A DIVERSE FOSSIL ASSEMBLAGE FROM THE EARLY NEOPROTEROZOIC REEFAL ASSEMBLAGE OF THE FIFTEENMILE GROUP, COAL CREEK INLIER, YUKON

Organic-walled fossils from well-constrained strata of the early Neoproterozoic are rare, especially in comparison with the abundance of younger Tonian, Cryogenian, and Ediacaran assemblages. We report organic-walled eukaryotic and bacterial fossils from macerated carbonates of the early Neoproterozoic (pre-811 Ma) Reefal Assemblage of the Fifteenmile Group in the Coal Creek Inlier, Yukon. The Reefal Assemblage consists of ~500 meters of mixed siliciclastic and carbonate rocks deposited in stromatolitic fore-reef, reef core and back-reef environments. Fossils were extracted from fore-reef organic-rich molar tooth micritic limestones more than 400 meters below the 811 Ma ash bed in the upper Reefal Assemblage. This assemblage thus predates the ‘scale’ microfossils found in the uppermost Reefal Assemblage.

Fossils include organic filaments, which may represent both bacterial and eukaryotic organisms, and acritarch taxa inferred to be eukaryotic. Non-branching septate filaments are similar to fossils from the Mesoproterozoic Lower Belt Supergroup (Montana), which have been interpreted as oscillatorian cyanobacterial sheaths. A variety of non-branching, non-septate filaments are similar to specimens found in early Neoproterozoic strata including the Little Dal Formation (Northwest Territory, Canada). The acritarch assemblage contains both smooth-walled specimens and those with asymmetrical processes, thus making it similar to assemblages from late Mesoproterozoic to early Neoproterozoic strata. Both groups of acritarchs display medial splitting and well-defined, uniform holes that represent possible excystment structures.

While fossils reported here are syngenetic, we also discovered significant fungal contamination in some samples. To determine the extent of contamination, we stained macerated samples with Calcifluor White, which binds to the chitin in fungal mycelia, and fluoresces under UV light. Using this technique, we were able to differentiate between contaminants and syngenetic material. The use of stains such as Calcifluor White can thus be a helpful aid in the process of determining potential contamination in macerated samples.