New anatomical information on <i><scp>A</scp>nomalocaris</i> from the <scp>C</scp>ambrian <scp>E</scp>mu <scp>B</scp>ay <scp>S</scp>hale of <scp>S</scp>outh <scp>A</scp>ustralia and a reassessment of its inferred predatory habits

Abstract

Abstract Two species of A nomalocaris co‐occur in the E mu B ay S hale ( C ambrian S eries 2, S tage 4) at B ig G ully, K angaroo I sland. Frontal appendages of A nomalocaris briggsi N edin, 1995, are more common than those of A nomalocaris cf. canadensis W hiteaves, 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 A nomalocaris canadensis , confirming that A nomalocaris lacks a tetraradial ‘ P eytoia’ oral cone and strengthening the case for the identity of the A ustralian specimens as A nomalocaris . Disarticulated anomalocaridid body flaps are more numerous in the E mu B ay S hale 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 A nomalocaris , 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, A nomalocaris briggsi is inferred to have been a predator of soft‐bodied animals exclusively and only A nomalocaris cf. canadensis may have been capable of durophagous predation on trilobites, although predation (including possible cannibalism) by R edlichia could also explain the coprolites and damage to trilobite exoskeletons found in the E mu B ay S hale.