Thoracotremata is a clade of crabs, comprising those crabs in which the genital openings in both sexes are on the sternum, rather than on the legs. It comprises 17 families in four superfamilies . Evolution Thoracotremata is the sister group to Heterotremata within the clade Eubrachyura, having diverged during the Cretaceous 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 below: The internal relationships within Thoracotremata are less certain, with many of the superfamilies found to be invalid. The proposed cladogram 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 and invalid. References Crabs