2014 Annual Science Report
Massachusetts Institute of Technology Reporting | SEP 2013 – DEC 2014
Early Animals: Taphonomic Controls on the Early Animal Fossil Record
We have been carrying out a number of studies in support of our objective to investigate the controls on the preservation of complex life on earth, with the ultimate aim of allowing the fossil evidence for the succession of events to be constrained and interpreted. These studies include investigating the connections between taphonomy and ecology of the Ediacara Biota based on the collection of fossil specimens and their careful examination, laboratory experiments designed to better understand how these fossils became preserved in different settings, and investigations of how Proterozoic eukaryotic microfossils are preserved, again from studying both new fossil material through a variety of approaches, and performing analog experiments in the laboratory.
Post-doc Lidya Tarhan’s research is tracking the interplay between animals and environments, and the emergence of metazoans as ecosystem engineers during their early radiation. She is also investigating the taphonomic processes responsible for the exceptional preservation of Neoproterozoic and early Paleozoic soft-bodied fossil assemblages. Lidya Tarhan’s research on the taphonomy and ecology of the Ediacara Biota—Earth’s earliest macroscopic eukaryotic communities—seeks to distinguish taphonomically-induced variance from true species-level diversity. For instance, the holdfast form-genus _Aspidella_—one of the most abundant Ediacaran fossils worldwide—is highly variable, but much of this variability appears to be taphonomic in origin (Tarhan et al. 2015, Precambrian Research).
Lidya Tarhan’s taphonomic research seeks to determine the biogeochemical conditions that prevailed during the development and preservation of early complex life as well as understanding the evolutionary diversification of the Ediacara Biota. We are testing the model that early-stage silicification was an essential aspect in preservation of the soft-bodied Ediacara Biota and other early Paleozoic soft-bodied faunas. Tarhan’s field and specimen based research on this question is complemented by laboratory experiments on decay and mineralization in collaboration with post-doc Sean McMahon and PI Derek Briggs. Tarhan is also using field observations to constrain the nature and heterogeneity of the Ediacara ‘microbial’ substrate and determine how it mediated the taphonomy and distribution of Ediacara macroorganisms. This allows her to reconstruct the paleoenvironmental conditions under which Ediacara organisms lived and were fossilized (Tarhan et al. in revision for Palaeogeography, Palaeoclimatology, Palaeoecology). She presented initial results on Ediacara substrates at the 2014 annual meeting of the Palaeontological Association.
Lidya Tarhan is also interested in the turnover between Ediacaran and Cambrian faunas. Shaanxilithes ningqiangensis, a cryptic tubular fossil, occurs during this critical boundary interval and may provide a useful index fossil for the latest Ediacaran. She has documented the biology, preservation and geochemistry of Shaanxilithes based on material from the Lesser Himalaya of India (Tarhan et al. 2014, Palaeontology).
Lidya Tarhan is continuing her research on the earliest records of animal infaunal activity, in order to constrain the co-development of early Phanerozoic seafloor ecosystems, substrates and biogeochemical cycling. Her results suggest that, contrary to longstanding assumptions, infaunal sediment mixing (bioturbation) was not initiated in the earliest Cambrian and that the extent and intensity of bioturbation remained far below that of today even as late as the Late Silurian (Tarhan et al. in revision, Nature Geoscience). Tarhan’s work has demonstrated that bioturbation exercised a first-order control upon early Paleozoic biogeochemical cycling, in particular the sulfur cycle: the protracted development of bioturbation may be responsible for the preponderance of lower Paleozoic exceptionally preserved fossil deposits in shelfal settings (Tarhan et al. 2014, Memoirs of the Association of Australasian Palaeontologists; Tarhan et al. 2014, Palaeogeography, Palaeoclimatology, Palaeoecology).
Post-doc Sean McMahon is conducting experiments on the taphonomy of a range of exceptionally well-preserved Neoproterozoic fossils, with a particular focus on the the organic-walled microorganisms of the Neoproterozoic and the enigmatic organisms of the Ediacaran.
We (McMahon, Tarhan, Briggs) are investigating how large soft-bodied organisms are preserved as casts and molds in coarse siliciclastic sediments, a phenomenon restricted to the Ediacaran and some settings in the Cambrian and Ordovician. The “death mask” model of Gehling (Palaios, 1999) invoked bacterially mediated precipitation of iron-sulfide cement along buried interfaces between microbial mats and macroorganisms, forming veneers that ultimately become permanent mold-bearing lithological partings.
Building on our previous experimental work at Yale on Ediacaran preservation (Darroch et al., Palaios, 2012), we are testing variations of this model using the sea anemone Nematostella and well-characterised, metabolically diverse, saltwater microbial mats. Mats were collected from tidal muds in the Barn Island salt marsh on the Connecticut coast. They are approximately one centimeter thick and have cracked, pustular surfaces that resemble Proterozoic microbially-induced sedimentary structures. The mats contain a redox-stratified assemblage of diatoms, cyanobacteria, purple sulfur bacteria, Beggiatoa-like filamentous sulfur bacteria, sulfate-reducers and other organisms. Macrofauna have been painstakingly removed, meiofauna are kept to the minimum possible, and genetic analysis of the remaining community of bacteria, archaea and eukaryotes is currently underway. We are also developing methods to grow new mats in the laboratory from this mixed natural inoculum.
The first round of experiments will investigate the role of silica concentration in the formation of “death masks”. These experiments are motivated by indications that silica may have been the original cement in many of the rocks yielding classic Ediacaran fossil assemblages. Nematostella specimens will be buried in coarse artificial sand (artificial soda glass), both with and without microbial mats and under negligible, moderate and high concentrations of silica (derived from TEOS). Samples will be taken at intervals of 1, 2 and 6 weeks, and impregnated with Spurr’s epoxy resin to allow thin-sectioning. We have successfully developed a sample preparation protocol that allows for water to be removed and resin injected without excessively disturbing an initially wet, delicate sample that contains both fragile organic tissues and sediment grains. The second round of experiments will vary the concentration of SO42−. Future experiments may also vary Fe3+, O2, microbial community structure, and the composition of the sedimentary substrate. We will use a variety of techniques to trace the interactions between seawater chemistry, microbial activity, the biostabilization and cementation of sediment, and ultimately the quality of preservation. A better understanding of these processes will elucidate the opening and closing of the “Ediacara-style” taphonomic window in geological time and increase the reliability of the interpretation of the morphology of Ediacara fossils. A preliminary presentation was made at the November 2014 NAI meeting at Harvard.
Graduate student Ross Anderson is investigating the taphonomic processes leading to the preservation of organic eukaryotic microfossils in Proterozoic carbonate rocks. Fieldwork in southwestern Mongolia in June 2014, in collaboration with Francis Macdonald (Harvard University), yielded samples from the crucial time interval between Cryogenian “Snowball Earth” ice ages. This period marks a possible extinction in the early eukaryotic record. We are testing the alternative explanation that the paucity of fossils known from this interval is a result of changing preservation potential. Anderson has been working to expand on preliminary data that indicates a correlation between fossil abundance/diversity and changing clay mineralogy in the rock. Clay minerals have previously been implicated in instances of Phanerozoic organic fossil preservation. The samples collected in Mongolia are currently being analyzed via thin sections and maceration for microfossil diversity in collaboration with Phoebe Cohen (Williams College) and Tanja Bosak (MIT), and for clay mineralogy via X-ray diffraction (XRD) in collaboration with Nicholas Tosca (University of Oxford). Anderson spent time in Tosca’s lab in August 2014 and will do so again in February and March 2015 to gather XRD data. Anderson presented preliminary data from this project at both the Annual Meeting of the Geological Society of America and the Annual Meeting of the Palaeontological Association.
To complement our work in Mongolia we are exploring whether the pattern of microfossil diversity and clay mineralogy persists in other basins during the crucial interglacial interval. This involves assessing microfossil abundance and clay mineralogy in carbonate rocks from Namibia in collaboration with Sara Pruss (Smith College), and Paul Hoffman and Francis Macdonald (Harvard University).
The Mongolian fieldwork is yielding possible new records of Proterozoic diversity. Preliminary analysis of early diagenetic black cherts from late Ediacaran strata shows that they preserve spheroidal and irregular structures ~100 μm in maximum dimension with some internal structure in addition to clasts of microbial mat filled with filamentous forms. Our future work will describe the diversity preserved within these cherts and coeval phosphorite horizons. Additionally, black shales that bound the crucial Cryogenian interglacial interval are being macerated in a search for possible new microfossil records. Black shales are an important reservoir for Proterozoic eukaryotic microfossils due to their potential to preserve morphological detail, allowing assignment of taxonomic affinity. These shales are also being analysed in collaboration with Noah Planavsky (Yale University) for evidence of levels of oxygen in the inter-glacial oceans.
Graduate student Ross Anderson has also initiated a project to investigate the taphonomic processes that lead to the preservation of eukaryotes as carbonaceous compressions in shales of the ~800 Ma Svanbergfjellet Formation in Spitsbergen in collaboration with Kristin Bergmann (Harvard University/MIT), and Andrew Knoll and Zach Adam (Harvard University). Samples are being analyzed lamina by lamina using bedding parallel thin-sections for microfossil eukaryotic diversity. Secondary ion mass spectrometry is being employed to measure the ratios of organic carbon and sulfur isotopes. These can reveal whether fossiliferous beds were relatively deprived of oxidants. Clay minerals are also being measured lamina by lamina in collaboration with Nicholas Tosca (Oxford University). These analyses are similar to those recently performed on Cambrian Burgess Shale-Type (BST) fossil assemblages where soft bodied animals are preserved as carbonaceous compressions. If the fossils of the Svanbergfjellet Formation were formed through a similar process to those of the Burgess Shale, the discovery will provide further evidence that the conditions for the exceptional preservation of soft-bodied animals are not exclusive to Paleozoic time. As such the lack of readily preserved animal faunas in Proterozoic strata cannot be explained by unfavorable preservational conditions but suggests that they had yet to evolve.
Ross Anderson has carried out fieldwork to determine the tectonic setting of the Avalon assemblage of Ediacaran macrofossils in Newfoundland, Canada. In collaboration with David Evans (Yale University), he conducted fieldwork around Bonavista and Trinity Bays to collect samples for paleomagnetic analysis. Preliminary data suggest that between 570 and 550 Ma the Avalonia terrane underwent rapid plate motions. Anderson presented preliminary data from this project at the Annual Meeting of the Geological Society of America.
In addition to experiments to investigate the preservation of Ediacara animals, post doc Sean McMahon is developing experimental protocols to investigate the apparent correlation between clay mineral assemblages and the preservation of eukaryotic microfossils in the Cryogenian carbonates of the Zavkhan Terrane, Mongolia. These fossils only occur in beds in which the clay fraction is dominated by berthierine or talc and are absent from those containing kaolinite. Previous work by Nick Butterfield (Cambridge) has drawn attention to the possibility that clay minerals may act directly to alter the molecular structure of degrading tissue and thus retard (or promote) decay. However, the fact that clays interact strongly with microorganisms in a variety of ways (e.g., electrostatic adsorption and inactivation of cells; crystal penetration of cells; leachate metal toxicity; pH and Eh buffering) has been largely overlooked in considerations of the taphonomy of organic-walled microfossils, despite the existence of a burgeoning literature on this subject. We (McMahon, Anderson, Briggs) are presently preparing to perform culture experiments to investigate interactions between clay minerals (including kaolinite and berthierine, the latter synthesized anaerobically “in-house”), tissue decay and the microbial communities that promote decay and diagenesis. Our results have the potential to reveal and explain biases in the Neoproterozoic fossil record and to illuminate the controls on critical taphonomic pathways.
Derek Briggs also reviewed the significance of the recent discovery of new Burgess Shale-type localities for Current Biology. He also published a paper on Konservat-Lagerstätten 40 years on: The exceptional becomes mainstream for a Paleontological Society short course held at the annual GSA meeting in Vancouver. Finally he wrote a commentary for an upcoming issue of the Philosophical Transactions of the Royal Society (celebrating the 350th anniversary of the founding of the journal) which explores how the redescription of the remarkable Burgess Shale animal Opabinia in 1975 led to a paradigm shift in our understanding of the Cambrian Explosion.
PROJECT MEMBERS:Derek Briggs
RELATED OBJECTIVES:Objective 4.1
Earth's early biosphere.
Production of complex life.