Evolutionary Ecology of the First Animal Ecosystems

Abstract

The late Ediacaran (~574 – 564 Ma) strata of Newfoundland, Canada preserve entire early animal communities in situ under volcanic ash. These communities lie at a key juncture between the preceding microbe-dominated world of the Proterozoic, and the subsequent animal-dominated world that we inhabit today. Understanding the composition of and processes within unfamiliar ecosystems - the life history characteristics of these organisms, and how they interacted with their environment and each other - is key to understanding what processes drove the evolution and diversification of animals through this transitional period. Ediacaran incipient animal communities, preceding the complexity of complex food webs, can also help establish the nature of universal ecological and evolutionary rules. Within this thesis I exploit under-accounted-for features of Ediacaran palaeocommunities preserved in Newfoundland to constrain and account for biases in palaeocommunity datasets, and to directly test associations between organisms and their local environment for the first time. To exploit these features, I develop and apply novel methodologies, including surface metrology, persistent homology and discrete phase modelling, which promise to have wide-ranging future applications both the field of early animal evolution and palaeobiology more generally. I uncover several important ecological processes operating in these early animal communities: in Chapter 2, I find tentative evidence of an association between newly settled juvenile organisms and microbial mat characteristics, supporting the claimed ecological importance of matgrounds in the Ediacaran and the ‘savannah’ hypothesis for the evolution of movement. In Chapter 3, I also find evidence for negative density-dependent mortality between heterospecifics in several communities, which suggest neutral dynamics and could be the consequence of processes such as scramble competition or disease. In Chapter 4, I find evidence of mixed reproductive strategies in early sessile animals that reflect evolutionary trade-offs, perhaps in response to environmental instability. Finally, I argue that the findings of this thesis, and indeed many unexpected features of the wider early animal fossil record, can be explained by the emergent properties of coloniality. This thesis provides novel insights into the evolutionary ecology of the first animal communities: how early animals interacted with their environment; the complexity of their reproductive strategies and how mortality processes operated in contrast to Phanerozoic.