Biomarkers and the dawn of animal life

Abstract

The macrofossils of the Ediacara biota (~571-539 Ma) marks the emergence of large, complex organisms in the palaeontological record. Preluding the radiation of modern animal phyla, they may hold important clues to the earliest evolution of animals. However, their phylogeny, ecology and even preservation remain largely unresolved. This thesis explores the questions on the biological origins of Ediacaran organisms based on a new technique, reconstructing their lipid composition using molecular fossils. Biomarker analysis showed that Ediacaran macrofossil Beltanelliformis represents large spherical colonies of cyanobacteria, while Dickinsonia, along with other dickinsoniid genera, belong among the oldest animals preserved in the rock record. Utilising the same technique, the thesis explores the diets of Ediacaran animals: the tube worm Calyptrina, currently interpreted as a relative of extant chemosymbiotic Siboglinidae, Kimberella, a mollusc-like bilaterian, and Dickinsonia. The results indicate that these animals shared a diet of green algae and bacteria, contradicting previous suggestions that Ediacaran tube worms were chemosymbiotic. In contrast, no traces of dietary molecules were found in Dickinsonia, which indicates that these organisms did not possess a digestive system, and instead fed on microbial mats using external digestion. A new model for the Ediacara biota preservation is proposed here. The model demonstrates that the preservation was promoted by unusually prolonged conservation of organic matter, coupled with differences in rheological behaviour of the over- and underlying sediments. In contrast to accepted models, cementation of overlying sand was not critical for fossil preservation, which is supported by the absence of cement in unweathered White Sea specimens and observations of soft sediment deformation in both White Sea and South Australian specimens. The rheological model, confirmed by laboratory simulations, implies that Ediacaran fossils do not necessarily reflect the external shape of the organism, but the morphology of a soft external or internal organic 'skeleton'. The thesis presents information on primary producers in the Ediacaran based on biomarker molecules that were extracted from sediments that host Ediacaran macrofossils. High relative abundances of algal steranes over bacterial hopanes suggest that the Ediacara biota inhabited nutrient replete environments with an abundance of algal food sources comparable to Phanerozoic ecosystems. Thus, in contrast to earlier predictions, organisms of the Ediacara biota inhabited nutrient-rich environments akin to those that later fuelled the Cambrian explosion. Finally, the thesis explores the preservation of palaeoecological biomarker parameters in relation to the interpretation of ancient biomarker record, including the controls on biomarker distributions in deposits that host the Ediacara biota. The study shows that core biomarker parameters can be affected by microbial degradation to the extent that ecological information is nearly obliterated, and provides new methodological approaches for the biomarker analysis. Thus, this thesis presents a compilation of advances in different aspects of the Ediacara biota studies and biomarker analysis. It provides new baselines for further interpretations of complex morphologies of Ediacaran macrofossils, as well as new methodologies and constrains on the interpretations of the biomarker record.