McLaughlin, Patsy A. Formerly, Shannon Point Marine Center, Western Washington University, Anacortes, Washington.
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A subclass of the crustacean class Malacostraca containing the extant order Leptostraca and the fossil order Archiostraca (see illustration). The Phyllocarida has a long fossil record, and many early fossil taxa were referred to this subclass. However, studies of presumed phyllocarids from the Burgess Shale have shown that only the archiostracans agree with the definition of the Phyllocarida. The apparent antiquity of the Phyllocarida, together with the foliacious structure of their thoracic appendages, gave rise to the hypothesis that fossil phyllocarids encompassed the ancestral stem of the Eumalacostraca. However, eumalacostracan stocks never show such phyllocarid features as thoracic filter-feeding mechanisms and a brood pouch formed with the setose tips of the thoracic endopods. Thus a consensus has been reached that the Phyllocarida represents an early, and perhaps dead-end, offshoot of the Malacostraca rather than the stem group for eumalacostracan evolution.
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