LIFE IN THE VICINITY OF A CAMBRIAN REEF: INSIGHTS FROM THE MID-DYERAN MCDAME KONSERVAT-LAGERSTAETTE OF BRITISH COLUMBIA, CANADA (Invited Presentation)

Burgess Shale-type (BST) deposits are critical to our understanding of the taxonomic and ecological diversification of Cambrian animals. However, most BST Konservat-Lagerstätten were deposited in quiet, relatively deep-water marine settings near the shelf break, resulting in an impoverished, somewhat biased picture of the diversity of faunal communities inhabiting the continental shelf at that time. We report the discovery of a new mid-Dyeran (Cambrian Stage 4) Konservat-Lagerstätte within the Rosella Formation (Cassiar Mountains, western Canada), which preserves a biota that lived adjacent to archaeocyathan reefs.

The Dease River section exposes a c. 60 m-thick succession of predominantly skeletal limestone, interrupted by shaly intervals yielding non-biomineralized fossils. Sedimentological and biostratinomic features of the shaly intervals suggest event-driven, energetic deposition in a muddy shelf environment near archaeocyath-rich bioherms. The biota comprises over 30 predominantly soft-bodied species, including panarthropods, scalidophorans, vetulicolians, annelids, brachiopods, cnidarians, entoprocts, hyoliths, and algae. Except for abundant, benthic olenelloid trilobites, panarthropods are rare and mostly represented by free-swimming forms such as radiodonts and isoxyids. The soft-bodied assemblage is dominated by ecologically diverse, millimetre to centimetre-sized worms, including tubicolous forms and others presumably preserved in lined burrows. Surprisingly, no sponge remains were found. The non-biomineralized fossils are preserved as carbonaceous compressions with subordinate apatite and pyrite. Millimetric to centimetric horizontal burrows occur across much of the shaly intervals, either within sediment or in direct association with non-biomineralized carapaces. Along with possible pellets, they document the activity of a vermiform epifauna to shallow infauna. The characteristics of the McDame Biota sharply contrast with most Cambrian BST assemblages, which suggests important variations in composition and structure of benthic assemblages along the shelf.