The morphology, mode of life, and affinities of Canadaspis perfecta (Crustacea: Phyllocarida), Middle Cambrian, Burgess Shale, British Columbia

A detailed description and reconstruction of Canadaspis perfecta demonstrates its status as the earliest well-preserved crustacean. The cephalon consisted of five somites (in addition to the eyes), the thorax eight, and the abdomen seven, excluding the telson. Two pairs of apparently uniramous antennae flanked a median cephalic spine. The mandible bore a massive incisor process posterior of a molar area made up of finer spines, and apparently lacked a palp. The first and second maxillae were essentially similar to the eight pairs of thoracopods, with a multisegmented inner ramus, and foliaceous outer ramus made up of wide filaments attached to a proximal lobe. A bivalved carapace covered the thorax; no rostral plate was present. The abdomen lacked appendages, apart from a pair of spinose ventral projections of the pre-telson somite. There was no caudal furca. The evidence suggests that C. perfecta fed on coarse particles, possibly with the aid of currents set up by the biramous appendages. The erection of a new order Canadaspidida and family Canadaspididae Novozhilov (in Orlov 1960) to include Canadaspis is vindicated, and they are re-defined and the subclass Phyllocarida amended to include them.