anomura

The name Anomura derives from an old classification in which reptant 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.{{cite book |author=Gary Poore |year=2004 |title=Marine Decapod Crustacea of Southern Australia: a Guide to Identification |publisher=CSIRO Publishing |isbn=978-0-643-09925-8 |chapter=Anomura – hermit crabs, porcelain crabs and squat lobsters |pages=215–287 |chapter-url=https://books.google.com/books?id=TptuZCY3OU0C&pg=PA215}}

The group has been moulded by several instances of carcinisation – the development of a crab-like body form.{{cite book |editor1=Joel W. Martin |editor2=Keith A. Crandall|editor2-link=Keith A. Crandall |editor3=Darryl L. Felder |year=2009 |title=Decapod Crustacean Phylogenetics |volume=18 |series=Crustacean issues |publisher=CRC Press |author1=Shane T. Ahyong |author2=Kareen E. Schnabel |author3=Elizabeth W. Maas |chapter=Anomuran phylogeny: new insights from molecular data |pages=399–414 |chapter-url=https://books.google.com/books?id=Bxs6SgSW2kQC&pg=PA399 |doi=10.1201/9781420092592 |isbn=978-1-4200-9258-5}} Thus, the king crabs (Lithodidae), porcelain crabs (Porcellanidae) and hairy stone crab (Lomisidae) are all separate instances of carcinisation.

As decapods (meaning ten-legged), anomurans have ten pereiopods, but the last pair of these is reduced in size, and often hidden inside the gill chamber (under the carapace) to be used for cleaning the gills.{{cite journal |author1=Jonas Keiler |author2=Stefan Richter |year=2011 |title=Morphological diversity of setae on the grooming legs in Anomala (Decapoda: Reptantia) revealed by scanning electron microscopy |journal=Zoologischer Anzeiger |volume=250 |issue=4 |pages=343–366 |doi=10.1016/j.jcz.2011.04.004}} Since this arrangement is very rare in true crabs (for example, the small family Hexapodidae),{{cite journal |title=Differentiation of the fossil Hexapodidae Miers, 1886 (Decapoda: Brachyura) from similar forms |author1=Carrie E. Schweitzer |author2=Rodney M. Feldmann |journal=Journal of Paleontology |volume=75 |issue=2 |year=2001 |pages=330–345 |url=http://decapoda.nhm.org/pdfs/10789/10789.pdf|doi=10.1666/0022-3360(2001)075<0330:DOTFHM>2.0.CO;2|s2cid=85997166 }} a "crab" with only eight visible pereiopods is generally an anomuran.

The infraorder Anomura belongs to the group Reptantia, which consists of the walking/crawling decapods (lobsters and crabs). There is wide acceptance from morphological and molecular data that Anomura and Brachyura ("true" crabs) are sister taxa, together making up the clade Meiura. Anomura likely diverged from Brachyura in the Late Triassic period, with the earliest discovered Anomuran fossil Platykotta akaina dating from the Norian–Rhaetian aged Ghalilah Formation of the United Arab Emirates.

The cladogram below shows Anomura's placement within the larger order Decapoda, from analysis by Wolfe et al. (2019).{{cite journal |last1=Wolfe |first1=Joanna M. |last2=Breinholt |first2=Jesse W. |last3=Crandall |first3=Keith A. |last4=Lemmon |first4=Alan R. |last5=Lemmon |first5=Emily Moriarty |last6=Timm |first6=Laura E. |last7=Siddall |first7=Mark E. |last8=Bracken-Grissom |first8=Heather D. |display-authors=6 |date=24 April 2019 |title=A phylogenomic framework, evolutionary timeline and genomic resources for comparative studies of decapod crustaceans |journal=Proceedings of the Royal Society B |volume=286 |issue=1901 |doi=10.1098/rspb.2019.0079 |doi-access=free |pmid=31014217 |pmc=6501934}}

{{Decapoda cladogram}}

Some of the internal relationships within Anomura are shown in the cladogram below, which shows Hippidae as sister to Paguroidea, and resolves Parapaguridae outside of Paguroidea:

{{Anomura cladogram}}

The infraorder Anomura contained seven extant superfamilies:{{cite journal |journal = Raffles Bulletin of Zoology |year=2009 |volume = Suppl. 21 |pages=1–109 |title = A classification of living and fossil genera of decapod crustaceans |author1=Sammy De Grave |author2=N. Dean Pentcheff |author3=Shane T. Ahyong |url=https://decapoda.nhm.org/pdfs/31051/31051.pdf|display-authors=etal}}{{cite journal |author1=Jérôme Chablais |author2=Rodney M. Feldmann |author3=Carrie E. Schweitzer |year=2011 |title=A new Triassic decapod, Platykotta akaina, from the Arabian shelf of the northern United Arab Emirates: earliest occurrence of the Anomura |journal=Paläontologische Zeitschrift |volume=85 |issue=1 |pages=93–102 |doi=10.1007/s12542-010-0080-y |bibcode=2011PalZ...85...93C |s2cid=5612385 |url=http://decapoda.nhm.org/pdfs/31744/31744.pdf }}{{cite journal |author1=K. E. Schnabel |author2=S. T. Ahyong |author3=E. W. Maas |year=2011 |title=Galatheoidea are not monophyletic – molecular and morphological phylogeny of the squat lobsters (Decapoda: Anomura) with recognition of a new superfamily |journal=Molecular Phylogenetics and Evolution |volume=58 |issue=2 |pages=157–168 |doi=10.1016/j.ympev.2010.11.011 |pmid=21095236}}{{cite WoRMS |author=WoRMS |year=2018 |title=Anomura |id=106671 |access-date=2018-09-28}}

The oldest fossil attributed to Anomura is Platykotta, from the Norian–Rhaetian (Late Triassic) Period in the United Arab Emirates.

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Category:Arthropod infraorders

Category:Norian first appearances

Category:Extant Late Triassic first appearances