Phyllocarida

A subclass of the crustacean class Malacostraca containing the extant order Leptostraca and the fossil order Archiostraca. 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.

Phyllocarids are distinct from other malacostracan crustaceans because of two other characteristics considered to reflect the primitive condition, which strengthen the hypothesis of early separation from the main evolutionary line. The first is the presence of a bivalve carapace. The second is an abdomen consisting of seven fully formed somites and terminating in a telson that bears caudal rami. See also Crustacea; Leptostraca; Malacostraca.