The rise of animals: challenging Darwin’s dilemma

Dr Frances Dunn
Research Fellow 2019

Dr Frances Dunn
University of Oxford

Our understanding of the evolutionary emergence of animals – one of the most profound events in Earth history – remains very poorly constrained. This is a problem of unusual significance because the animal fossil record is often used as a proxy for understanding the evolutionary process itself by asking questions concerning, for example, the nature of explosive radiations or mass extinctions. Members of most of the animal Phyla appeared during one such radiation event at the beginning of the Cambrian Period (541–485 Ma), and this has historically been termed the Cambrian Explosion. However, evolution by natural selection demands that Precambrian (the vast stretch of time from 4600–541 Ma) animals were not only present, but diverse. This is problematic because convincing Precambrian animal fossils are exceptionally rare.

Members of the Ediacaran macrobiota are enigmatic icons of this problem, and will be the focus of this study. Like their Cambrian counterparts, fossils of this biota appear suddenly at ~571 Ma, and dominate ecosystems for ~30 million years before disappearing from the rock record just before the dawn of the Cambrian. They possess strange and unfamiliar anatomies that are not easily resolvable with living organisms, and so while they are often invoked as early animals, they have also been interpreted as representing a wide variety of other multicellular organisms.

“This study – which aims to integrate animal fossil records across the Precambrian and Cambrian – represents the first of its kind”

I will examine the Precambrian record of animals using 3-D modelling techniques, and the study of ontogeny – how organisms develop from embryo to adult – to better constrain the evolutionary affinities of putative early animals. I will also conduct fieldwork to collect new data at fossil sites including the ~565 million year old UNESCO world heritage site Mistaken Point in Newfoundland, Canada. I will then use these data to address how morphological disparity has changed through time. Understanding morphological disparity is of importance because it fundamentally underpins evolutionary radiation events, but previous studies have only included fossils as far back as the Cambrian, when modern animal ‘body plans’ (e.g. that of an annelid worm or an arthropod) are already set. By ignoring the Precambrian rise of animals these models will remain fundamentally incomplete.

This study – which aims to integrate animal fossil records across the Precambrian and Cambrian – represents the first of its kind.