COPROLITES OF THE RAVENS THROAT RIVER LAGERSTÄTTE: UNDERSTANDING THE MIDDLE CAMBRIAN FOOD CHAIN AND ENVIRONMENT

Burgess Shale-type (BST) Lagerstätten are early Paleozoic deposits with extraordinary preservation of soft tissues. While the fossils have been extensively studied, their mode of preservation is still controversial. The Ravens Throat River Lagerstätte, of Drumian age in the Mackenzie Mountains, is not metamorphosed and therefore ideal for investigating the ecological and taphonomic factors behind BST preservation. Host rock calcareous mudstones were deposited on the eastern flank of the Selwyn Basin of northwestern Canada. The deposit is relatively specimen-rich but species-poor which probably reflect the deep-water environment of deposition. In contrast to other well-known BST deposits, this Lagerstätte conserves mainly robust parts, although the sediments also contain large coprolites that are markedly carbonaceous. Three different coprolite morphotypes are exhibited by 127 recovered specimens, all consisting of a black carbon film that itself consists typically of variably distinct, flattened pellet aggregates. On the bedding plane most coprolites are circular and up to 5 cm in diameter, while some appear to be elongated and others partly disaggregated. They occur throughout the main fossiliferous bed of the deposit, and are found in association with trilobites, agnostoid arthropods and hyoliths. In many cases these associated fossils are preserved fully articulated and appear to be in feeding position. The origin of the coprolites is uncertain, because the specimens are relatively large, and no apex predators have been recovered from the deposit, but the composition of the coprolites does offer some clues. One unique feature of the Ravens Throat River Lagerstätte is the unexpectedly high concentration of Ag in the fossiliferous bed (up to 0.45 ppm), and even higher concentrations in some coprolites. Elemental Ag occurs as crystals ranging in size from a few to several hundred micrometers. Both algae and bacteria have the potential to scavenge noble metals out of the water column. The extensive accumulation of Ag in the coprolites in this Lagerstätte may be explained by enrichment in the adjacent Selwyn Basin due to exhalative mineralization during the Cambrian.