The order Decapoda falls within the class Malacostraca, a group of crustaceans that comprises twice as many species as all other crustacean classes combined. Malacostracans are distinguished by bodies divided into 19 segments (five head, eight thoracic, and six abdominal segments) and the location of their genital openings. Many of the divisions within the Malacos-traca are based in part on the number of thoracic appendages that have been integrated into the head region to function as mouthparts; these appendages are termed maxillipeds. In the decapods, the first three thoracic appendages are maxillipeds, leaving five pairs of legs or ten feet for walking. The order name Decapoda literally means "ten-footed."
The two suborders within the Decapoda are based in part on differences in the gill structure. The gills of the Dendro-branchiata consist of bundles of branching filaments, while those of the Pleocyemata are unbranched— either as filaments (trichobranchs) or more commonly as unbranched leaflike plates (phyllobranchs). The two groups also differ in their method of reproduction: female Pleocyemata brood their eggs on their abdominal appendages until they hatch, while female Dendrobranchiata spawn their eggs into the sea.
Many of the subsequent divisions within the Pleocyemata can be recognized by the number and position of appendages with claws. The classification of crustaceans remains in flux, however, reflecting the great degree of uncertainty as of 2003 about the relationships among the groups. The following list represents one of the more common classifications of the major groups within the Decapoda:
• Suborder Dendrobranchiata
• Superfamily Penaeoidea, the penaeid shrimps
• Superfamily Sergestoidea, the sergestids
• Suborder Pleocyemata
• Infraorder Caridea, the true shrimps
• Infraorder Stenopodidea, the boxer shrimps
• Infraorder Astacidea, the clawed lobsters and crayfish
• Infraorder Thalassinidea, the ghost and mud shrimps
• Infraorder Palinura, the spiny and slipper lobsters
• Infraorder Anomura, the hermit crabs, king crabs, and porcelain crabs
• Infraorder Brachyura, the true crabs
The earliest decapod to appear in the fossil record is Palaeopalaemon newberryi, which lived during the late Devonian period, about 360 million years ago (mya). It shares characters of both lobsters and shrimps, leaving unanswered questions about its taxonomic affiliations. The Dendro-branchiata are considered the most primitive extant decapods, but early fossils of dendrobranchs are lacking. Fossil representatives of all of the infraorders except the Stenopo-didea are known from the Triassic (213 mya) and Jurassic (145 mya) periods onward, and the Brachyura have shown extensive radiation (diversification) since the Eocene epoch, 33.7 mya.
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