Thoracotremata is a

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

crab Crabs are decapod crustaceans of the infraorder Brachyura (meaning "short tailed" in Greek language, Greek), which typically have a very short projecting tail-like abdomen#Arthropoda, abdomen, usually hidden entirely under the Thorax (arthropo ...

s, comprising those crabs in which the genital openings in both sexes are on the

sternum The sternum (: sternums or sterna) or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major bl ...

, rather than on the legs. It comprises 17 families in four superfamilies .

Evolution

Thoracotremata is the

sister group In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...

to Heterotremata within the clade Eubrachyura, having diverged during the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

period. Eubrachyura itself is a subset of the larger clade Brachyura, which consists of all "true crabs". A summary of the high-level internal relationships within Brachyura can be shown in the

cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

below: The internal relationships within Thoracotremata are less certain, with many of the superfamilies found to be invalid. The proposed

below is from analysis by Tsang ''et al'', 2014:

Superfamilies and families

* Ocypodoidea ** Camptandriidae ** Dotillidae ** Heloeciidae ** Macrophthalmidae ** Mictyridae ** Ocypodidae ** Ucididae ** Xenophthalmidae * Pinnotheroidea ** Pinnotheridae * Cryptochiroidea ** Cryptochiridae * Grapsoidea ** Gecarcinidae ** Glyptograpsidae ** Grapsidae ** Plagusiidae ** Sesarmidae ** Varunidae ** Xenograpsidae However, recent studies have found the superfamilies Grapsoidea and Ocypodoidea to be

polyphyletic A polyphyletic group is an assemblage that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor. The term is often applied to groups that share similar features known as Homoplasy, homoplasies ...

and invalid.

References

Crabs {{Eubrachyura-stub