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.
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.
