During the Mesoproterozoic (1.6-1 Ga), the supercontinent Rodinia was formed (around 1.1 Ga), atmospheric oxygen levels continued to rise, and stromatolites formed large and widespread reefs. Unicellular, eukaryotic organisms probably arose either in the late Paleoproterozoic or early Mesoproterozoic, and continued to diversify throughout this time period. Coccoid microfossils show an increase in size during the Mesoproterozoic and biotas exhibit an increasingly diverse assemblage of cyanobacterial remains, both filamentous and coccoid. As noted earlier, cyst-forming, planktonic, eukaryotic algae (acritarchs) are believed to have emerged in the Mesoproterozoic, ~ 1.4 Ga ago (Knoll, 1985a), with the earliest evidence for multicellularity not long thereafter. Oehler (1977) described a deep-water biota from the Mesoproterozoic of Australia. The assemblage was dominated by filamentous bacteria, unlike those known from shallow-water deposits or stromatolitic assemblages. Oehler suggested that these organisms lived below the photic zone under anoxic conditions.

Earliest Multicellular Life

Many textbooks, journal papers, and online sites identify the origin of multicellular life as the famous 610 Ma Ediacaran biota—an assemblage of strange, soft-bodied metazoans that are preserved in only a limited number of sites around the world. These reports tend to forget the fact that irrefutable evidence for multicellular algae existed long before the Ediacaran. In 1990, Butterfield et al. described a permineral-ized red alga similar to living members of the Bangiophyceae (Rhodophyta) from the Hunting Formation of arctic Canada. This formation consists of shallow-water carbonates and includes stromatolites, as well as cyanobacterial fossils; the rocks that contain the algae are dated at ~ 1200 Ma (late Mesoproterozoic) (Butterfield, 2000). The fossils of Bangiomorpha pubescens (FIGS. 4.43, 4.44) consist of filaments made up of stacked, discoidal cells enclosed within a sheath; many have a holdfast at their base. The filaments, which are up to 2 mm long and range from 15 to 45 |m in diameter, show radial cell division within the filament, a feature characteristic of bangiophytes. As Butterfield (2000) noted in the formal description of B. pubescens, the fossil is so similar to the modern red alga Bangia as to be ' 'indistinguishable." Based on the different morphology of filaments and circular objects within them, Butterfield suggests that B. pubescens fossils also show the earliest fossil evidence of sexual reproduction. The shallow-water carbonates that include these red algae contain a total of four fossil assemblages, three of which are dominated by mat-forming, photosynthetic prokaryotes (Butterfield, 2001). Each assemblage appears to occupy a specific niche within a larger ecosystem. In the Bangiomorpha assemblage, however, mat-forming prokaryotes are excluded, perhaps because of the vertical growth of the algae. Butterfield (2001) suggested that this biota documents the oldest case of competitive exclusion of prokaryotes by multicellular eukaryotes. He hypothesized that such exclusion may explain the relatively rapid, even explosive, diversification of eukaryotes in the Neoproterozoic.

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