Anomalocarid
From Freepedia
Anomalocarids (meaning "odd 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 meters in length), and most of them were probably active carnivores (although recent thought posits one genus, Laggania, as a plankton-eating animal).
Anomalocarids were flat, free swimming, segmented animals with two shrimp-like appendages forward of the mouth. Their mouth is a peculiar structure resembling a pineapple slice with multiple layers of hard, sharp teeth in the central orifice. The mouth was actually more rectangular than round, and the teeth did not meet in the middle. This would still allow it to 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.
After death this large organism tended to disintegrate and fall apart into separate chunks, and completely intact fossil remains 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 "shrimp", were always found without "heads"), the mouth was thought to have been a fossilized jellyfish, and the body was not associated with either. Since the pieces were re-assembled 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 anamalocarids thrived in the Early and Mid Cambrian and then apparently died out.
Classification
Three genera of anomalocarids are known: Anomalocaris, Laggania, and Parapeytoia.
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.
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.
