Autapomorphies of the endophragmal system in trichodactylid freshwater crabs (Crustacea: Decapoda: Eubrachyura).

An examination of the thoracic endophragmal system in trichodactylid freshwater crabs revealed several derived features that appear restricted to this family. Trichodactylids have a highly reduced sella turcica with a rim-like overall configuration; as a consequence of sella turcica reduction the median septum is dorsoventrally diminished and exposed along its length. In addition, interosternite 7/8 is medially erased with the remnants restricted to the lateral regions of the sternum. The configurations of the sella turcica and interosternite 7/8 in trichodactylids underscore the monophyly of this family. The Dilocarcininae is characterized by having interosternites 5/6 through 7/8 restricted to the lateral sectors of the sternum, the lateral remnants of interosternite 4/5 forming prominent, rib-like ridges oriented diagonally to the longitudinal axis of the crab, and the median septum forming with interosternite 4/5 a Y-shaped pattern. These autapomorphies of the endophragmal system separate the trichodactylids from all other eubrachyurans and emphasize the morphological distinctness of the group. A description of the trichodactylid endophragmal junction plate is also provided.