Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting tail-like abdomen, usually hidden entirely under the thorax. They live in all the world's oceans, in freshwater, and on land, are generally covered with a thick exoskeleton, and have a single pair of pincers on each arm. They first appeared during the Jurassic period, around 200 million years ago.
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.
Cancridae is a family of crabs. It comprises six extant genera, and ten exclusively fossil genera, in two subfamilies:
Retroplumidae is a family of heterotrematan crabs, placed in their own (monotypic) superfamily, Retroplumoidea.
Dromioidea is a superfamily of crabs mostly found in Madagascar. Dromioidea belongs the group Dromiacea, taxonomically ranked as a section, which is the most basal grouping of Brachyura crabs. Dromiacea likely diverged from the rest of Brachyura around the Late Triassic or Early Jurassic, and the earliest fossils attributable to the Dromioidea date from the Late Jurassic.
Etyiidae is a prehistoric family of dromiacean crabs only known from Cretaceous and a few Paleocene fossils.
Dromiacea is a group of crabs, ranked as a section. It contains 240 extant and nearly 300 extinct species. Dromiacea is the most basal grouping of Brachyura crabs, diverging the earliest in the evolutionary history, around the Late Triassic or Early Jurassic. Below is a cladogram showing Dromiacea's placement within Brachyura:
Carcineretidae is a prehistoric family of heterotrematan crustaceans. They are only known from Cretaceous fossils. These crabs are tentatively placed in the superfamily Portunoidea and resemble the swimming crabs (Portunidae) in having some paddle-shaped pereiopods. But it is not certain that this placement is correct, as the Carcineretidae also show some similarities to the Matutidae of superfamily Leucosioidea and the Goneplacidae of superfamily Xanthoidea.
Homolodromiidae is a family of crabs, the only family in the superfamily Homolodromioidea. In contrast to other crabs, including the closely related Homolidae, there is no strong linea homolica along which the exoskeleton breaks open during ecdysis. The family comprises two genera, Dicranodromia, which has 18 species, and Homolodromia, with five species.
Carpilioidea is a superfamily of crabs containing a single extant family, Carpiliidae and three extinct families. The modern range of the family includes the Indo-Pacific, Western Atlantic and Caribbean Sea. The fossil record of the group extends back at least as far as the Paleocene.
Goneplacidae is a family of crabs of the order Decapoda and the superfamily Goneplacoidea. It includes the following genera:
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.
Componocancer roberti is an unusual species of fossil crab described in 2008. It lived in the Albian age in what is now Montana. The species is unlike any other described crab, and is therefore placed in its own family and superfamily.
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.
Dakoticancroidea is a superfamily of fossil crabs, containing six species in five genera, divided into two families. The family Dakoticancridae is only known from North American rocks of Late Cretaceous age, while the single species in the Ibericancridae was found in Spain.
Pseudozioidea is a superfamily of crabs, formerly treated in the Eriphioidea, Carpilioidea, Xanthoidea, Pilumnoidea and Goneplacoidea. A number of fossils from the Eocene onwards are known from the family Pseudoziidae. Eleven genera are recognised in three families:
Hepatus is a genus of crabs in the family Aethridae, containing seven extant species, plus some fossil species:
The Galatheoidea are a superfamily of decapod crustaceans comprising the porcelain crabs and some squat lobsters. Squat lobsters within the three families of the superfamily Chirostyloidea are not closely related to the squat lobsters within the Galatheoidea. The fossil record of the superfamily extends back to the Middle Jurassic genus Palaeomunidopsis.
Cyclida is an extinct order of crab-like fossil arthropods that lived from the Carboniferous to the Jurassic and possibly Cretaceous. Their classification is uncertain, but they are generally interpreted as crustaceans, likely belonging to the superclass Multicrustacea.
The Cretaceous crab revolution was a major diversification event of brachyuran 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.
