Paleobiology of the late Mesoproterozoic (ca. 1200 Ma) Hunting Formation, Somerset Island, arctic Canada

Silicified shallow water carbonates of the late Mesoproterozoic (ca. 1200 Ma) Hunting Formation of Somerset Island, arctic Canada, preserve four distinct fossil assemblages. Three of these are dominated by prokaryotic fossils, whereas the fourth includes abundant populations of the red alga Bangiomorpha pubescens. as well as two problematic forms interpretable as possible multicellular eukaryotes. A few of the spheroidal microfossils show features suggestive of eukaryotes. but the majority are small simple, probable cyanobacteria; filamentous and stalk-forming cyanobacteria are also present. The different assemblages document distinct environments within a warm, intertidal to supratidal carbonate facies. Physical attributes, such as relative exposure, salinity, and substrate were clearly the determining factors for the majority of these environments and their respective communities. In the Bangiomorpha assemblage, however. it was the organisms themselves that controlled community structure. Vertically oriented populations of Bangiomorpha appear to have successfully excluded benthic microbial mats and. in the process, produced an environmentally heterogeneous environment populated by a high diversity, low dominance community. As the oldest recorded instance of competitive superiority by a multicellular eukaryote over prokaryotes, this occurrence also has implications for understanding ecosystem evolution and secular change in carbonate sedimentation. In particular, it provides key evidence for the hypothesis that the rise of multicellular algae contributed to major environmental shifts through the Mesoproterozoic-Neoproterozoic transition. Two new taxa are described, Bicamera stigmata n. gen., n. sp. and Salome nunavutensis n. sp. (C) 2001 Elsevier Science B.V. All rights reserved.