The Palaeobiology of Avalonian (Ediacaran) Rangeomorphs

• Main Author: Kenchington, Charlotte G.
• Format: Thesis
• Language: English
• Published: 2016
• Subjects: 04 - Palaeobiology; Newfoundland
• Online Access: http://eprints.esc.cam.ac.uk/3671/; http://eprints.esc.cam.ac.uk/3671/1/Charlotte%27s%20K%20beats.pdf; http://eprints.esc.cam.ac.uk/3671/2/CGK%20cover.png; http://eprints.esc.cam.ac.uk/3671/3/cgk%20image.jpg

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 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.