The few species that survived were the starting point for all of today's vertebrates

A mass extinction of fish 360 million years ago hit the reset button on Earth's life, setting the stage for modern vertebrate biodiversity. The mass extinction scrambled the species pool near the time at which the first vertebrates crawled from water towards land.

Those few species that survived the bottleneck were the evolutionary starting point for all vertebrates — including humans — that exist today, according to results of a study published recently in the journal Proceedings of the National Academy of Sciences(PNAS).

“Everything was hit; the extinction was global,” said Lauren Sallan of the University of Chicago and lead author of the paper. “It reset vertebrate diversity in every single environment, both freshwater and marine, and created a completely different world.”

The Devonian Period, which spanned from 416 to 359 million years ago, is also known as the Age of Fishes for the broad array of species present in Earth's aquatic environments.

Armored placoderms such as the gigantic Dunkleosteus and lobe-finned fishes — similar to the modern lungfish — dominated the waters, while ray-finned fishes, sharks and tetrapods were in the minority, according to Maureen Kearney, program director in the National Science Foundation (NSF)'s Division of Environmental Biology.

But between the latest Devonian Period and the subsequent Carboniferous period, placoderms disappeared and ray-finned fishes rapidly replaced lobe-finned fishes as the dominant group, a demographic shift that persists to today.

Scientists have long theorized that the Late Devonian Kellwasser event —considered to be one of the ‘Big Five' extinctions in Earth's history — was responsible for a marine invertebrate species shake-up.

But an analysis of the vertebrate fossil record by Sallan and Coates pinpointed a critical shift in diversity to the Hangenberg extinction event 15 million years later, according to a University of Chicago press release.

Prior to the extinction, lobe-finned forms such as Tiktaalik and the earliest limbed tetrapods such as Ichthyostega had made the first tentative “steps” toward a land-dwelling existence.

But after the extinction, a long stretch of the fossil record known as ‘ Romer's Gap,' is almost barren of tetrapods, a puzzle that had confused paleontologists for many years. Sallan and Coates' data suggest that the 15-million-year gap was the hangover after the traumatic Hangenberg event.

“Something that's seen after an extinction event is a gap in the records of survivors,” Sallan said. “You have a very low diversity fauna, because most things have been killed off.”

When tetrapods finally recovered, those survivors were likely the great-great-grandfathers to the vast majority of land vertebrates present today.

Modern vertebrate traits — such as the motif of five-digit limbs that is shared by all mammals, birds and reptiles in utero — may have been set by this early common ancestor, the authors propose.

The analysis benefitted from recent advances in filling in the vertebrate fossil record, Coates said.

What remains mysterious is exactly what happened 360 million years ago to trigger this mass extinction.

Other researchers have found evidence of substantial glacier formation at the end of the Devonian period, which would have dramatically lowered sea levels and affected life. The first appearance of forest-like environments in some regions might also have produced atmospheric changes catastrophic to animal life. The research also raises questions about the pattern of evolution after the extinction event.

Regardless of these questions, the consequences are still being felt hundreds of millions of years later. — Our Bureau

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