New Anatomical Information on 'Anomalocaris' from the Cambrian Emu Bay Shale of South Australia and a Reassessment of its Inferred Predatory Habits

Abstract

Two species of 'Anomalocaris' co-occur in the Emu Bay Shale (Cambrian Series 2, Stage 4) at Big Gully, Kangaroo Island. Frontal appendages of 'Anomalocaris briggsi' Nedin, 1995, are more common than those of 'Anomalocaris cf. canadensis' Whiteaves, 1892, at a quarry inland of the wave-cut platform site from which these species were originally described. An oral cone has the three large, node-bearing plates recently documented for 'Anomalocaris canadensis', confirming that 'Anomalocaris' lacks a tetraradial 'Peytoia' oral cone and strengthening the case for the identity of the Australian specimens as 'Anomalocaris'. Disarticulated anomalocaridid body flaps are more numerous in the Emu Bay Shale than in other localities, and they preserve anatomical details not recognized elsewhere. Transverse lines on the anterior part of the flaps, interpreted as strengthening rays or veins in previous descriptions of anomalocaridids, are associated with internal structures consisting of a series of well-bounded, striated blocks or bars. Their structure is consistent with a structural function imparting strength to the body flaps. Setal structures consisting of a series of lanceolate blades are similar to those of other anomalocaridids and are found in isolation or associated with body flaps. A single specimen also preserves putative gut diverticula. The morphology of the appendages, oral cone, gut diverticula and compound eyes of 'Anomalocaris', along with its large size, suggests that it was an active predator, and specimens of coprolites containing trilobite fragments and trilobites with prominent injuries have been cited as evidence of anomalocaridid predation on trilobites. Based on frontal appendage morphology, 'Anomalocaris briggsi' is inferred to have been a predator of soft-bodied animals exclusively and only 'Anomalocaris cf. canadensis' may have been capable of durophagous predation on trilobites, although predation (including possible cannibalism) by 'Redlichia' could also explain the coprolites and damage to trilobite exoskeletons found in the Emu Bay Shale.