Anomalocarid
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 Artist's reconstruction of an anomalocarid hunting a trilobite.
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Extinct (fossil)
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Anomalocarids ("Strange shrimp") are a group of very early marine animals known from fossils found in Cambrian deposits in China, North America, and Australia. Anomalocarids are the largest Cambrian animals known (some Chinese forms grew up to 2 m, and most of them were probably active carnivores (although recent thought posits one genus, Laggania, as a plankton-eating animal).
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[edit] Characteristics
Anomalocarids were flat free-swimming segmented animals, which in front of their mouths had two appendages that look like the bodies of shrimps. The mouth is a peculiar circular structure like a pineapple slice, but with a ring of hard sharp teeth in the central orifice. The mouth was more rectangular than round, and the teeth did not meet in the middle. This would still let it crack open shells of small arthropods and other like animals, such as trilobites. Indeed, many trilobites have been found with bite marks on them. Anomalocarids also had large eyes and a body half-flanked with a series of swimming lobes.
Parapeytoia yunnanensis, one species of anomalocarid, may even have legs[1].
Compared with many of the other sea-dwelling creatures of its time, anomalocarids were extremely agile. The flaps along its body could probably be moved in a wave-like formation, allowing it to move at great speeds or to 'hover'. This motion could be compared to present-day Batoidea (rays). The shell of the anomalocarids was more flexible than those of its prey, allowing it easier movement.
After death this large organism tended to disintegrate and fall apart into separate chunks; the same happened with its moulted skins. Completely intact fossil remains of it are very rare. When the fossils were originally described, the jointed arms in front of the mouth were classified as separate arthropods (a large mystery before the fossils were fully reassembled was why these fossils, mistaken as "shrimps", were always found without "heads"), the mouth was thought to have been a fossilized jellyfish called "Peytoia", and the body, thought to be a sponge named "Laggania" was not associated with either. Since the pieces were reassembled in the 1980s, a number of genera and species have been described that differ in the details of the grasping appendages, as to whether a tail is present, mouth location, and other features.
The name Anomalocaris (meaning "strange shrimp") originally referred to the detached arms (which were the first part to be named), and was later used for the whole animal because of the biological name priority rules. Curiously enough, when fully assembled, these animals do strongly resemble (in outside appearance) a gigantic brine shrimp with a pair of finger-like appendages near its mouth.
The anomalocarids thrived in the Early and Mid Cambrian and then apparently died out.
[edit] Classification
Three genera of anomalocarids are known: Anomalocaris, Laggania, and Amplectobelua. A variety of other related animals including Parapeytoia, Pambdelurion and Kerygmachela are sometimes classified as anomalocarids, but probably belong to different clades.
Compared with Anomalocaris species, Laggania species lacked tail structures and had a considerably larger head with the eyes placed behind instead of in front of the mouth, which would have been disadvantagous for active hunting. Because of these characteristics, some scientists have described Laggania as a cruising, plankton feeder. Amplectobelua species, in contrast to Anomalocaris, were smaller and had a much wider body front with eyes placed lateral to the mouth.
The only plausible close relatives of the anamalocarids are the opabinids, another group of enigmatic early forms. The anomalocarids and opabinids are usually considered to be allied to the arthropods, but they clearly are not crown group arthropods. In some taxonomies they are placed as stem group arthropods; in others they are given their own phylum, Dinocarida. Some fossils suggest that Anamalocarids and other Dinocarids share a common ancestry with Onychophora, due to the similarities in the fossils to both groups.[2]
[edit] See also
[edit] References
- Briggs, Derek; Collier, Frederick; Erwin, Douglas. The Fossils of the Burgess Shale. Smithsonian Books, 1995.
- James W. Valentine. On the Origin of Phyla. University Of Chicago Press, 2004.
- Tim Haines & Paul Chambers. The Complete Guide to Prehistoric Life. BBC Books, 2005.
- Conway Morris, Simon. The Crucible of Creation. Oxford University Press, 1998.
