The early Cambrian Emu Bay Shale radiodonts revisited: morphology and systematics

Abstract

Two species of Radiodonta (stem-group Euarthropoda) from the Emu Bay Shale (Cambrian Series 2, Stage 4), Kangaroo Island, South Australia, are revised based on new field collections and insights from recent phylogenetic analyses and advances in knowledge of radiodonts globally. Anomalocaris briggsi Nedin, 1995, the most common Emu Bay Shale radiodont, is designated the type species of a new monotypic genus of Tamisiocarididae, Echidnacaris gen. nov. The less common species, previously identified as Anomalocaris aff. canadensis Whiteaves, 1892, is formally named Anomalocaris daleyae sp. nov. Oral cones are assigned to both Echidnacaris briggsi comb. nov. and A. daleyae based on that of the latter species being found in association with pairs of frontal appendages. The Echidnacaris briggsi oral cone is the best preserved for the family Tamisiocarididae; it is triradial, with three large plates and a more pervasive ornament of nodes than in any other known radiodont. Shared characters of the Echidnacaris and Anomalocaris oral cones add support for a sister group relationship between Tamisiocarididae and Anomalocarididae. Unique eye characters documented in E. briggsi, such as being sessile and encircled by an eye sclerite, are unknown in the other tamisiocaridids, Tamisiocaris and Houcaris, and are tentatively regarded as diagnostic for Echidnacaris. An ovate head element resembling that of Tamisiocaris borealis is assigned to E. briggsi, informed by the sister group relationship between these taxa. Isolated radiodont body flaps and sets of setal blades in the Emu Bay Shale cannot be confidently assigned to a species, although relative abundance suggests that many or most are likely E. briggsi. The inner attachment margin of the body flaps is sharply defined and may represent a suture at which flaps are shed in moulting.