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The Decapod Carapace

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.

Explanation of lettering:

Transverse grooves:

a: Branchio-cardiac groove - variable in length, this may or may not extend very far ventrally. There may also be a antero-ventral extension.
c: Post-cervical groove - located behind the cervical groove, this divides the cardiac region into an anterior and posterior part.
e: Cervical groove - divides the gastric and cardiac regions.

Longitudinal grooves

i: Inferior groove - arising from the branchio-cardiac groove, this extends to the side margins of the carapace.
u: Urogastric groove - the posterior branch of the postcervical groove; if present, this is found towards the dorsal surface and is directed more to the midline than the rear (a helpful way of avoiding confusion with the branchio-cardiac groove.
c': Intercervical groove - an anterior projection of the cervical groove.
b1: Hepatic groove - a groove that begins with the antero-ventral extension of the branchio-cardiac groove and joins the post-cervical and cervical grooves.
b: Antennal groove - arising near the antennal spine, this extends backwards to the cervical or hepatic grooves.
d: Gastro-orbital groove - a short projection towards the orbit which branches from the cervical groove.

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.