The order Anaspidacea contains four families that have sometimes been divided into two suborders, the Anaspidinea and Stygocaridinea. The family Anaspididae has been known since the Triassic and exhibits strong relationships to the extinct order Palaeocaridacea. Anaspidaceans are generally thought to be among the most primitive of eumalacostracan crustaceans (that is, members of the subclass Eumalacostraca), especially because their thoracic limbs possess both epipods and exopods. However, details of their circulatory system, foregut morphology, and other aspects of their internal anatomy suggest that anaspidaceans are derived from ancestors leading to the eucaridan decapods. The lack of a carapace has caused debate about their exact placement among eumalacostracan crustaceans, but anaspidaceans are generally thought to be ancestral to at least the line leading to decapods, if not some of the other eumalacostracan groups as well.
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