21 October 2020
Refining Earth history records
Published online 8 July 2020
Study clarifies the chronology of geochemical events that occurred about 570 million years ago.
The Ediacaran (635 to 541 million years ago) was a pivotal period in Earth’s evolution and in the rise of animals characterized by tubular and frond-shaped organisms. The interplay of geochemical and climatic upheavals that occurred during this period has yet to be fully assessed.
Now, researchers in the USA and Oman report that, contrary to earlier conjectures, one of the largest disturbances to the carbon cycle in Earth’s history is not responsible for Ediacaran extinction or the explosion of species that followed in the Cambrian period. The disturbance, called the Shuram carbon isotope excursion, involved a shift in the ratio of carbon-13 to carbon-12.
By examining ancient rock samples from sites in Oman and Canada, the team deduced radio-isotopic ages for the beginning and end of the Shuram event. The event’s timing, now estimated at 574 to 567 million years ago, supports the notion that it occurred after the Gaskiers glaciation, a period of widespread glacial deposits in the late Ediacaran Period. The same timing inferred from the samples in both countries suggests Shuram was a global, synchronous event.
“Previously, it had been suggested that the Shuram event could have been an extinction mechanism responsible for clearing the way for the rise of some animals or that it had changed the paleoenvironment to better suit particular animals,” says co-lead author Alan Rooney of Yale University. “However, our age constraints for the Shuram event and fossiliferous units reveal that macroscopic life with frondose organisms evolved before, were globally distributed, and then survived the Shuram event.”
The researchers say the newly established timeline provides essential context for testing hypotheses related to extreme carbon isotope excursions and their impacts on the habitats and evolution of complex life.
“This is a really important paper,” says Simon Darroch, a geobiologist at Vanderbilt University, USA, who was not involved in the study. “The Shuram has always been an enigmatic event, hovering close to the time when complex life first appears in the fossil record. But we’ve never really had a handle on when this event happened, or what it represents. Rooney et al have finally managed to date this event... These new dates conflict with several prominent hypotheses surrounding the origins of complex life. To me, this suggests that palaeontologists interested in picking apart why complex life evolved when it did should perhaps start thinking more about ecological, rather than environmental, drivers for the rise of animals and the origin of the modern biosphere.”
Rooney, A. et al. Calibrating the coevolution of Ediacaran life and environment. PNAS https://doi.org/10.1073/pnas.2002918117 (2020).