Nature

An underwater revolution millions of years ago rewrote the script of the ocean

Look far enough back in time, and a pattern may emerge. After studying thousands of ancient fossils, paleontologist Jack Sepkoski identified just such a thing in 1981: an epic sequence of life and death, etched into the skeletons of the last 500 million years.

The late Sepkoski, a professor at the University of Chicago, discovered what became known as the three great evolutionary faunas of marine animals – a trio of successive explosions in biodiversity in the ocean over the course of the Phanerozoic Eon.

These giant bloomings of marine life were bookended by catastrophes of world-changing scale: extinction-level events precipitating mass animal die-offs – simultaneously clearing the stage for new creatures to emerge and prosper in the spaces they left behind.

But it doesn’t have to happen that way, a new study suggests. Equally powerful forces – capable of shaping macroevolutionary processes with planetary implications – don’t always require asteroids or supervolcanoes.

Sometimes the fire comes from within.

“The fossil record tells us that some of the key transitions in the history of life were rapid changes triggered by abrupt external factors,” explains paleontologist Michal Kowalewski from the University of Florida.

“But this study shows that some of those major transitions were more gradual and may have been driven by biological interactions between organisms.”

The case in this point is what’s known as the Mesozoic Marine Revolution. Commencing roughly 150-200 million years ago, this transition represents all the macroevolutionary changes that took place as marine predators like bony fish, crustaceans, and predatory snails increased in numbers, forcing their invertebrate prey, such as mollusks, to adapt defenses against boring and shell-crushing attacks.

In the new research, which used modeling to demonstrate the network of relationships between giant assemblages of prehistoric marine lifeforms, the team found that the Mesozoic Marine Revolution effectively represents a fourth, unrecognized chapter of surging biodiversity within the Phanerozoic – equal in its power to the three great evolutionary faunas Sepkoski identified decades ago.

“We are integrating the two hypotheses – the Mesozoic Marine Revolution and the three great evolutionary faunas into a single story,” explains first author and paleontologist Alexis Rojas from Umeå University in Sweden.

“Instead of three phases of life, the model shows four.”

Ultimately, although the Mesozoic Marine Revolution was characterized by gradual ecological changes produced by marine life interactions over millions of years, the researchers say it nonetheless triggered a prolonged biotic transition comparable in magnitude to the end-Permian transition.

This episode, often called the Great Dying, occurred approximately 250 million years ago and was Earth’s most severe mass extinction event, wiping out approximately 80 percent of all marine species (and 70 percent of terrestrial vertebrates).

In the aftermath, life rebounded with the third great evolutionary fauna, known as the Modern fauna period, per Sepkoski’s framework.

But according to Rojas, Kowalewski, and their team, the Modern period intersected with the Mesozoic Marine Revolution, contributing to a recognizable transition in biodiversity in Earth’s marine life during the mid-Cretaceous period, about 129 million years ago.

“What we actually built is an abstracted fossil record that provides a unique perspective of the organization of marine life,” Rojas says.

“At the most basic levels, this map shows ocean regions with particular animals,” he adds. “The building blocks of our study are the individual animals themselves.”

The findings are reported in Communications Biology.

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