Anomalocaris ("Anomalous shrimp") is an extinct genus of anomalocaridids, which are, in turn, thought to be closely related to the Arthropoda.
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The anomalocaris went extinct because of major climate decline. It was a swimming creature which possibly used flexible lobes on the sides of its body to propel itself through the water. Its large head had one pair of large, possibly compound, eyes, and a disk-like mouth that resembled a slice of pineapple. Two large 'arms' with barb-like spikes were positioned in front of the mouth; Anomalocaris probably used these to grab prey and bring it to its mouth.For the time in which it lived Anomalocaris was a truly gigantic creature, reaching lengths from an already large 60 cm (2 ft) to a staggering 2 m (6 ft 8 in). Anomalocaris has been misidentified several times. Its name originates from a description of a detached 'arm', mistaken for a separate creature. The mouth was mistaken for a jellyfish and called Peytoia, and the body was classified as a sponge named Laggania. According to ICZN rules, the oldest name takes priority, which in this case would be Anomalocaris. The name Laggania was later used for another genus of anomalocarid. "Peytoia" has been modified into Parapeytoyia, a genus of Chinese anomalocarid.Anomalocaris had a cosmopolitan distribution in Cambrian seas, and has been found from early to mid Cambrian deposits from Canada, China, Utah and Australia, to name but a few.A long-standing view holds that Anomalocaris fed on hard-bodied animals, including trilobites. While its mid-gut glands strongly suggest a predatory lifestyle, its ability to penetrate mineralised shells has come under fire in recent years.[14] Some Cambrian trilobites have been found with round or W-shaped "bite" marks, which were identified in shape with the mouthparts of Anomalocaris.Stronger evidence that Anomalocaris ate trilobites comes from fossilised faecal pellets, which contain trilobite parts and are so large that the anomalocarids are the only organisms large enough to have produced them.[12] However, since Anomalocaris lacks any mineralised tissue, it seemed unlikely that it would be able to penetrate the hard, calcified shell of trilobites.One possibility is that anomalocarids fed by grabbing one end of their prey in their jaws while using their appendages to quickly rock the other end of the animal back and forth. This produced stresses that exploited the weaknesses of arthropod cuticle, causing the prey's exoskeleton to rupture and allowing the predator to access its innards.This behaviour is thought to have provided an evolutionary pressure for trilobites to roll up, to avoid being flexed until they snapped.However, the lack of wear on anomalocarid mouthparts suggests they did not come into regular contact with mineralised trilobite shells. Computer modeling of the Anomalocaris mouthparts suggests they were in fact better suited to sucking on smaller, soft-bodied organisms (and could not have been responsible for many trilobite deformations).

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