Affinity of Ediacaran skeletal fauna and their environmental context
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Date
14/03/2022Author
Shore, Amy J.
Metadata
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.