Rachel Wood on the Emergence of Complex Life in the Precambrian
Rachel Wood is Professor of Carbonate Geoscience at the University of Edinburgh. She has uncovered fossils that suggest that the fuse of the so-called Cambrian explosion of life was lit in the Ediacaran, the geological period that preceded the Cambrian. Here she is on a geological field trip in an extremely remote part of Siberia.
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Podcast Illustrations
All images courtesy of Rachel Wood unless otherwise noted.
Geological time scale spanning the entire 4.56-billion-year history of the Earth. This podcast focuses on events that took place at the very end of the Precambrian during the Ediacaran period and in the first period of the Phanerozoic eon - the Cambrian.
Courtesy of IUGS
Siberian Field Work
Rachel Wood and her team travelled down the Yudoma River to reach the key outcrops with Ediacaran fossils.
Outcrop of lower Cambrian rocks with abundant conical-shelled fossils.
Closeup of a fossil-rich exposure in the Cambrian outcrop shown above. These fossils are known as Hyoliths, and are probably related to brachiopods and bryozoans.
Field Work in Namibia
The Swartpunt site records the youngest rocks close to the Precambrian (Ediacaran)/Cambrian boundary. The site is famous for the preservation of soft bodied Ediacaran fossils, as well as the skeletal fossils Cloudina and Namacalathus.
At this site, called Grens, Rachel Wood collected soft-bodied Ediacaran fossils, as well as samples for carbon isotope and redox analysis.
Ediacaran Fossils
Cloudina – a skeletal fossil that appears in rocks globally around 550 million years ago, and goes extinct around 540 million years ago. There is debate as to whether it was related to cnidarians or annelid worms.
Exceptionally well preserved Cloudina fossil from Namibia.
Courtesy of Geoff Wood
Namacalathus - with excellent preservation of soft body parts. This is the creature described in the podcast as being goblet-shaped.
Namapoikia, an Ediacaran sponge, often found together with Cloudinia. It has only been found in a single locality in Namibia, and has an age of 547 million years.
The pale structure in the foreground is an Ediacaran soft-bodied fossil in this Newfoundland outcrop.
Modern stromatolites growing at Carbla Point, Australia. Shallow seas may have looked like this before the rise of animals.
Timeline spanning the latest two periods of the Precambrian (Cryogenian and Ediacaran) and the Cambrian (shaded green) showing examples of animals found in the fossil record at various times (a). Middle track (b) shows carbon isotope variations, which indicates that the carbon cycle was unstable and in a state of flux. Bottom track (c) shows ocean oxygenation levels obtained by analyzing iron compounds in the rocks with oxygen-deficient (anoxic) water shown in black, anoxic and sulfidic (euxinic) water shown in red hatch, and oxygen-rich (oxic) water shown in blue. Dissolved oxygen in the oceans probably reached a threshold, or series of thresholds in the Ediacaran that allowed animals to diversify by meeting their increased metabolic demands as they became more active. The blue “spikes” labeled OOE1-4 (ocean oxic events) indicate oxygenation episodes that appear to coincide with the carbon isotope variations. The episodic increase in oxygenation continued throughout the Ediacaran and probably well beyond.
Wood et al., Nature Ecology & Evolution, Vol. 3, 528, 2019