From ScienceDaily:
Oxygen has provided a breath of fresh air to the study of the Earth’s evolution some 400-plus million years ago.
A team of researchers, including a faculty member and postdoctoral fellow from Washington University in St. Louis, found that oxygen levels appear to increase at about the same time as a three-fold increase in biodiversity during the Ordovician Period, between 445 and 485 million years ago, according to a study published Nov. 20 in Nature Geoscience.
…
This explosion of diversity, recognized as the Great Ordovician Biodiversification Event, brought about the rise of various marine life, tremendous change across species families and types, as well as changes to the Earth, starting at the bottom of the ocean floors. Asteroid impacts were among the many disruptions studied as the reasons for such an explosion of change. Edwards, Fike and others wanted to continue to probe the link between oxygen levels in the ocean-atmosphere and diversity levels of animals through deep time.
…
“This study suggests that atmospheric oxygen levels did not reach and maintain modern levels for millions of years after the Cambrian explosion, which is traditionally viewed as the time when the ocean-atmosphere was oxygenated,” Edwards said. “In this research, we show that the oxygenation of the atmosphere and shallow ocean took millions of years, and only when shallow seas became progressively oxygenated were the major pulses of diversification able to take place.” Paper. (paywall) – Cole T. Edwards, Matthew R. Saltzman, Dana L. Royer, David A. Fike. Oxygenation as a driver of the Great Ordovician Biodiversification Event. Nature Geoscience, 2017; DOI: 10.1038/s41561-017-0006-3 More.
So that means that the remarkable Cambrian explosion did not even have the benefit of the large amount of oxygen that is commonly assumed. One senses that there are more surprises to come.
See also: Animal parenting at 508 mya
New book: Our long-held beliefs about the history of life are wrong?
and
The “boring billion” years: New hypothesis suggests oxygen shortage stalled life (So oxygen is important but how and under what circumstances?)