A MIDDLE CAMBRIAN MASS-KILL RECORDED IN THE PENGUIN COVE FORMATION, WESTERN NEWFOUNDLAND

A large (~80 x 40 x 20 cm), loose block of the Penguin Cove Formation in the North Brook Anticline of western Newfoundland is host to an unusual layer, crowded with complete specimens of the Middle Cambrian dorypygid trilobite Kootenia. At least forty individuals were counted in a 32 x 19 cm area on the block; most of these vary in length from 3.6 to 4.2 cm. The block came from a nearby outcrop of heterolithic, largely unfossiliferous, thinly bedded succession of mudstone, shale, siltstone and sandstone with a few beds of hummocky sandstone, rare limestone laminae and a cluster of thrombolitic dolostone mounds. Trilobite remains typically are preserved as incomplete, disarticulated molts or fragments. Consequently, these complete, articulated trilobites represent a death assemblage. The mass mortality possibly occurred because of oxygen-deprivation, caused most likely by either a catastrophic release of toxic gas from the sea floor, or the trilobites' chance entry into a massive body of anoxic water, or exposure to fresh water brought to the shelf during fluvial input onto the shelf. Most of the Kootenia on the large block are of similar size, indicating that they probably were of about the same age. This suggests that, like some modern day shrimp, Kootenia may have lived in 'schools'. The trilobites indicate that the rocks of North Brook – those in the immediate vicinity of the block, at least – were deformed by pure shear (irrational or coaxial strain); the principal strain Y'/X' ratio is between 0.67 and 0.68.