Look back in time far enough and a pattern may emerge. After studying thousands of ancient fossils, paleontologist Jack Sepkoski discovered just such a thing in 1981: an epic sequence of life and death etched into the skeletons of the past 500 million years.
The late Sepkoski, a professor at the University of Chicago, discovered what came to be known as the three great evolutionary fauna of marine animals – three consecutive explosions in ocean biodiversity over the course of the Phanerozoic Eon.
This gigantic marine life boom was made by catastrophes of a world-changing scale: extinction-level events that triggered massive animal die-off – while simultaneously clearing the stage for new creatures to emerge and thrive in the spaces they left behind.
But it doesn’t have to be that way, a new study suggests. Equally powerful forces – capable of shaping macroevolutionary processes with planetary implications – do not always require asteroids or super volcanoes.
Sometimes the fire comes from within.
“The fossil record tells us that some of the most important transitions in the history of life were rapid changes brought about by abrupt external factors,” explains University of Florida paleontologist Michal Kowalewski.
“But this study shows that some of those important transitions were more gradual and may have been caused by biological interactions between organisms.”
The case at this point is what is known as the Mesozoic Marine Revolution. This transition, which began about 150-200 million years ago, represents all the macroevolutionary changes that took place when marine predators such as bony fish, crustaceans and predatory snails increased in numbers, forcing their invertebrate prey, such as mollusks, into their defenses against dull and grenade-smashing attacks.
In the new study, which used modeling to demonstrate the network of relationships between giant assemblies of prehistoric marine life forms, the team found that the Mesozoic Marine Revolution actually represents a fourth, unrecognized chapter of the growing biodiversity within the Phanerozoic – Equal in its strength three great evolutionary faunas identified by Sepkoski decades ago.
“We integrate the two hypotheses – the Mesozoic Marine Revolution and the three great evolutionary faunas into one story,” explains lead author and paleontologist Alexis Rojas of Umeå University in Sweden.
“Instead of three life stages, the model shows four.”
Ultimately, although the Mesozoic Marine Revolution was characterized by gradual ecological changes brought about by interactions in marine life over millions of years, the researchers say it nonetheless produced a long-lasting biotic transition comparable in magnitude to the late Permian transition.
Often referred to as the Great Dying, this episode occurred about 250 million years ago and was the most severe mass extinction on Earth, wiping out about 80 percent of all marine species (and 70 percent of terrestrial vertebrates).
In the aftermath, life bounced back with the third major evolutionary fauna, known as the modern fauna period, according to 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 marine life biodiversity on Earth during the mid-Cretaceous Period, approximately 129 million years ago.
“What we’ve actually built is an abstracted fossil record that provides a unique perspective on the organization of marine life,” says Rojas.
“At the most basic levels, this map shows ocean regions with certain animals,” he adds. “The building blocks of our research are the individual animals themselves.”
The findings are reported in Communication biology