Often the only thing preserved in a decapod fossil is the carapace. The carapace in decapods extends laterally and covers the gills, surrounding them in a branchial chamber (branchiostegite). The anterior part of the carapace is joined to the thorax by a muscle that passed along a groove on the outer surface of the carapace, the branchio-cardiac groove, one of three transverse furrows on the carapace. In front are the post-cervical groove and the cervical groove and it is thought that these three grooves may represent internal segmentation between respectively the thorax and the maxillule; the maxillule and maxilla; and the maxilla and either the mandible or the antennal segment.
It is important to note, however, that the presence of a furrow does not necessarilly indicate an associated muscle or segment boundary, but the funbction of the grooves is still uncertain. The carapace grooves have been given a lettered code which most authors admit is less than perfect. However, it is handy to keep the system for ease in communication, and it is outlined below.
One of the trends seen in the evolution of the Brachyura (crabs) is a reduction in carapace grooves, with all but the Dromiacea losing the cervical and post-cervical grooves (see below).
To learn more about the trends in decapod evolution and to see the development of these grooves, click here.