PROBABLE PREDATION ON UPPER CAMBRIAN TRILOBITES AND ITS RELEVANCE FORTHE EXTINCTION OF SOFT-BODIED BURGESS SHALE-TYPE ANIMALS

The lower Rabbitkettle Formation of northwestern Canada is a monofacia l Upper Cambrian unit of variably calcareous, argillaceous siltstone a nd fine-grained sandstone with rare bioclastic grainstone, deposited o n a gentle slope below fair-weather wave base with no discernible fluc tuation in water depth. The trilobite fauna is a mixture of pandemic a gnostoids and Lau rentian polymeroids. including protaspides and meras pides, and individuals are disarticulated, non-abraded and mostly orie nted convex-upward. Bioclasts are interpreted as in situ elements affe cted only by weak bottom currents and storm-induced turbulence. A majo r proportion of the larger (greater than or equal to 5 mm across) poly meroid cranidia and pygidia in the lower parr (Marjuman) of the format ion are broken; large thoracic segments are often broken at the axial furrow and some broken free cheeks occur, but essentially no broken ag nostoids or hypostomes were observed. Trilobites are not broken in upp er beds (Steptoean), above the base of the Glyptagnostus reticulatus Z one. Physical breakage cannot be dismissed entirely, but most damage i s interpreted to be due to size-selective predation, possibly through lethal blows similar to those delivered by some extant stomatopod crus taceans. A possible culprit may be an animal akin to Yohoia, known fro m the Middle Cambrian Burgess Shale. The distribution of attacked tril obites serves as a prosy for the presence and disappearance of soft-bo died carnivores. In the Rabbitkettle Formation, it suggests that Burge ss Shale-type animals may have persisted into the Late Cambrian but su ffered extinction at the Marjuman-Steptoean 'biomere' event when most trilobite species vanished.