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| Anomalocaris Fossil range: Early to mid Cambrian: Chengjiang–Burgess shale |
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 Above: A complete specimen of Anomalocaris
Below: Reconstruction of Anomalocaris, seen from below. The "tail fins" in this reconstruction should be placed ventrally (on top of the organism), and not at the rear as illustrated.  |
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Anomalocaris ("Anomalous 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.1 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.21
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Anatomy
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.3 Each lobe sloped below the one more posterior to it,4 and this overlapping allowed the lobes on each side of the body to act as a single "fin", maximising the swimming efficiency.3 The construction of a remote-controlled model showed this mode of swimming to be intrinsically stable,5 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 it tail, and had at least 11 lobes in total.4 The more posterior lobes are difficult to discriminate, making a total count slightly difficult to reach.4 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.1 The mouth could constrict to crush prey, but never completely close, and the tooth-like prongs continued down the walls of the gullet.6 Two large 'arms' (up to seven inches in length when extended6) with barb-like spikes were positioned in front of the mouth, and were probably used these to grab prey and bring it to its mouth.2 The tail was large and fan-shaped, and along with undulations of the lobes, was probably used to propel the creature through Cambrian waters.178 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.1
Discovery
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.6 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.6 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.6 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 discovered the unequivocally connected arm thought to be a shrimp tail and mouth thought to be a jellyfish.16 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.6 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 phyla that existed in the Cambrian era6 (discussed in his book Wonderful Life), a conclusion disputed by other paleontologists.1
Ecology
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.591011
It appears to have fed on other animals, including trilobites. Some Cambrian trilobites have been found with W-shaped "bite" marks which seemed to match the mouthparts of Anomalocaris.10 However, since Anomalocaris lacks any mineralised tissue, it seemed unlikely that it would be able to penetrate the tough, calcified shell of trilobites.10 It turns out 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.10 This behaviour is thought to have provided an evolutionary pressure for trilobites to roll up, to avoid being flexed until they snapped.10
Further 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.10
Gallery
 Image of the first complete Anomalocaris fossil found, residing in the Royal Ontario Museum |
 Anomalocaris canadensis fossil appendix in Cosmocaixa, Barcelona. |
 An example of Anomalocaris whiteaves on display at the Paleozoological Museum of China in Beijing. |
 Anomalocaris model, Natural History Museum |
See also
Classification is discussed at Anomalocarididae. Other relevant articles are:
Footnotes
- ^ a b c d e f g Conway Morris, S. (1998). The crucible of creation: the Burgess Shale and the rise of animals. Oxford [Oxfordshire]: Oxford University Press. pp.56–9. ISBN 0-19-850256-7.
- ^ a b Whittington, H.B.; Briggs, D.E.G. (1985). "The largest Cambrian animal, Anomalocaris, Burgess Shale, British Columbia". Philosophical Transactions of the Royal Society of London B. 309: 569--609. doi:.
- ^ a b Usami, Y. (2006), "Theoretical study on the body form and swimming pattern of Anomalocaris based on hydrodynamic simulation", Journal of Theoretical Biology 238 (1): 11–17, doi:
- ^ a b c Whittington, H.B.; Briggs, D.E.G. (1985), "The Largest Cambrian Animal, Anomalocaris, Burgess Shale, British Columbia" (free full text), Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 309 (1141): 569–609, doi:, http://adsabs.harvard.edu/abs/1985RSPTB.309..569W
- ^ a b Briggs, Derek E. G. (1994), "Giant Predators from the Cambrian of China", Science 264 (5163): 1283–1284
- ^ a b c d e f g h Gould, Stephen Jay (1989). Wonderful life: the Burgess Shale and the nature of history. New York: W.W. Norton. pp.194–206. ISBN 0-393-02705-8.
- ^ "The Anomalocaris homepage". Retrieved on 2008-03-20.
- ^ Usami, Yoshiyuki (2006). Theoretical study on the body form and swimming pattern of Anomalocaris based on hydrodynamic simulation. Journal of Theoretical Biology. Volume 238, Issue 1, pp. 11-17
- ^ Briggs, D.E.G.; Mount, J.D. (09/01/1982), "The occurrence of the giant arthropod Anomalocaris in the Lower Cambrian of southern California, and the overall distribution of the genus", Journal of Paleontology (Paleontological Soc) 56 (5): 1112–1118, http://jpaleontol.geoscienceworld.org/cgi/content/abstract/56/5/1112
- ^ a b c d e f Nedin, C. (1999), "Anomalocaris predation on nonmineralized and mineralized trilobites", Geology 27 (11): 987–990, doi:, http://geology.geoscienceworld.org/cgi/content/abstract/27/11/987
- ^ Briggs, D.E.G.; Robison, R.A. (1984), Exceptionally preserved nontrilobite arthropods and Anomalocaris from the Middle Cambrian of Utah, The Paleontological Institute, The University of Kansas, http://kuscholarworks.ku.edu/dspace/handle/1808/3656
References
- Paul Chambers; Haines, Tim (2005). The Complete Guide to Prehistoric Life. Buffalo, N.Y: Firefly Books. ISBN 1-55407-181-X. OCLC 85767395.
External links
- Anomalocaris 'homepage' with swimming animation
- Burgess Shale: Anomalocaris canadensis (proto-arthropod), Smithsonian.
Wikipedia content modification information:
- This page was last modified on 8 November 2008, at 07:46.
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