Eubrachyura is a group of crustacean

Crustacean

Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

s (ranked

Taxonomic rank

In biological classification, rank is the level in a taxonomic hierarchy. Examples of taxonomic ranks are species, genus, family, and class. Each rank subsumes under it a number of less general categories...

 as a "section") comprising the more advanced crab

Crab

True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" , or where the reduced abdomen is entirely hidden under the thorax...

s. It is divided into two subsections, based on the position of the genital openings in the two sexes. In the Heterotremata

Heterotremata

Heterotremata is a clade of crabs, comprising those crabs in which the genital openings are on the sternum in females, but on the legs in males. It comprises 68 families in 28...

, the openings are on the legs

Arthropod leg

The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments are of Latin origin, and may be confused with terms for bones: coxa , trochanter , femur, tibia, tarsus, ischium, metatarsus, carpus, dactylus ,...

 in the males, but on the sternum

Sternum (arthropod)

The sternum is the ventral portion of a segment of an arthropod thorax or abdomen.In insects, the sterna are usually single, large sclerites, and external...

 in females, while in the Thoracotremata

Thoracotremata

Thoracotremata is a clade of crabs, comprising those crabs in which the genital openings are on the sternum, rather than on the legs in both sexes. It comprises 17 families in four superfamilies .-Superfamilies and...

, the openings are on the sternum in both sexes. This contrasts with the situation in other decapods

Decapoda

The decapods or Decapoda are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crayfish, crabs, lobsters, prawns and shrimp. Most decapods are scavengers. It is estimated that the order contains nearly 15,000 species in around 2,700 genera, with...

, in which the genital openings are always on the legs. Heterotremata is the larger of the two groups, containing the species rich superfamilies Xanthoidea

Xanthoidea

Xanthoidea is a superfamily of crabs, comprising the three families Xanthidae, Panopeidae and Pseudorhombilidae. Formerly, a number of other families were included in Xanthoidea, but many of these have since been removed to other superfamilies. These include Carpilioidea, Eriphioidea, Hexapodoidea,...

 and Pilumnoidea

Pilumnoidea

Pilumnoidea is a superfamily of crabs, whose members were previously included in the Xanthoidea. The three families are unified by the free articulation of all the segments of the male crab's abdomen and by the form of the gonopods...

 and all the freshwater crab

Freshwater crab

There are around 1,300 species of freshwater crabs, distributed throughout the tropics and subtropics, divided among eight families. They show direct development and maternal care of a small number of offspring, in contrast to marine crabs which release thousands of planktonic larvae. This limits...

s (Gecarcinucoidea

Gecarcinucoidea

Gecarcinucoidea is a superfamily of freshwater crabs. Its members have been grouped into families in various ways, with some authors recognising families such as "Deckeniidae", "Sundathelphusidae", but only two families are currently recognised: Gecarcinucidae and Parathelphusidae....

, Potamoidea

Potamoidea

Potamoidea is a superfamily of freshwater crabs, comprising the two families Potamidae and Potamonautidae. Two previously recognised families, Deckeniidae and Platythelphusidae, are now treated as parts of the family Potamonautidae....

). The fossil record of Eubrachyura extends back to the Bathonian

Bathonian

In the geologic timescale the Bathonian is an age or stage of the Middle Jurassic. It lasted from approximately 167.7 Ma to around 164.7 Ma...

 (Middle Jurassic

Middle Jurassic

The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from 176-161 million years ago. In European lithostratigraphy, rocks of this Middle Jurassic age are called the Dogger....

).