Only 170 million years ago, new plankton evolved. Their demand for carbon and calcium permanently transformed the seas as homes for life.
Quanta Magazine said:...
About 250 million years ago, at the end of the Permian period, life on Earth suffered an unprecedented blow. Something — massive volcanic activity in what are now the Siberian Traps is the likeliest culprit — devastated the global ecosystem and eliminated 90% or more of all marine species and upward of 70% of those on land. The nearly cleaned slate marked the dawn of a new era: the Mesozoic.
Life was slow to recover, and for the first few tens of millions of years, the seas were dominated by groups of animals like the tough, thick-shelled brachiopods, ammonites and mollusks, some of the scant survivors of the Permian extinction. In the middle of the Mesozoic, however, the ocean began to teem with agile animals that had much thinner shells. “If you look at the fossil record, we see animals becoming bigger, becoming more active — predators become more fearsome,” Eichenseer said.
This dramatic shift in marine life came to be known as the Mesozoic marine revolution. And it was in this time of renewal that a new kind of life emerged: the calcifying plankton — a diverse classification of tiny organisms that build shells or skeletons of calcium carbonate.
It’s not clear why groups like coccolithophores (single-celled plantlike organisms) and foraminifera (tiny amoeboid animals in shells) evolved when they did, roughly 80 million years after the mass extinction. The advent of biomineralization in the early Cambrian opened up significant new options for living things, so in that sense, it’s not surprising that new species evolved and took advantage of the opportunity to calcify. Hard shells do help protect an organism against predation and other physical harm, after all. They can also act as a protective barrier against DNA-damaging ultraviolet rays. Moreover, at that time, there was a lot of dissolved carbonate in the water (the corals and bivalves that lived before the calcifying plankton didn’t remove very much of it), so the abundance of carbonate as a resource could have facilitated the development of calcification pathways.
But calcifiers face serious risks. As a group, they’re very sensitive to changes in ocean chemistry. To make their shells, they must pull calcium and carbonate ions from seawater — a process that becomes harder when that water becomes more acidic. Worse, if the water becomes acidic enough, those calcium carbonate shells start to dissolve.
That’s why mass marine extinctions have often been linked to geological or climatic events that cause the pH of the ocean to fall. Volcanic activity, in particular, tends to release a lot of carbon dioxide into the atmosphere, and as the gas diffuses into the oceans, it turns waters more acidic. Between the Ordovician period, about 485 million years ago, and the early Jurassic, about 170 million years ago, there was abundant volcanic activity, which led to the extinction of many, many marine species. During that time, organisms thrived or died mostly based on factors beyond their control.
But since the middle of the Jurassic, that hasn’t really been the case. “There was still volcanism, but that was usually not anymore accompanied by an extinction event,” Eichenseer explained. He suspects that the credit for life’s new resilience probably goes to those plankton — especially the ones that died.
...