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ANOMALOCARIS PREDATION ON MINERALIZED AND NON-MINERALIZED TRILOBITES FROM THE EARLY CAMBRIAN EMU BAY SHALE, SOUTH AUSTRALIA

CHRISTOPHER NEDIN

Department of Geology and Geophysics, University of Adelaide, South Australia 5005

Nedin, C. 1997. Anomalocaris predation on mineralized and non-mineralized trilobites from the Early Cambrian Emu Bay Shale, South Australia. Second International Trilobite Conference, Brock University, St. Catharines, Ontario, Canada, Abstracts with Program, p. 37.

ABSTRACT

The giant Cambrian arthropod Anomalocaris has been cast as the villain in several cases of trilobite predation, based mainly on its size, the possession of presumed raptorial appendages and its unusual jaw structure. Pre-eminent is the unusual jaw morphology, seen as the only known coeval structure capable of inflicting characteristically arcuate-, wedge- or "W"-shaped bite marks exhibited by some trilobite fossils. However, doubt remains as to the duophagus abilities of Anomalocaris. Specifically, how could the jaw apparatus inflict such wounds on biomineralized skeletons, since the jaws, whilst stout, appear to have been poorly schlerotized, and the teeth could probably not occlude? Indeed it has been suggested that anomalocaridids preyed primarily on the abundant non-mineralized fauna.

New insights into the predation method of Anomalocaris comes from the Early Cambrian (Botomian) Emu Bay Shale, which includes the mineralised and non-mineralised trilobites Redlichia and Naraoia and two species of Anomalocaris, with the first body fossil record of predation on non-mineralised organisms. A specimen of Naraoia exhibits a large, wedge-shaped bite mark on the right side. Opposite and slightly anterior of the bite mark, the left lateral margin also exhibits damage, being pinched inwards. This is interpreted as a 'holding scar' caused by one of the grasping appendages of Anomalocaris, where it held the specimen while delivering the bite. Examination of other fossils, similarly predated upon, confirms the presence of 'holding scars' in association with the bite mark.

The configuration of the predation damage to Naraoia allows comparison with the structure and rheology of modern 'composite' arthropod cuticle, indicating that the appendages applied significant force and were thus actively involved in the post-capture predation process, rather than passively holding the prey. This process probably involved manipulation of the prey exoskeleton by the appendages, pivoting about a point where the jaws locked onto the organism. Such flexure would reverse the normal stress fields experienced by the layers of the cuticle, weakening them considerably. In this weakened state, pressure from the jaws would probably have been enough to fracture the mineralised cuticle, imparting a durophagus ability to Anomalocaris.
Only one of the Anomalocaris species found in the Emu Bay Shale appears capable of such a pradation strategy, suggests niche partitioning was already present within the anomalocaridids by the upper Early Cambrian.

The possibility that a non-mineralised form such as Anomalocaris could practice durophagy has major implications for the importance of predation in the aquisition of mineralised exoskeletons and the escalation of predator/prey strategies. Especially considering that there is one defence against which this type of predation would be inaffective - enrollment.