Ocean biology experienced dramatic evolutionary shift 170 million years ago

The evolution of life in Earth's oceans changed dramatically around 170 million years ago, according to new research.

Prior to the shift, the evolutionary success of marine life forms was dictated primarily by biochemical constraints. Climate and ocean chemistry were the main drivers of evolution.

But during the middle of the Jurassic period, calcium carbonate-secreting plankton showed up in large numbers. Their arrival coincided with the emergence of new evolutionary drivers, like predator-prey relationships.

The abundance of plankton provided a reliable source of nutrition for larger organisms, but more importantly, the deposition of calcium carbonate on the seafloor helped stabilize ocean chemistry. This stability, authors contend in the new study, set the stage for the rapid diversification of marine life during the second half of the Jurassic.

"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," Kilian Eichenseer, PhD candidate at the University of Plymouth, said in a news release. "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."

Scientists knew that for as long as multicellular life has been around, its evolution has been dictated by a combination of biological and non-biological factors. The interplay between these two kinds of factors, however, wasn't well understood.

To study this unknown, scientists analyzed the fossil record, plotting changes in the abundance of aragonite and calcite, the minerals that make up seashells. Using a model of past chemical and climate conditions in the ocean, scientists simulated the amounts of aragonite and calcite that formed inorganically over the last 500 million years. Scientists compared the simulation results with the rates of seashell mineral composition in the fossil record.

The analysis revealed a close relationship between ocean chemistry and the success of shell-secreting species. Marine species that secreted the minerals that were abundant in Earth's oceans, not surprisingly, had an evolutionary advantage.

But according to the latest study, published this week in the journal Nature Geoscience, the emergence of calcifying plankton allowed calcium carbonate to proliferate in the open ocean, minimizing the evolutionary influence of sudden changes in climate and ocean history.

"During the Earth's history there have been several major events that shaped the evolution of life on our planet, such as the five big mass extinctions or the radiation of complex animals during the Cambrian explosion," said Uwe Balthasar, a lecturer in palaeontology at the University of Plymouth.

"Our research identifies a previously overlooked event of this magnitude around 170 million years ago when the emergence of calcium carbonate-secreting plankton lifted constraints on the evolution of other marine organisms that we did not know existed. As a result, life in the ocean has diversified to levels far beyond what existed before."

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