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 called the three great evolutionary faunas of marine animals – a trio of successive explosions in biodiversity within the ocean over the course of the Phanerozoic.

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

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

Sometimes the hearth comes from within.

"The fossil record tells us that a number of the key transitions within 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 a number of those major transitions were more gradual and should are driven by biological interactions between organisms."

The case during this point is what's referred to as the Mesozoic Marine Revolution. Commencing roughly 150-200 million years ago, this transition represents all the macroevolutionary changes that occurred as marine predators like fish, crustaceans, and predatory snails increased in numbers, forcing their invertebrate prey, like 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 2 hypotheses – the Mesozoic Marine Revolution and also the three great evolutionary faunas into one 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 many years, the researchers say it nonetheless triggered a chronic biotic transition comparable in magnitude to the end-Permian transition.

This episode, often called the good 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, referred to as the fashionable fauna period, per Sepkoski's framework.

But per Rojas, Kowalewski, and their team, the fashionable 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 gives a singular perspective of the organization of marine life," Rojas says.

"At the foremost 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.