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Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (abdomen), usually hidden entirely under the thorax. They live in all the world's oceans, in fresh water, and on land, are generally covered with a thick exoskeleton, and have a single pair of pincers. Many other animals with similar names – such as hermit crabs, king crabs, porcelain crabs, horseshoe crabs, and crab lice – are not true crabs, but many have evolved features similar to true crabs in a process of carcinisation.
Squat lobsters are dorsoventrally flattened crustaceans with long tails held curled beneath the cephalothorax. They are found in the two superfamilies Galatheoidea and Chirostyloidea, which form part of the decapod infraorder Anomura, alongside groups including the hermit crabs and mole crabs. They are distributed worldwide in the oceans, and occur from near the surface to deep sea hydrothermal vents, with one species occupying caves above sea level. More than 900 species have been described, in around 60 genera. Some species form dense aggregations, either on the sea floor or in the water column, and a small number are commercially fished.
Anomura is a group of decapod crustaceans, including hermit crabs and others. Although the names of many anomurans include the word crab, all true crabs are in the sister group to the Anomura, the Brachyura.
Porcelain crabs are decapod crustaceans in the widespread family Porcellanidae, which superficially resemble true crabs. They have flattened bodies as an adaptation for living in rock crevices. They are delicate, readily losing limbs when attacked, and use their large claws for maintaining territories. They first appeared in the Tithonian age of the Late Jurassic epoch, 145-152 million years ago.
Cancer is a genus of marine crabs in the family Cancridae. It includes eight extant species and three extinct species, including familiar crabs of the littoral zone, such as the European edible crab, the Jonah crab and the red rock crab. It is thought to have evolved from related genera in the Pacific Ocean in the Miocene.
Metacarcinus starri is an extinct species of crab in the family Cancridae, subfamily Cancrinae. The species is known solely from the early Miocene, Clallam Formation and the underlying Pysht Formation deposits on the Olympic Peninsula of Washington state, United States.
Hexapodidae is a family of crabs, the only family in the superfamily Hexapodoidea. It has traditionally been treated as a subfamily of the family Goneplacidae, and was originally described as a subfamily of Pinnotheridae. Its members can be distinguished from all other true crabs by the reduction of the thorax, such that only seven sternites are exposed, and only four pairs of pereiopods are present. Not counting the enlarged pair of claws, this leaves only six walking legs, from which the type genus Hexapus, and therefore the whole family, takes its name. Some anomuran "crabs", such as porcelain crabs and king crabs also have only four visible pairs of legs. With the exception of Stevea williamsi, from Mexico, all the extant members are found either in the Indo-Pacific oceans, or around the coast of Africa.
Metacarcinus is a genus of crabs formerly included in the genus Cancer. It includes nine exclusively fossil species and five extant species, of which four are also known from the fossil record. The monophyly of this genus was not supported by a molecular study using the cytochrome oxidase I gene.
Eocarcinus praecursor is a Jurassic species of decapod crustacean, sufficiently distinct from its relatives to be placed in its own family (Eocarcinidae). Often considered the oldest true crab, it was considered by a 2010 study to be an early member of the Anomura. However, a reanalysis in 2020 again found it to be the earliest known stem-group crab.
Glaessneropsoidea is a superfamily of fossil crabs. They are found in rocks from Late Jurassic age to Late Cretaceous. The 45 species in the superfamily are divided among 11 genera in four families:
Anatolikos is a genus of two species of crabs in the family Cancridae. They are recorded from Japan and Taiwan. Two fossil species are known, one from Japan and one from Mexico.
Aciculopoda is an extinct prawn which existed in what is now Oklahoma approximately 360 million years ago. It was described in 2010 on the basis of a single fossil from Oklahoma. The single species, Aciculopoda mapesi, was named by Rodney Feldmann and Carrie Schweitzer in honour of Royal Mapes, a paleontologist who discovered the type specimen. It is only the third unambiguous fossil decapod from before the Mesozoic.
Platykotta akaina is a species of decapod crustacean from the Triassic of the United Arab Emirates. It is the oldest known fossil from the infraorder Anomura, and is most closely related to Eocarcinus praecursor.
Aenigmastacus crandalli is a species of fossil freshwater crayfish. It was found in Eocene lake deposits in British Columbia, and was described in 2011. It is the first member of the Gondwana-distributed family Parastacidae to be found in the Northern Hemisphere, and is the only species in the genus Aenigmastacus. Twelve specimens are known, with a total body length of 3–5 cm (1.2–2.0 in). On some specimens, details of the internal anatomy can be seen due to the exceptional preservation.
Tanzanonautes tuerkayi is a species of fossil freshwater crab from Tanzania, the only species in the genus Tanzanonautes. It is the oldest known freshwater crab, and probably dates from the Oligocene; the next oldest specimens are from the Miocene. A number of fragmentary remains were discovered in the Songwe Valley part of the East African Rift in Mbeya Region, Tanzania, in fragile sandstone sediments. The animal had a carapace around 50 millimetres (2.0 in) across the widest part, which is around 15 mm (0.6 in) from the front of the carapace; the carapace is 44 mm (1.7 in) from front to back, and the slightly inward-curving rear edge of the carapace is 28 mm (1.1 in) across. The genus name refers to Tanzania, the country where the fossil was found, while the specific epithet commemorates Michael Türkay, an authority on freshwater crabs. The genus Tanzanonautes is assigned to the family Potamonautidae, although the characters mentioned in the family's diagnosis are missing from the fragmentary fossils available.
Tricarina is a genus of crustaceans of uncertain phylogenetic placement, known from a single incomplete fossil specimen from the Cretaceous of western Iran. It has a flattened body with three longitudinal ridges, which give it its name.
Mesoparapylocheles is an extinct hermit crab genus which existed during the Mesozoic in what is now Europe. It was described by René H.B. Fraaije, Adiël A. Klompmaker and Pedro Artal in 2012. The type species is Mesoparapylocheles michaeljacksoni from the Albian or Cenomanian of Spain; it was named after the singer Michael Jackson. Genus also includes two species from the Late Jurassic (Kimmeridgian) of Germany and additional two species from the Late Jurassic (Tithonian) of Austria.
The Munididae are a family of squat lobsters, taxonomically separated from the family Galatheidae in 2010.
Cancrocaeca xenomorpha is a species of troglobitic (cave-dwelling) freshwater crab from Sulawesi, the only species in the monotypic genus Cancrocaeca. It has been described as the world's "most highly cave-adapted species of crab".
The Cretaceous crab revolution refers to a major diversification event of Brachyura crabs that took place during the Cretaceous Period, from 145 to 66 million years ago. Nearly 80% of modern groups of crabs originated during this event. The Cretaceous Crab Revolution is a smaller component of the greater Mesozoic Marine Revolution.
References
- 1 2 3 4 5 6 Carrie E. Schweitzer & Rodney M. Feldmann (2010). "Earliest known Porcellanidae (Decapoda: Anomura: Galatheoidea) (Jurassic: Tithonian)" (PDF). Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen . 258 (2): 243–248. doi:10.1127/0077-7749/2010/0096.
- ↑ Carrie E. Schweitzer & Rodney M. Feldmann (2009). "New species of Longodromitidae Schweitzer and Feldmann, 2009, from the Ernstbrunn Formation, Late Jurassic (Tithonian), Austria" (PDF). Annalen des Naturhistorischen Museums in Wien . 117A: 207–224.
- ↑ Robins, Cristina M; Klompmaker, Adiël A (2019-11-14). "Extreme diversity and parasitism of Late Jurassic squat lobsters (Decapoda: Galatheoidea) and the oldest records of porcellanids and galatheids". Zoological Journal of the Linnean Society. 187 (4): 1131–1154. doi:10.1093/zoolinnean/zlz067. ISSN 0024-4082.