ON THE FEEDING BEHAVIOR OF ANOMALOCARIS: A SPECTRUM OF ADAPTATIONS ELUCIDATED BY THE FORM OF SPINES ON THE ‘GREAT APPENDAGE’ OF A NEW FORM FROM THE KINZERS FORMATION (EARLY CAMBRIAN) OF SOUTH-EASTERN PENNSYLVANIA

On account of its large size and morphology, Anomalocaris has been interpreted as the top predator in soft-bottom marine faunas of the Early and early Middle Cambrian. For a while, it was supposed to have preyed on the abundant trilobites of that time. However, given the form of its mouth and the mode of operation of its teeth, it is now thought to have been incapable of breaking through the tough carapace of a trilobite (Hagadorn, 2009). Moreover, the ‘great appendages’ are not designed as claws to be used in manipulating and crushing prey. Functional analysis suggests instead that Anomalocaris fed mainly on the worms of several kinds that were so abundant in Burgess Shale-type faunas. Worms would have been a natural target for a predator that could find them and extract them from their burrows. The form of the ‘great appendages’ and their spines is consistent with their use in searching for shallow infaunal prey. Likewise, the form of the mouth was well suited to extract and ingest worms.

Four species of Anomalocaris have so far been recognized, notably from the Burgess Shale of British Columbia, the Maotianshan Shale at Chengjiang, and the Emu Bay Shale in Australia. Anomalocaris pennsylvanica occurs in the Kinzers Formation of Lancaster County, Pennsylvania. Now, a recent find appears to represent a previously undescribed species. The new specimen, a single ‘great appendage’, is of particular interest on account of the nature of its ventral spines. These spines are longer than those of any species so far described, they are narrow and uniformly spaced, and they lack any evidence of auxiliary spines.

The variety of forms of ventral spines attached along the ’great appendage’ of different species of Anomalocaris implies a range of actions. Long flexible spines have properties that would have enabled the appendages to sweep loose sediment; short rigid spines would have been effective in probing coarser or stiffer materials. The new specimen represents an end member on this spectrum of forms; spines of established species represent the middle and the other end of this range. Recognition of an end member on a spectrum of ’great appendage’ form and function opens a window on the behavior of Anomalocaris species, adapted to feed on worms and other organisms living in substrates of different consistencies.