plankton/© Vera Kuttelvaserova, Adobe Stock
About 170 mya ago. From ScienceDaily:
The ocean as we understand it today was shaped by a global evolutionary regime shift around 170 million years ago, according to new research.
Until that point, the success of organisms living within the marine environment had been strongly controlled by non-biological factors, including ocean chemistry and climate.
However, from the middle of the Jurassic period onwards (some 170 million years ago), biological factors such as predator-prey relationships became increasingly important. …
PhD candidate Kilian Eichenseer, the study’s lead author, explained the impact of calcifying plankton: “Today, huge areas of the ocean floor are covered with the equivalent of chalk, made up of microscopic organisms that rose to dominance in the middle of the Jurassic period. The chalky mass helps to balance out the acidity of the ocean and, with that balance in place, organisms are less at the mercy of short-term perturbations of ocean chemistry than they might have been previously. It is easier to secrete a shell, regardless of its mineralogy, if the ocean chemistry is stable.” …
The results show that up until the middle of the Jurassic period, around 170 million years ago, the ecological success of shell-secreting marine organisms was tightly coupled to their shell composition: organisms that secreted the mineral that was environmentally favoured had an evolutionary advantage.
However, the Earth-Life system was revolutionised forever by the rise of calcifying plankton, which expanded the production of calcium carbonate from continental shelves to the open ocean.
This ensured that the evolutionary impact of episodes of severe climate changes, and resulting ocean acidification, was less severe than comparable events earlier in Earth history. Paper. paywall – Kilian Eichenseer, Uwe Balthasar, Christopher W. Smart, Julian Stander, Kristian A. Haaga, Wolfgang Kiessling. Jurassic shift from abiotic to biotic control on marine ecological success. Nature Geoscience, 2019; DOI: 10.1038/s41561-019-0392-9 More.
It would be an interesting thought experiment to consider what the oceans would be like if the formation of shells was difficult. It’s still hard to picture the idea that predation wasn’t as important as dissolution.
See also: Many Plankton Behave Like Both Plants And Animals, Challenging Biological Concepts