Affinity of Ediacaran skeletal fauna and their environmental context

Abstract

The Ediacaran-Cambrian boundary (~541 million years ago (Ma)) signifies the start of the ‘Cambrian Explosion’ of animals, and by 520 Ma most major phyla had emerged. However, characteristics associated with the Cambrian Explosion, such as motile behaviours and biomineralisation, originated during the Ediacaran. It is thought that oxygenation is the key driver of the rise of metabolically costly forms of life. Yet, the drivers behind the oxygenation of Ediacaran basins are not well known. This thesis contributes to the understanding of the drivers behind the rise of skeletal animals during the Ediacaran as well as offering insight into their morphology, affinity and mode of life. New phosphorus speciation data were collected from siliciclastic samples of the terminal Ediacaran-Cambrian Nama Group, Namibia (ca. 550-538 Ma), from shelf transects of the two subbasins in order to determine regional nutrient cycling. This was achieved by combining redox, nutrient cycling, and biotic distribution and diversity data to understand the controls behind oxygenation through time. Limited phosphorus cycling, prior to 547 Ma, may have supported the ferruginous conditions, with possible influence from upwelling from the deep ocean. However, the reduction in continental run off caused the Nama Basin to transition from unstable redox conditions to more stable oxic conditions at ~547 Ma with full oxic conditions across the basin by ~542 Ma. The decrease in recycling of bioavailable phosphorus into the water column allowed for the development of more stable oxic conditions. This in turn allowed for the radiation of mobile taxa and biomineralising taxa, both metabolically costly forms a life, allowing them to inhabit deeper areas of the Nama Basin. The changing redox conditions determined the availability of habitable areas along the shelf of the Nama Basin. In oxygenated, or transiently-oxygenated, areas of the shelf Cloudina was able to form reef-frameworks, often in association with microbial mats. Coeval Cloudina across the Zaris Subbasin share similar features, such as Cloudina-associated cements and paired lamina, implying calcification was biologically-controlled where laminae acted as part of the ‘biomineralisation toolkit’. Cloudina-associated cements may have formed during life as they form prior to breakage, transportation, and abiotic cement formation. However, the mineralisation of Cloudina must have been environmentally controlled as evidenced by the variation of paired lamina thickness and Cloudina wall thickness across the shelf. The variation in thickness may have been due to physical factors, such as hydrodynamic energy, or chemical factors, such as seaswater pH. The affinity of Ediacaran fauna are greatly contested due to the general absence of preserved diagnostic features and soft tissue. However, this thesis presents new findings of polytomous branching in cloudinomorphs within the Omkyk Member of the Nama Group. Polytomous branching is a feature attributed to non-bilaterian taxa and so could suggest that these cloudinomorphs are of cnidarian origin and are part of a potentially polyphyletic group. In addition, the discovery of a Lagerstätte within the Omkyk Member shows soft-tissue preservation of in-situ Namacalathus where a combination of features, such as a U-shaped gut and organic-rich pores within the skeleton, could suggest a lophotrochazoan affinity. Although molecular phylogenies predict an older origin, lophotrochozoan fossils were previously known only from the Early Cambrian and so this discovery provides a potential link between the Ediacaran and Cambrian biotas.