Growth and Development in the Ediacaran Macrobiota

Abstract

The evolutionary emergence and subsequent radiation of the animals remains one of the most significant events in Earth history. The fossil record has historically been considered to give up uncontroversial evidence for the existence of animals only at the Precambrian–Cambrian boundary, ~539 million years ago. However, molecular clocks consistently indicate a much more ancient origin for this group, deep in the Tonian or Cryogenian Periods, 700–800 million years ago. This mismatch is a problem of unusual significance because the animal fossil record is often used as a proxy with which to test hypotheses on the evolutionary process itself. Until we understand these early stages in animal evolution, such hypotheses will remain unvalidated. Some of the best Precambrian candidate animal fossils are members of the Ediacaran Macrobiota, a soft-bodied assemblage of organisms that appear in the fossil record ~571 million years ago, but have a controversial reseach history.

This thesis turns to the study of growth and development as an underexplored avenue with which to shed new light on the affinities of these fossils. I examine populations of taxa from multiple localities, and use the study of comparative morphology, including both X-ray microtomography and synchrotron radiation X-ray microtomography in order to create new models of anatomy and, using these data, quantify morphogenetic patterns. These new data allow phylogenetic analyses to be undertaken in order to resolve the positions of my studied taxa. I find support for a crown-group metazoan affinity for members of both the rangeomorphs and the arboreomorphs. These data provide further support to the hypothesis that the ‘Cambrian Explosion’ of metazoan taxa has a protracted Neoproterozoic fuse. Perhaps the Cambrian radiation of animal groups represents one of a series of metazoan radiations that began in the late Neoproterozoic.

Date of Award	25 Jun 2019
Original language	English
Awarding Institution	University of Bristol
Sponsors	British Geological Survey
Supervisor	Alex G S C Liu (Supervisor), Philip C J Donoghue (Supervisor) & Philip R Wilby (Supervisor)