Literature Review: Proterozoic Eon

by Carl Strang

The Proterozoic Eon spanned the immense period of time from 2.5 billion to 542 million years ago. It has captured the imaginations of many researchers, because its rocks have teased them with clues that hint at amazing stories, such as the first eukaryotic life forms, a billion-year stall-out of life’s evolution, a globe-covering ice age (“snowball Earth”), and the first appearance of multicellular organisms, which may or may not be connected to those we have today. Here are my notes from last year on some studies of that eon.

There was no terrestrial life in the Proterozoic, but this was the time when the Chicago region’s crust joined the North American continent, appending itself to the southern boundary of the Canadian Shield.

There was no terrestrial life in the Proterozoic, but this was the time when the Chicago region’s crust joined the North American continent, appending itself to the southern boundary of the Canadian Shield.

Northwestern University. 2014. “Mysterious Midcontinent Rift is a geological hybrid.” ScienceDaily, <www.sciencedaily.com/releases/2014/10/141016132850.htm>. This article described a collaborative project, still unpublished, focusing on the mid-continent rift that left Lake Superior as its most visible feature. The rift was underway in the mid-Proterozoic when it filled with magma and stopped opening. More magma subsequently poured out on top of it, pushing the original body down and thickening the crust there. The feature thus combines rift characteristics with those of a large igneous province, and contains more volcanic rock than any other mid-continent rift on the planet. Incidentally the pieces of volcanic rock we find in local glacial drift came from that source.

Sánchez-Baracaldo, Patricia, Andy Ridgwell, and John A. Raven. 2014. A Neoproterozoic transition in the marine nitrogen cycle. Current Biology, DOI: 10.1016/j.cub.2014.01.041  From a ScienceDaily article. They used molecular clock estimation to place the appearance of nitrogen fixing by cyanobacteria at 800 million years ago. This may have removed the nutrient limitation that was holding life back, setting the stage for proliferation both of biomass and of evolutionary potential. However, this timing also is just before the Snowball Earth glaciation event, and the authors suspect that the algal bloom might have sequestered enough carbon to be a trigger for that event.

Hoyal Cuthill, Jennifer F., and Simon Conway Morris. 2014. Fractal branching organizations of Ediacaran rangeomorph fronds reveal a lost Proterozoic body plan. PNAS, DOI: 10.1073/pnas.1408542111  From a ScienceDaily article. They looked at the 3-dimensional structure of Ediacaran life forms (referred to as rangeomorphs), and found that their fractal designs efficiently filled the space around them. They argue that these were animals, living too deep in the sea for photosynthesis, which absorbed dissolved nutrients directly from the water. This was possible until predators, filter feeders and more mobile life forms rendered this subsistence style unsupportable.

Liu, Alex, et al. 2014. Haootia quadriformis n. gen., n. sp., interpreted as muscular Cnidarian impression from the Late Ediacaran period (approx. 560 Ma). Proceedings of the Royal Society B, DOI: 10.1098/rspb.2014.1202  From a ScienceDaily article. They described an Ediacaran fossil from Newfoundland, 560 million years old, concluding that it was a cnidarian with muscle tissue, the earliest animal with muscle.

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