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2015 Annual Science Report

Massachusetts Institute of Technology Reporting  |  JAN 2015 – DEC 2015

Early Animals: Taphonomic Controls on Fossil Record

Project Summary

This project has focused on Neoproterozoic life with an emphasis on factors influencing fossil preservation. A combination of field and experimental approaches has been used to study preservation of Ediacara-type fossils and to test the prevailing ‘death-mask’ hypothesis that considers iron sulfides to have been a primary agent. Results so far indicate that ferruginization was a late-stage process and not consistent with this model suggesting an important role for early silicification. Initial experimental results show that microbial mats are prone to silicification and that their presence in association with invertebrate carcasses inhibits decay and enhances the preservation of soft-bodied organisms. An investigation of factors controlling the preservation of eukaryotic microfossils in Proterozoic rocks is also underway. Experimental data indicate that certain clays inhibit the growth of decay bacteria such as Pseudoaltermonas.

New fossil assemblages from grey shales and cherts have been discovered from this same interval – a significant development because very few fossils have been described from rocks between the two Snowball Earth ice ages. The preponderance of exceptional preservations in the Cambrian and subsequent early Paleozoic may be explained in part by a delay in intense mixing of marine shelf sediments by bioturbators, which did not develop until the Devonian. This slow onset of thorough mixing may also have contributed to the late rise of sulfate in the oceans and a mid-Paleozoic drop in oxygen levels.

4 Institutions
3 Teams
11 Publications
4 Field Sites
Field Sites

Project Progress

Ross Anderson’s research: Anderson investigates the taphonomic processes leading to the preservation of organic eukaryotic microfossils in Proterozoic carbonate rocks. His focus is on eukaryotic microfossils collected from sections in carbonate rocks between the Cryogenian glaciations in Mongolia and Namibia in collaboration with Francis Macdonald (Harvard). Preliminary studies suggest that taphonomy could be a major factor behind the apparent diversity drop following the Sturtian ice age: the abundance and type of clay minerals appears to affect preservation. In collaboration with Nicholas Tosca (Oxford), he has produced a substantial dataset (~400 carbonate samples) of X-ray diffraction results from this interval. We are also collaborating with Phoebe Cohen (Williams), Sara Pruss (Smith), and Tanja Bosak (MIT) to refine our understanding of the distribution of eukaryotic testate fossils in these same rocks.

Early diagenetic black cherts from the Ediacaran Shurgaat Formation preserve diverse microfossil communities, which likely reflect the nature of Ediacaran microbial mat communities. Grey shales from directly below the Khongoryn Marinoan-equivalent diamictite have also yielded microfossils. This discovery is particularly significant because very few fossils have been described from rocks between the two snowball Earth ice ages.

In collaboration with Kristin Bergmann (MIT), Andy Knoll (Harvard), Zach Adam (Harvard), and Nicholas Butterfield (Cambridge) we have collated a sample suite from the three most diverse Proterozoic shale assemblages (~800 Ma Wyniatt and Svanbergfjellet Formations, and the ~1 Ga Lakhanda Formation). Detailed lamina-by-lamina microstratigraphic analyses of fossil diversity and mineralogy will be performed on bedding planes via bedding parallel thin-sections and microscope linked Fourier transform infrared spectroscopy.

A systematic reinvestigation of the preservation of Burgess Shale type fossils of complex organisms in the Cambrianis underway and will explore any correlation with mineralogy for comparison with our results from the Proterozoic. Samples from the Kaili, Marjum, Wheeler, and Burgess shales have been obtained in collaboration with Robert Gaines (Pomona), Jean-Bernard Caron (ROM) and Doug Erwin (Smithsonian). Their clay mineralogy is being analyzed in collaboration with Nicholas Tosca (Oxford).

Sean McMahon’s Research: McMahon is conducting experiments on the relationship between clay mineral assemblages and the preservation of eukaryotic microfossils. The results will illuminate our understanding of the controls on organic preservation in Neoproterozoic and younger sedimentary rocks with implications for understanding important biases in the early fossil record. For example, the inhibitory effect of iron-rich clay minerals (glauconite) on saprophytic bacteria may help to explain the peculiar fidelity with which soft-bodied animals were fossilized in the Cambrian and Ordovician, when deposition of these minerals was enhanced. Preliminary results of Sean McMahon’s experiments suggest that a small suspended load of glauconite (0.5 mg/mL) strongly inhibits the growth of Pseudoaltermonas bacteria, which are significant participants in the decomposition of marine animals.

A collaboration is underway with plant biochemist Nicole Clay (Yale) to investigate whether antimicrobial compounds in tissues influence postmortem decay rates. Varieties of the bryophyte Physcomitrella and the spikemoss Selaginella are being cultivated and decayed in naturalistic conditions. Differences in decay rate will be related to the suite of antimicrobial secondary metabolites detected in the tissues of these plants by our collaborators in Nicole Clay’s lab.

Lidya Tarhan’s research: Tarhan continues to investigate the paleoenvironments, paleoecology and taphonomy of Ediacaran and early Paleozoic ecosystems. Using a combination of paleontological and sedimentological data obtained in the field, and geochemical modelling, she investigated the development of bioturbation (sediment mixing by burrowing animals). She found that, contrary to given wisdom, the development of intensively mixed sediments in shallow marine shelfal environments was delayed until at least the Devonian and this protracted evolution of bioturbation may have played an important role in delaying the rise of sulfate in the global ocean. It may also have been responsible for a mid-Paleozoic drop in atmospheric oxygen (Tarhan et al., 2015, Nature Geoscience).

Tarhan is also investigating the nature and heterogeneity of the Ediacara ‘microbial’ substrate and how it impacted the habitat and preservation of Ediacara macroorganisms. She is using a combination of field-, hand sample- and petrographic-scale sedimentological techniques, as well as exploring the utility of geochemical proxies (with colleagues at Yale, the University of California, Riverside, the South Australia Museum, and the European Institute for Marine Studies).

In collaboration with Derek Briggs, Tarhan is investigating the coevolution of Ediacaran and early Paleozoic paleoenvironments, substrates and ecosystems through field work in the Death Valley and White-Inyos regions. This work is being done under the auspices of a NASA Exobiology grant, in collaboration with colleagues at the University of California, Riverside and Colorado College.