Temporal range: Cambrian Stage 3–Devonian
|Life sized model of Peytoia nathorsti measuring around 60 cm|
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 large Ordovician specimens extended this somewhat. The later Devonian Schinderhannes, a great appendage arthropod that retains many anomalocaridid features, 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 compound 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 (precipitating quite a mystery; the fossils were mistakenly identified as "shrimp", but always with their "heads" missing), 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 detached arms (which were the first part to be named) similar to those of Peytoia. Curiously enough, when fully assembled, these animals outwardly resemble gigantic brine shrimp with a pair of finger-like appendages near the mouth.
The anomalocaridids thrived in the Early and Mid Cambrian, and are rarer in later deposits mainly due to the scarcity of post-Cambrian lagerstatten. Nevertheless, they are still present in Ordovician lagerstatten.
Five genera of anomalocarids are known: Anomalocaris, Peytoia, 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, Peytoia 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 Peytoia 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.
See also 
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- Neolatin compound word from Greek ἀνώμαλος anomalos and καρίς karis (gen.: καρίδος), meaning "strange shrimp". Note that while "Anomalocarid" is a widely used alternative spelling, the double "id" at the end is technically the correct form, for the reasons given in Xianguang, H.; Bergström, J.; Jie, Y. (2006). "Distinguishing anomalocaridids from arthropods and priapulids". Geological Journal 41 (3–4): 259. doi:10.1002/gj.1050.
- Briggs, Derek; Collier, Frederick; Erwin, Douglas. The Fossils of the Burgess Shale. Smithsonian Books, 1995.
- James W. Valentine. On the Origin of Phyla. University Of Chicago Press, 2004.
- Tim Haines & Paul Chambers. The Complete Guide to Prehistoric Life. BBC Books, 2005.
- Conway Morris, Simon. The Crucible of Creation. Oxford University Press, 1998.
- Budd, John S.; Peel (1998). Palaeontology 41: 1201–1213 http://palaeontology.palass-pubs.org/pdf/Vol%2041/Pages%201201-1213.pdf
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- Kühl, G.; Briggs, D. E. G.; Rust, J. (Feb 2009). "A Great-Appendage Arthropod with a Radial Mouth from the Lower Devonian Hunsrück Slate, Germany". Science 323 (5915): 771–3. Bibcode:2009Sci...323..771K. doi:10.1126/science.1166586. ISSN 0036-8075. PMID 19197061.
- Van Roy, P.; Briggs, D. E. G. (2011). "A giant Ordovician anomalocaridid". Nature 473 (7348): 510–513. doi:10.1038/nature09920.
- Robert R. Gaines; Derek E.G. Briggs; Patrick J. Orr (2012). "Preservation of giant anomalocaridids in silica-chlorite concretions from the Early Ordovician of Morocco" (Abstract). PALAIOS 27. Unknown parameter
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- Dzik, J. and Lendzion, K. 1988. The oldest arthropods of the East European Platform. Lethaia, 21, 29–38.
- Anomalocaris at trilobites.info