Anomalocaris ("abnormal shrimp") is an extinct genus of anomalocaridid, which are, in turn, thought to be closely related to the arthropods. The first fossils of Anomalocaris were discovered in the Ogygopsis shale by Joseph Frederick Whiteaves, with more examples found by Charles Doolittle Walcott in the famed Burgess Shale. Originally several fossilized parts discovered separately (the mouth, feeding appendages and tail) were thought to be three separate creatures, a misapprehension corrected by Harry B. Whittington and Derek Briggs in a 1985 journal article.
Anomalocaris is thought to have been a predator. It propelled itself through the water by undulating the flexible lobes on the sides of its body. Each lobe sloped below the one more posterior to it, and this overlapping allowed the lobes on each side of the body to act as a single "fin", maximising the swimming efficiency. The construction of a remote-controlled model showed this mode of swimming to be intrinsically stable, meaning that Anomalocaris need not have had a complex brain to cope with balancing while swimming. The lateral lobes overlapped. The widest part of the body was on the third to fifth lobe; it narrowed towards its tail, and had at least 11 lobes in total. The more posterior lobes are difficult to discriminate, making an accurate count slightly difficult. Anomalocaris had a large head, a single pair of large, possibly compound eyes, and an unusual, disk-like mouth. The mouth was composed of 32 overlapping plates, four large and 28 small, resembling a pineapple ring with the center replaced by a series of serrated prongs. The mouth could constrict to crush prey, but never completely close, and the tooth-like prongs continued down the walls of the gullet. Two large 'arms' (up to seven inches in length when extended) with barb-like spikes were positioned in front of the mouth. The tail was large and fan-shaped, and along with undulations of the lobes, was probably used to propel the creature through Cambrian waters. Stacked lamella of what were probably gills attached to the top of each lobe.
For the time in which it lived Anomalocaris was a truly gigantic creature, reaching lengths of up to one meter.
Anomalocaris has been misidentified several times, in part due to its makeup of a mixture of mineralized and unmineralized body parts; the mouth and feeding appendage was considerably harder and more easily fossilized than the delicate body. Its name originates from a description of a detached 'arm', described by Joseph Frederick Whiteaves in 1892 as a separate crustacean-like creature due to its resemblance to the tail of a lobster or shrimp. The first fossilized mouth was discovered by Charles Doolittle Walcott, who mistook it for a jellyfish and placed the genus Peytoia. Walcott also discovered a second feeding appendage but failed to realize the similarities to Whiteaves discovery and instead identified it as feeding appendage or tail of the extinct Sidneyia. The body was discovered separately and classified as a sponge in the genus Laggania; the mouth was found with the body, but was interpreted by its discoverer Simon Conway Morris as an unrelated Peytoia that had through happenstance settled and been preserved with Laggania. Later, while clearing what he thought was an unrelated specimen, Harry B. Whittington removed a layer of covering stone to discover the unequivocally connected arm thought to be a shrimp tail and mouth thought to be a jellyfish. Whittington linked the two species, but it took several more years for researchers to realize that the continuously juxtaposed Peytoia, Laggania and feeding appendage actually represented a single, enormous creature. According to International Commission on Zoological Nomenclature 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 Parapeytoia, a genus of Chinese anomalocarid. Anomalocaris is placed in the extinct family Anomalocaridae, and is now considered to be related to modern arthropods.
Stephen Jay Gould cites Anomalocaris as one of the fossilized extinct species he believed to be evidence of a much more diverse set of phyla that existed in the Cambrian era (discussed in his book Wonderful Life), a conclusion disputed by other paleontologists.
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. 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. 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 anamolacarids 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 anomalocaridid mouthparts suggests they did not come into regular contact with mineralised trilobite shells. Computer modelling 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).