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Geology Club Seminar

Thursday, January 10, 2019
4:00pm to 5:00pm
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Arms 151 (Buwalda Room)
Complex life in the Boring Billion
Heda Agíc, UC Santa Barbara,

The evolution of Eukaryota, one of the three domains of life, characterized by a complex cell with a nucleus, represents a major transition in the history of life on Earth. Palaeontological record and molecular clock estimates indicate that this step took place by the late Paleoproterozoic Era, around 1.6 billion years ago. Although the microbial eukaryotes expanded at this time, the fossil record shows c. 600-million-year delay until the appearance of multicellularity (seen in the red algae), and even further lag until the evolution of macroscopic forms such as animals, plants, and fungi.

This period of ostensible evolutionary and environmental stability from the Mesoproterozoic to the late Neoproterozoic glaciations and the subsequent appearance of the Ediacara biota is often termed the "Boring Billion". Anoxic conditions and nutrient limitation prevalent around this time have been often invoked to have acted as a brake on the diversification of eukaryotic lineages. Recent palaeontological, sedimentological, and geochemical investigations of the "Boring Billion" units, allow us to examine the possible drivers and constraints of the evolution of early eukaryotes, and if their fossil record represents a true evolutionary stasis, response to challenging environmental conditions, or sampling biases.

In this talk, I will present data from two case studies, from the siliciclastics of the early Mesoproterozoic Ruyang Group in northern China and the late Mesoproterozoic Bylot Supergroup in Arctic Canada. These successions contain rocks deposited under various conditions with diverse assemblages of organic-walled microfossils. In addition to studying the fossils' morphological complexity, I will examine trends in eukaryote species richness and abundance in relation to offshore and nearshore settings, as well as variations in water column redox recorded by iron speciation and trace metal enrichment analyses. These results suggest that microbial eukaryotes were abundant throughout water column, but preferred shallow environments, and that the distribution of oxygen per se may not have controlled the distribution of eukaryotes.