Dinocarida (Order: Radiodonta, stem group arthropods)
Anomalocaris is an anomalocaridid. Anomalocaridids have been variously regarded as basal stem-lineage euarthropods (e.g., Daley et al., 2009), basal members of the arthropod group Chelicerata (e.g., Chen et al., 2004), and as a sister group to the arthropods (e.g., Hou et al., 2006).
Anomalocaris – from the Greek anomoios, “unlike,” and the Latin caris, “crab” or “shrimp,” thus, “unlike other shrimp.”
canadensis – from Canada, the country where the Burgess Shale is located.
Lectotype – GSC3418 in the Geological Survey of Canada, Ottawa, Canada.
Burgess Shale and vicinity: none.
Other deposits: A. pennsylvanica from the Early Cambrian Kinzers Formation in Pennsylvania (Resser, 1929); A. saron (Hou et al., 1995) from the Early Cambrian Chengjiang biota; A. briggsi (Nedin, 1995) from the Early Cambrian Emu Bay Shale of Australia.
Middle Cambrian, Bathyuriscus–Elrathina Zone (approximately 505 million years ago).
The Collins, Raymond and Walcott Quarries on Fossil Ridge. The Trilobite Beds, Tulip Beds (S7) and the Collins Quarry on Mount Stephen. Additional localities on Mount Field, Mount Stephen, near Stanley Glacier and in the Early Cambrian Cranbrook Shale, Eager Formation, British Columbia.
Brief history of research:
Anomalocaris has a complex history of description because parts of its body were described in isolation before it was realized they all belonged to the same animal. The frontal appendage of Anomalocaris was described by Whiteaves (1892) as the body of a shrimp. The mouth parts were described by Walcott (1911) as a jellyfish called Peytoia nathorsti. A full body anomalocaridid specimen was originally described as the sea cucumber Laggania cambria (Walcott, 1911), and re-examined by Conway Morris (1978) who concluded it was a superimposition of the “jellyfish” Peytoia nathorsti on top of a sponge. Henriksen (1928) attached Anomalocaris to the carapace of Tuzoia, but Briggs (1979) suggested instead that it was the appendage of an unknown arthropod, an idea that turned out to be correct.
In the early 1980s, Harry Whittington was preparing an unidentified Burgess Shale fossil from the Geological Survey of Canada by chipping away layers of rock to reveal underlying structures, when he solved the mystery of Anomalocaris's identity. Much to his surprise, Whittington uncovered two Anomalocaris “shrimp” attached to the head region of a large body, which also had the “jellyfish” Peytoia as the mouth apparatus. Similar preparations of other fossils from the Smithsonian Institution in Washington DC revealed the same general morphology, including the Laggania cambria specimen Conway Morris (1978) thought to be the superimposition of the Peytoia jellyfish on a sponge, which was actually a second species of Anomalocaris. Thus, Whittington and Briggs (1985) were able to describe two species: Anomalocaris canadensis, which had a pair of the typical Anomalocaris appendages, and Anomalocaris nathorsti, which has a different type of frontal appendage and includes the original specimen of Laggania cambria. Bergström (1986) re-examined the morphology and affinity of Anomalocaris and suggested it had similarities to the arthropods.
Collecting at the Burgess Shale by the Royal Ontario Museum in the early 1990s led to the discovery of several complete specimens, which Collins (1996) used to reconstruct Anomalocaris canadensis with greater accuracy. This led to a name change of Anomalocaris nathorsti to Laggania cambria. Anomalocaris has since been the subject of many studies discussing its affinity (e.g., Hou et al., 1995; Chen et al., 2004; Daley et al., 2009), ecology (e.g., Rudkin, 1979; Nedin, 1999) and functional morphology (e.g., Usami, 2006).
Anomalocaris is a bilaterally symmetrical and dorsoventrally flattened animal with a non-mineralized exoskeleton. It has a segmented trunk, with at least 11 lateral swimming flaps bearing gills, and a prominent tailfan, which consists of three pairs of prominent fins that extend upward from the body. Paired gut glands are associated with the body segments in some specimens. The head region bears one pair of anterior appendages, two eyes on stalks, and a ventrally oriented circular mouth apparatus with many spiny plates. The frontal appendages are elongated and have 14 segments, each with a pair of sharp spikes projecting from the ventral surface. The stalked eyes are dorsal and relatively large. The ventral mouth apparatus has 32 rectangular plates, four large and 28 small, arranged in a circle, with sharp spines pointing into a square central opening. The most complete Anomalocaris specimen is 25 cm in length, although individual fragments suggest individuals could reach a larger size, perhaps up to 100 cm.
The Anomalocaris frontal appendage is extremely common at the Mount Stephen Trilobite Beds, and several hundred specimens of isolated frontal appendages and mouth parts have been collected from Mount Stephen and the Raymond Quarry on Fossil Ridge. These parts are relatively rare at Walcott Quarry, where fewer than 50 specimens are known (Caron and Jackson, 2008). Several dozen disarticulated assemblages and five complete body specimens are known from the Raymond Quarry.
Nektobenthic or nektonic, mobile
The streamlined body would have been ideal for swimming. Undulatory movements of the lateral flaps propelled the animal through the water column and might have also served in gill ventilation. While swimming, Anomalocaris's frontal appendages would hang below the body, but it would thrust its head and appendages forward 180° to attack prey as needed.
A predatory lifestyle is suggested by the large eyes, frontal appendages with spines, gut glands, and spiny mouth apparatus. The circular mouth part is unique in the animal kingdom. It seems unlikely that it was used to bite prey by bringing lateral plates into opposition, rather, it grasped objects either by pivoting the plates outwards or contracting them inward. It has been suggested that Anomalocaris may have preyed on trilobites because some Cambrian trilobites have round or W-shaped healed wounds, interpreted as bite marks (Rudkin, 1979), and large fecal pellets composed of trilobite parts have been found in the Cambrian rock record; anamalocaridids are the only known animals large enough to have produced such pellets. The anomalocaridids could have fed by grasping one end of the trilobite in the mouth apparatus and rocking the other end back and forth with the frontal appendages until the exoskeleton cracked (Nedin, 1999). However, the unmineralized mouth apparatus of Anomalocaris would have probably been too weak to penetrate the calcified shell of trilobites in this manner, and the mouth parts do not show any sign of breakage or wear. Thus, Anomalocaris may have been feeding on soft-bodied organisms including on freshly moulted "soft-shell" trilobites (Rudkin, 2009).