|
The Anomalocaridids〔Neolatin compound word from Greek ''ἀνώμαλος anomalos'' and ''καρίς karis'' (''gen.: καρίδος''), meaning "strange shrimp". Note that while "Anomalocarid" is a widely used alternative spelling, the double "id" at the end is technically the correct form, for the reasons given in .〕 comprise a group of very early marine animals known primarily from fossils found in Cambrian deposits in China, United States, Canada, Poland and Australia. They were long thought to be restricted to this Cambrian time range, but the discovery of large Ordovician specimens has extended this somewhat. The later Devonian ''Schinderhannes'' shows many anomalocaridid features. Although originally interpreted as an anomalocaridid-like arthropod, some recent studies suggest that it may represent an anomalocaridid: if so it would extend the group's record by some hundred million years: the non-mineralised anomalocaridid structure means they are absent from the intermediate fossil record.〔 Anomalocaridids are the largest Cambrian animals known—some Chinese forms may have reached 2 m (7 ft) in length—and most of them were probably active carnivores. ==Description== Anomalocaridids were flat, free-swimming, segmented animals that possessed two grasping appendages in front of their mouths (the "great appendages"). These consisted of a dozen or more jointed segments, which could be curled like fingers to grasp prey. Each segment bore a pair of spines, which were sometimes armed with smaller spinelets, that could be used to impale their food. The head was covered with a carapace formed by a series of chitinous plates, and the mouth was a circular structure resembling 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. The skeletons of Anomalocaridids were sclerotized but not mineralized—the great appendages, head, and jaws had a tough chitin exoskeleton like an insect's, but not the hard, calcium carbonate skeleton seen in arthropods such as trilobites and crabs. Anomalocaridids also had large compound eyes,〔(''Nature'': "An eye-opening fossil" )〕 a feature that they share with arthropods. The body was flanked with a series of flexible swimming lobes. On top of the back, there are a series of long, narrow, blade-shaped structures, which may have functioned as gills. Anomalocaridids were not particularly large by modern standards, but during the Early Cambrian, they would have been among the largest creatures on Earth. Their raptorial great appendages and large, compound eyes suggest that they were apex predators, lying at the top of the Cambrian food chain. The sclerotized nature of the frontal appendages and jaws would have made them effective in piercing and shredding soft-bodied or tough prey items such as worms, primitive arthropods, and other anomalocaridids. However, the unmineralized skeleton would have made it difficult, although perhaps not impossible, to pierce the hard carapaces of animals like trilobites. Unlike their lobopod relatives, such as ''Kerygmachela'', anomalocaridids lacked legs and were specialized for swimming. Anomalocaridids seem to have been swimming predators, similar to the fish and squid that would replace them in the Devonian. Compared with many of the other sea-dwelling creatures of their time, anomalocaridids must have been fast and agile. The flaps along its body could probably be moved in a wave-like formation, allowing it to swim quickly or to 'hover', as in Batoidea (rays) or cuttlefish. After death this large organism tended to disintegrate and fall apart into separate pieces; the same happened to its moulted skins. 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 (precipitating quite a mystery; the fossils were mistakenly identified as "shrimp", but always with their "heads" missing), 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 detached arms (which were the first part to be named) similar to those of ''Peytoia''. Curiously enough, when fully assembled, these animals outwardly resemble gigantic brine shrimp with a pair of finger-like appendages near the mouth. The anomalocaridids thrived in the Early and Mid-Cambrian, and are rarer in later deposits mainly due to the scarcity of post-Cambrian Lagerstätten. Nevertheless, they are still present in Ordovician Lagerstätten and a few anomalocaridids were able to survive as late as the Devonian. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「anomalocaridid」の詳細全文を読む スポンサード リンク
|