The fossil record of the Conchostraca is one of the oldest among the Branchiopoda, extending from the Lower Devonian. This record consists mostly of fossils of carapaces, and since the major diagnostic features of the clam shrimps relate to this carapace, these fossils are easily classified as conchos-tracans. Nevertheless, these characteristics do not provide details of the evolution within the group.
The Conchostraca and Cladocera belong to the subclass Diplostraca, which, together with the subclasses Calamanos-traca and Sarsostraca, form the three living groups of the class Branchiopoda; two fossil groups, Kazacharthra and Lipos-traca, also belong to this class. The term Branchiopoda literally means "gilled feet," and branchiopods are consequently said to breathe through their feet, which are leaflike and divided into lobes, each containing a gill plate. The presence of gills on the feet of the Branchiopoda is almost the only common characteristic among the diverse members of this group, although minor similarities can be found in the organization of the trunk segments and trunk limbs.
In 1986 the conchostracans were divided into five families of bivalve crustaceans: Lynceidae, Limnadiidae, Cyclestheri-idae, Cyzicidae, and Leptestheriidae. However, the relationships among these families are not well understood and many authors believe that these relationships may not even exist. In fact, the validity of the term Conchostraca as a taxonomic name is debatable, since no evidence has been found that supports a monophyletic origin for clam shrimps. Nevertheless, up to the present, no author has presented a phylogenetically supported or generally accepted reclassification for this group.
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