to present-day Batoidea (rays), or perhaps cuttlefish. The shell of the anomalocarids was more flexible than those of its prey, allowing it easier movement.
After death this large organism tended to disintegrate and fall apart into separate chunks; the same happened with its moulted skins . Completely intact fossil remains of it are very rare. When the fossils were originally described, the jointed arms in front of the mouth were classified as separate arthropods (a large mystery before the fossils were fully reassembled was why these fossils, mistaken as "shrimps", were always found without "heads"), the mouth was thought to have been a fossilized jellyfish called "Peytoia", and the body, thought to be a sponge named "Laggania" was not associated with either. Since the pieces were reassembled in the 1980s, a number of genera and species have been described that differ in the details of the grasping appendages, as to whether a tail is present, mouth location, and other features.
The name Anomalocaris (meaning "strange shrimp") originally referred to the detached arms (which were the first part to be named), and was later used for the whole animal because of the biological name priority rules. Curiously enough, when fully assembled, these animals do strongly resemble (in outside appearance) a gigantic brine shrimp with a pair of finger-like appendages near its mouth.
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 only plausible close relatives of the anamalocarids are the opabinids, another group of enigmatic early forms. The anomalocarids and opabinids are usually considered to be allied to the arthropods, but they clearly are not crown group arthropods. In some taxonomies they are placed as stem group arthropods; in others they are given their own phylum, Dinocarida. Some fossils suggest that Anamalocarids and other Dinocarids share a common ancestry with Onychophora, due to the similarities in the fossils to both groups . For example Onychophora and Dinocarid joints and segments tend not to be as well-defined and not as clearly separated as most arthropods.
Anomalocaris is the star of a Walking with Monsters segment.