Anomalocaris canadensis

Anomalocaris ("abnormal shrimp") is an extinct genus of anomalocaridid, a family of animals thought to be closely related to ancestral 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 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. Anomolocaris is thought to be one of the earliest examples of an apex predator, though others have been found in older Cambrian lagerstätten deposits.

Anomalocaris canadensis lived in the Burgess Shale in relatively great numbers, though comparable fossils have been found elsewhere, suggesting a more expansive range over the Laurentian continent. In the Burgess Shale, Anomalocaris is more common in the older sections, notably the Mount Stephen trilobite beds. However, in the younger sections, such as the Phyllopod bed, Anomalocaris could reach much greater sizes; roughly twice the size of its older, trilobite bed relatives. These rare giant specimens have previously been referred to a separate species, Anomalocaris gigantea; however, the validity of this species has been called into question, and is currently dubious. Other species attributed to Anomalocaris live in vastly different environments. Anomalocaris saron and Anomalocaris kunmingensis lived in the Maotianshan Shales, a shallow tropical sea in what is now modern China. Anomalocaris briggsi lived in a comparable environment; the shallow, tropical waters of Cambrian Australia. The Maotianshan Shale and the Emu Bay Shale are very close in proximity, being separated by a small landmass, far from the Burgess Shale.

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 anomalocaridid mouth was discovered by Charles Doolittle Walcott, who mistook it for a jellyfish and placed it in 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; a 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. The two genera are now placed into the family Anomalocaridae. Because Peytoia was named first, it became the correct name for the entire animal.

For the time in which it lived, Anomalocaris was gigantic, up to 1 metre (3.3 feet) long. 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", maximizing the swimming efficiency. The construction of a remote-controlled model showed this mode of swimming to be intrinsically stable, implying that Anomalocaris would not have needed a complex brain to manage balance while swimming. The body was widest between the third and fifth lobe and narrowed towards the tail; it had at least 11 lobes in total. It is difficult to distinguish lobes near the tail, making an accurate count difficult. Anomalocaris had an unusual disk-like mouth. The mouth was composed of several plates organized in triradially. 3 of the plates were quite large. 3-4 medium sized plates could be found between each of the large plates, and several small, wrinkled plates between them. The mouth resembled a pineapple ring with the center replaced by a series of serrated prongs. The mouth could constrict to crush prey, but never completely close; there was always an opening about 5 millimeters in diameter when the mouth was shut; a centimetres in exceptionally large individuals. Two large Great Appendages (up to 18 centimetres (7.1 inches) in length when extended) were positioned in front of the mouth, at the front of the head. Along with all other known Anomalocarids, Anomalocaris possessed a sclerite on the top of its head, called a head shield, or dorsal carapace. The head shield of Anomalocaris was shaped like an oval, with a distinct rim on the outer edge. Anomalocaris canadensis possessed 14 segments on each appendage, each one tipped with two barb-like ventral spines. The ventral spines themselves were both equipped with 2 smaller auxiliary spines, …

A long-standing view holds that Anomalocaris fed on hard-bodied animals, including trilobites—making it one of the first predators—and its mid-gut glands strongly suggest a predatory lifestyle. However, its ability to penetrate mineralised shells has been questioned. Some Cambrian trilobites have been found with round or W-shaped "bite" marks, which were identified as being the same shape as the mouthparts of Anomalocaris. Stronger evidence that Anomalocaris ate trilobites comes from coprolite, which contain trilobite parts and are so large that the anomalocarids are the only known organism from that period 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. Rather, the coprolites may have been produced by a different organisms, such as the trilobites of the genus Redlichia. The lack of wear on anomalocaridid mouthparts suggests they did not come into regular contact with mineralised trilobite shells, and were possibly better suited to feeding on smaller, soft-bodied organisms by suction, since they would have experienced structural failure if they were used against the armour of trilobites. A. canadensis may have been capable of feeding on organisms with hard exoskeletons due to the short, robust spines on its appendages. However, as opposed to Peytoia whose mouthpiece is more rectangular with short protruding spines, the mouthpiece of A. canadensis has a smaller and more irregular diamond-shaped opening, not permitting strong biting motions, and indicating a suction-feeding behavior to suck in softer organisms. Phylogenetic analyses have also found "A." briggsi to belong to the closely related, filter-feeding family Cetiocaridae, which suggests that may have been a filter-feeder as well.