| Anomura | |
|---|---|
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| Thehermit crabDardanus megistos | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Class: | Malacostraca |
| Order: | Decapoda |
| Suborder: | Pleocyemata |
| Clade: | Reptantia |
| Infraorder: | Anomura Macleay, 1838 |
| Superfamilies | |
Anomura (sometimesAnomala) is a group ofdecapodcrustaceans, includinghermit crabs and others. Several groups such as theking crabs andporcelain crabs within the Anomura have independently undergonecarcinisation, acquiring the armoured crab body plan with a concealed tail. It is thesister group to the "true crabs" orBrachyura.[2]
The name Anomura derives from an old classification in whichreptant decapods were divided into Macrura (long-tailed), Brachyura (short-tailed) and Anomura (differently-tailed). The alternative name Anomala reflects the unusual variety of forms in this group; whereas all crabs share some obvious similarities, the various groups of anomurans are quite dissimilar.[3]
The group has been moulded by several instances ofcarcinisation – the development of a crab-like body form.[4] Thus, theking crabs (Lithodidae),porcelain crabs (Porcellanidae) andhairy stone crab (Lomisidae) are all separate instances of carcinisation.[4]
As decapods (meaningten-legged), anomurans have tenpereiopods, but the last pair of these is reduced in size, and often hidden inside the gill chamber (under thecarapace) to be used for cleaning the gills.[5][3] Since this arrangement is very rare intrue crabs (for example, the small familyHexapodidae),[6] a "crab" with only eight visible pereiopods is generally an anomuran.[3]
Theinfraorder Anomura belongs to the groupReptantia, which consists of the walking/crawling decapods (lobsters and crabs). There is wide acceptance frommorphological andmolecular data that Anomura andBrachyura ("true" crabs) aresister taxa, together making up theclade Meiura.[4] Anomura likely diverged from Brachyura in theLate Triassic period, with the earliest discovered Anomuran fossilPlatykotta akaina dating from theNorian–Rhaetian agedGhalilah Formation of theUnited Arab Emirates.[1]
Thecladogram below shows Anomura's placement within the largerorderDecapoda, from analysis by Wolfeet al. (2019).[7]
| Decapoda |
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Some of the internal relationships within Anomura are shown in the cladogram below, which shows Hippidae assister to Paguroidea, and resolves Parapaguridae outside of Paguroidea:[7]
Brachyura ("true" crabs) | |||||||||||||||||||||||||||||||||||||||||||||||||
| Anomura |
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The infraorder Anomura contained seven extant superfamilies:[8][9][10][11]
The oldestfossil attributed to Anomura isPlatykotta, from theNorian–Rhaetian (Late Triassic) Period in theUnited Arab Emirates.[9]
Subgroups of orderDecapoda | |||||||||||||||||||||||||
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| Dendrobranchiata | 
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| Pleocyemata |
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