Anomalocaridids[derivation 1] 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 (7 ft) in length — and most of them were probably active carnivores.
Anomalocaridids were flat, free-swimming, segmented animals that possessed two appendages in front of their mouths that resembled the bodies of shrimp. The mouth was a circular structure resembling a pineapple slice, but with a ring of hard sharp teeth in the central orifice. The mouth was more rectangular than round, and the teeth did not meet in the middle. It has been hypothesised that the mouth enabled anomalocaridids to eat hard-shelled organisms such as trilobites; for a full discussion of this matter, see Anomalocaris. Anomalocarids also had large eyes and a body half-flanked with a series of swimming lobes.
Parapeytoia yunnanensis, one species of anomalocaridid (many scientists debate whether or not Parapeytoia was a true anomalocaridid, 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, anomalocaridids 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 cuticle of the anomalocaridids 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 pieces; the same happened to its moulted skins . Completely intact fossil remains 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 "shrimp", 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.
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