2013 Annual Science Report
Massachusetts Institute of Technology Reporting | SEP 2012 – AUG 2013
Early Animals: Taphonomic Controls on the Early Animal Fossil Record
Our objectives are to investigate the controls on the preservation of complex life on earth to allow the fossil evidence for the succession of events to be constrained and interpreted. We are concerned with how changing diversity correlates to specific environmental events during the late Neoproterozoic and earliest Phanerozoic. Are the correlations we draw between evolutionary patterns and environmental events real or an artifact of changing preservation potential, that is, taphonomy?
Several paleontological and geochemical projects are focused on understanding the fossil record of eukaryotic organisms and complex animals through the Neoproterozoic Era and into the Cambrian Period. Understanding taphonomic biases is fundamental to reconstructing temporal patterns in biodiversity with respect to environmental change and to refining biostratigraphy. These projects involve a combination of focused fieldwork and laboratory analyses and explore organic preservation and early diagenetic mineralization.
Postdoc Lidya Tarhan has just joined the Briggs lab. Her focus is the interplay between animals and environments, and the emergence of metazoans as ecosystem engineers during the earliest stages of animal history. She is also investigating the role preservational processes play in modifying both the paleobiological and sedimentary records.
Dr Tarhan’s Ph.D. research (at University of California, Riverside) focused on the earliest records of animal infaunal activity, specifically the timing and mechanisms of the transition from matground to mixground substrates (the “agronomic revolution”). The initiation of active infaunal lifestyles profoundly impacted the co-development of early Phanerozoic seafloor ecosystems, substrates and biogeochemical cycling. Dr Tarhan has investigated early and middle Cambrian trends in infaunalization and their biogeochemical and taphonomic implications as well as patterns in the early history and effects of infaunalization through the lower Paleozoic.
Dr Tarhan has investigated the impact of taphonomy on apparent diversity among Ediacaran biotas. Her work on Aspidella – one of the most abundant Ediacaran fossils worldwide – showed that much of its variability is preservational. Her research at Yale will investigate the role of marine biogeochemical cycling in promoting Ediacara-style early silicification and fossilization.
Graduate student Simon Darroch is investigating patterns of spatial diversity across mass extinction events and their significance for interpreting the impact of environmental changes on the modern biota. He has also investigated the Ediacaran biotas from Mistaken Point, Newfoundland, based on data collected in the field, revealing a population structure compatible with continuous reproduction. This may reflect the absence of a seasonal trigger for reproduction prior to the explosion of mesozooplankton near the base of the Cambrian. Darroch’s research on Ediacaran fossils also includes a novel experimental study showing how microbial mats promote mineralization in association with decaying invertebrate carcases in sediment.
Graduate student Victoria McCoy is investigating exceptional fossilization within carbonate concretions, particularly how concretion formation affects decay and mineralization to promote soft tissue preservation. This includes statistical analyses of data from the literature on concretion sites, geochemical analyses of fossiliferous concretions, and laboratory experiments to test how some of the factors affecting concretion formation impact decay and mineralization. Her current focus is the Mazon Creek concretionary Lagerstӓtten.
Graduate student Ross Anderson conducted fieldwork in the Zavkhan Terrane of southwestern Mongolia in collaboration with team member Francis Macdonald during summer 2013. The Zavkhan Terrane exposes a stratigraphic sequence from a ~700 Ma snowball Earth glacial diamictite to sedimentary rocks from the early Cambrian. The discovery by the MIT/Harvard team of a rich Neoproterozoic fossil record between Sturtian and Marinoan glacial diamictites in this locality makes Mongolia an ideal place to study taphonomic biases. Three stratigraphic sections are under investigation to determine the relationship between microfossil preservation, facies and geochemistry. Samples were obtained from carbonates: 1. containing and 2. lacking eukaryotic microfossils and 3. across a transition where eukaryotic microfossils are thought to disappear. Samples will be dissolved to assess microfossil diversity and thin-sections are in preparation. We will use EDS mapping and XRD to identify clay mineralogy. We are also investigating the mineralogy of small shelly fossils (SSFs) from the Cambrian of the Zavkhan Terrane to understand their preservation in a stratigraphic framework. Shaley lithologies from this interval provide the prospect of comparing the record of small carbonaceous fossils with that of SSFs.
Anderson is also investigating new records of eukaryotic diversity in Neoproterozic metamorphosed shales from the Dalradian Supergroup (Bonahaven Formation) of Islay, Scotland (with team member Andrew Knoll). He is measuring Sr isotopes and trace elements in the sequence underlying the Sturtian glacial diamictite to constrain the record of environmental change. A sequential leaching method (in collaboration with Zhengrong Wang at Yale) removes diagenetic signatures, revealing the primary isotopic signal.
Anderson is also investigating the formation of ooids in the Great Salt Lake, Utah (as part of the International Geobiology Course). Clumped isotope paleothermometry constrained ooid formation to summer months. Preliminary analyses using 16S-rRNA and organic geochemistry are being used to determine whether formation includes a biological component which would provide an important indicator of microbial activity in the rock record.