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 freshwater, and on land, are generally covered with a thick exoskeleton, and have a single pair of pincers. They first appeared during the Jurassic Period.
Hermit crabs are anomuran decapod crustaceans of the superfamily Paguroidea that have adapted to occupy empty scavenged mollusc shells to protect their fragile exoskeletons. There are over 800 species of hermit crab, most of which possess an asymmetric abdomen concealed by a snug-fitting shell. Hermit crabs' soft (non-calcified) abdominal exoskeleton means they must occupy shelter produced by other organisms or risk being defenseless.
The Achelata is an infra-order of the decapod crustaceans, holding the spiny lobsters, slipper lobsters and their fossil relatives.
The Glypheoidea, is a group of lobster-like decapod crustaceans which forms an important part of fossil faunas, such as the Solnhofen limestone. These fossils included taxa such as Glyphea, and Mecochirus, mostly with elongated chelipeds. This group of decapods is a good example of a living fossil, or a lazarus taxon, since until their discovery in the 1970s, the group was considered to have become extinct in the Eocene. The superfamily Glypheoidea comprises five families. The two extant species, Neoglyphea inopinata and Laurentaeglyphea neocaledonica, are both in the family Glypheidae.
Aeger is a genus of fossil prawns. They first occur in the Early Triassic, and died out at the end of the Late Cretaceous. A total of 21 species are known.
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.
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:
Polychelida is an infraorder of decapod crustaceans. Fossil representatives are known dating from as far back as the Upper Triassic. A total of 38 extant species, all in the family Polychelidae, and 55 fossil species have been described.
Eubrachyura is a group of decapod crustaceans comprising the more derived crabs. It is divided into two subsections, based on the position of the genital openings in the two sexes. In the Heterotremata, the openings are on the legs in the males, but on the sternum in females, while in the Thoracotremata, the openings are on the sternum in both sexes. This contrasts with the situation in other decapods, in which the genital openings are always on the legs. Heterotremata is the larger of the two groups, containing the species-rich superfamilies Xanthoidea and Pilumnoidea and all the freshwater crabs. The eubrachyura is well known for actively and constantly building its own burrows. The fossil record of the Eubrachyura extends back to the Cretaceous; the supposed Bathonian representative of the group, Hebertides jurassica, ultimately turned out to be Cenozoic in age.
Erymidae is a family of decapod crustaceans known only from fossils. They survived for 100 million years, from the Permo-Triassic boundary to the Albian. Eleven genera are recognised:
Corystidae is a family of crabs, in its own superfamily, Corystoidea. It includes what was once thought to be the oldest Eubrachyuran fossil, Hebertides jurassica, thought to be dating from the Bathonian ; the species was subsequently reinterpreted as being Cenozoic in age. Corystidae contains ten extant and five extinct species in eight genera:
Acanthochirana is an extinct genus of prawn that existed during the upper Jurassic period. It was named by E. Strand in 1928, and its type species is Acanthochirana cordata. They are distinguished from the related genus Aeger by the presence of teeth on the rostrum, which are absent in Aeger.
Aegeridae is a family of fossil prawns, one of the earliest Mesozoic shrimp families. It contains the genera Aeger, Acanthochirana, Anisaeger and Distaeger. The main diagnostic character of Aegeridae is the presence of numerous spines or thin setae on the third maxilliped.
Antrimpos is an extinct genus of crustacean which existed during the Triassic and Jurassic periods. It contains 15 species, including Antrimpos speciosus.
Carpopenaeus is an extinct genus of prawn, which existed during the Upper Jurassic and Cretaceous periods. It contains three species.
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:
Longichela orobica is an extinct species of prawn which lived in the Norian, and is the only species in the genus Longichela.
Acanthinopus gibbosus is an extinct species of shrimp placed in its own genus, Acanthinopus, which has not been assigned to a family. It was found in Norian sediments of the Zorzino Limestone in northern Italy.
Hefriga is an extinct genus of shrimp in the order Decapoda. It contains three species, and lived in the Jurassic period.
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.
