Look far enough back in time, and a pattern may emerge. After studying thousands of ancient fossils, paleontologist Jack Sepkoski in 1981 identified exactly one such thing: an epic sequence of life and death, engraved in the skeletons of the past 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 ocean biodiversity over the course of the Phanerozoic Eon.
These giant blossoms of marine life have been documented by catastrophes on a global scale: extinction-level events that are causing the mass extinction of animals, while at the same time clearing the stage for new creatures to emerge and flourish in the spaces they have left behind. .
But that does not have to be the case, suggests a new study. Equally powerful forces – which can form macro-evolutionary processes with planetary implications – do not always require asteroids or supervolcanoes.
Sometimes the fire comes from within.
“The fossil record tells us that some of the most important transitions in life history were rapid changes caused by sudden external factors,” explains paleontologist Michal Kowalewski of the University of Florida.
“But this study shows that some of the major transitions were more gradual and possibly driven by biological interactions between organisms.”
The case at this point is what is known as the Mesozoic Marine Revolution. This transition represents about 150-200 million years ago and represents all the macro-evolutionary changes that took place when marine predators such as bonefish, crustaceans and predators increased in numbers, forcing their invertebrate prey, such as mollusks, to defend against sowing and shell-crushing attacks.
In the new research, which used modeling to demonstrate the network of relationships between giant assemblages of prehistoric life forms in the ocean, the team found that the Mesozoic Marine Revolution represents a fourth, unrecognizable chapter of rising biodiversity within the Phanerozoic – equal to its ability as the three great evolutionary faunas that Sepkoski identified decades ago.
“We integrate the two hypotheses – the Mesozoic Marine Revolution and the three great evolutionary faunas into a single story,” explains the first author and paleontologist Alexis Rojas of Umeå University in Sweden.
“Instead of three life stages, the model shows four.”
Finally, although the Mesozoic Marine Revolution was characterized by gradual ecological changes produced over millions of years by interactions with marine life, the researchers say that it nevertheless caused a prolonged biotic transition comparable to the end of the Permian transition.
This episode, often called the Great Dying, occurred about 250 million years ago and was Earth’s largest mass extinction event, wiping out about 80 percent of all marine species (and 70 percent of terrestrial vertebrates).
In the aftermath, life recovered with the third great evolutionary fauna, known as the modern fauna period, according to Sepkoski’s framework.
But according to Rojas, Kowalewski, and their team, the modern era intersected with the Mesozoic marine revolution, which contributed 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 absorbed fossil record that offers a unique perspective on the organization of marine life,” says Rojas.
“At the most basic levels, this map shows ocean regions with specific animals,” he adds. “The building blocks of our study are the individual animals themselves.”
The findings are presented in Communication Biology.