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(Ediacaran) Rangeomorphs

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(Ediacaran) Rangeomorphs THE PALAEOBIOLOGY OF AVALONIAN (EDIACARAN) RANGEOMORPHS Charlotte Guenevere Kenchington Selwyn College University of Cambridge December 2015 This dissertation is submitted for the degree of Doctor of Philosophy SUMMARY: THE PALAEOBIOLOGY OF AVALONIAN (EDIACARAN) RANGEOMORPHS CHARLOTTE G. KENCHINGTON The Earth has supported life for most of its 4.5 billion year history, but the first macroscopic organisms only appeared some 600 million years ago, in the Ediacaran. Their world was fundamentally different from the one we know today, and many aspects of their biology and ecology remain a mystery. The late Ediacaran fossil assemblages of Avalonia represent some of the oldest evidence for complex macroscopic life, and are dominated by rangeomorphs, a group characterised by their self-similar branching architecture. In this thesis, I investigate several aspects of the preservation, classification and ecology of these enigmatic deep marine organisms. The biotas of Charnwood Forest host several taxa which are new to science. Five of these are described here, and include two new genera, Orthiokaterna fordi gen. et sp. nov., and Undosyrus nemoralis gen. et sp. nov., and three new species: Primocandelabrum anatonos sp. nov., P. boytoni sp. nov., and P. katatonos sp. nov.. The Primocandelabrum species in particular encompass a great deal of variation in both branching characters and overall morphology. By using a novel multivariate statistical approach to analyse multiple characters in tandem, individual taxa can be discriminated from one another. Much of the observed variation is interpreted as intra-specific. This level of variation within a single taxon has not previously been recognised in rangeomorphs, and is likely attributable to (eco?)phenotypic rather than ontogenetic variability. Orthiokaterna displays eccentric branches, interpreted as a growth response to mechanical damage, reflecting a greater degree of growth plasticity than that recognised in other rangeomorphs, while Undosyrus had an external sheath, interpreted as modified rangeomorph elements serving a protective role. Even without knowing the phylogenetic relationships of rangeomorphs, it is possible to resolve key aspects of their palaeoecology. The response(s) of communities in Charnwood Forest and Newfoundland to both ambient disturbance and to more substantial events is investigated by combining detailed petrographic analysis of the host sediments with multivariate statistical techniques. I demonstrate that higher taxonomic diversity is correlated with low–intermediate physical disturbance; that upright taxa (e.g. Charnia) dominate surfaces which experienced small-scale, sub-lethal sedimentation events and comparably high background sediment input; and that flat-lying forms (e.g. Fractofusus) preferentially occur on surfaces with low sediment input. The population demographics of several taxa also show evidence of multimodality: in some (including Charnia and Primocandelabrum), bimodality was I induced by culling of part of an incumbent population by a substantial disturbance event, followed by re-colonisation; in others (e.g. Fractofusus), overlapping cohorts reflect non-continuous or pulsed reproduction. Disturbance (ambient and discrete events) demonstrably influenced community succession, with early-colonising taxa dominating horizons with low overall levels of disturbance, and those able to survive disturbance events dominating recovery populations and horizons with higher levels of disturbance. Based on their inferred life history traits and their environmental preferences, I propose a model of ecological succession for rangeomorph communities. II DECLARATION Declaration of Originality This dissertation is the result of my own work and includes nothing which is the outcome of work done in collaboration except as declared below and specified in the text. It is not substantially the same as any that I have submitted, nor is it being concurrently submitted for a degree or diploma or other qualification at the University of Cambridge or any other University or similar institution. Published work arising from the dissertation Parts of Chapter 3 are based on material presented in Kenchington & Wilby (2014), paper entitled Of time and taphonomy: preservation in the Ediacaran and published in The Paleontological Society Papers, volume 20. Parts of the interpretation in Chapter 9 (and Fig. 9.13) are based on work I published in collaboration with P.R. Wilby and R.L. Wilby (Wilby et al. 2015), paper entitled: Role of low intensity environmental disturbance in structuring the earliest (Ediacaran) macrobenthic tiered communities, Palaeogeography, Palaeoclimatology, Palaeoecology, volume 434. Figure 10.1 was published in Mitchell et al. 2015, paper entitled: Reconstructing the reproductive mode of an Ediacaran macro-organism, published in Nature, volume 524. Figure 10.3 was published in Liu et al. 2015a, paper entitled: Remarkable insights into the paleoecology of the Avalonian Ediacaran macrobiota, Gondwana Research, volume 27. Statement of Length This dissertation is within the prescribed length as defined by the Degree Committee for Earth Sciences and Geography. III IV CONTENTS SUMMARY: THE PALAEOBIOLOGY OF AVALONIAN (EDIACARAN) RANGEOMORPHS ....................................I DECLARATION ..................................................................................................................................... III FIGURES ..............................................................................................................................................XI TABLES .............................................................................................................................................XIII ACKNOWLEDGEMENTS .......................................................................................................................XIV 1. INTRODUCTION: THE EDIACARAN AND AVALONIA ...............................................................................1 1.1 Global context for life Ediacaran ......................................................................................... 1 1.1.1 Glaciations and oxygenation ...................................................................................3 1.2 Life in the Ediacaran ............................................................................................................ 3 1.2.1 Rangeomorphs ......................................................................................................... 5 1.2.2 Arboreomorphs....................................................................................................... 10 1.2.3 Discoidal forms ......................................................................................................10 1.2.4 Other taxa ...............................................................................................................10 1.3 Rangeomorph growth and functional morphology .............................................................11 1.4 The Ecology of Avalonian Rangeomorphs .........................................................................12 2. GEOLOGICAL CONTEXT FOR THE EDIACARAN OF AVALONIA ............................................................. 14 2.1 Ediacaran Stratigraphy of Avalonia ....................................................................................14 2.1.1 The Geological Setting of Charnwood Forest ........................................................14 2.1.1.1 Age constraints for Charnwood Forest ........................................................ 16 2.1.1.2 Fossil occurrences in Charnwood Forest .....................................................17 2.1.1.3 Detailed lithological analysis of the Bradgate Formation ............................19 2.1.2 The Geological Setting of the Ediacaran of Newfoundland .................................. 20 2.1.2.1 Age constraints for Newfoundland .............................................................. 24 2.1.2.2 Deep-water fossil occurrences in Newfoundland ........................................ 25 2.2 Comparing the Newfoundland and Charnwood Forest successions. ........................25 3. THE TAPHONOMY OF AVALONIAN RANGEOMORPHS ........................................................................... 27 3.1. Introduction ....................................................................................................................... 27 3.2 Metamorphism .................................................................................................................... 28 3.3. Mouldic preservation: mechanisms and mineralisation .................................................... 28 3.3.1 The “death mask” hypothesis .................................................................................29 3.3.2 The timing of pyritisation: new evidence from Charnwood Forest and Newfoundland ................................................................................................................. 30 3.3.3 Phosphatisation ...................................................................................................... 33 3.3.4 Discussion .............................................................................................................. 36 3.3.4.1 The location of pyritisation and variability of preservation .........................36 3.3.4.2 The timing of pyritisation ........................................................................... 37 V 3.3.4.3 The importance of different mineral phases in Avalonian
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