Mass death paved the way for the Age of Fishes

About 445 million years ago, our planet completely changed. Massive glaciers formed over the supercontinent Gondwana, sucking up sea water like an icy sponge. Now called the Late Ordovician mass extinction (LOME), Earth’s first major mass extinction wiped out about 85 percent of all marine species as the ocean chemistry radically changed and Earth’s climate turned bitter cold. 

However, with great biological havoc also comes opportunity. During all of this upheaval, one group evolved to dominate all others—jawed vertebrates. This ultimately put life on a forward path that can be traced up to today, according to a study published today in the journal Science Advances.

“We have demonstrated that jawed fishes only became dominant because this event happened,” Lauren Sallan, a study co-author and evolutionary biologist at Okinawa Institute of Science and Technology in Japan, said in a statement. “And fundamentally, we have nuanced our understanding of evolution by drawing a line between the fossil record, ecology, and biogeography.”

Earth’s first mass extinction

During the Ordovician period (roughly 486 to 443 million years ago) Earth looked very different than it does now. A southern supercontinent called Gondwana, dominated the planet and was surrounded by vast, shallow seas. There was no ice on the North or South Pole and the water was warm due to a greenhouse climate. Small plants and many-legged arthropods began to thrive on the coasts, and the water surrounding them were teeming with lifeforms that looked like something from a science fiction. Large-eyed, lamprey-like conodonts looped around sea sponges. Tiny trilobites scuttled among shelled mollusks. Sea scorpions as big as humans and nautiloids with 16-foot-tall shells scoured the water in search of prey. 

In between these creatures were the ancestors of gnathostomes, or jawed vertebrates. Gnathostomes would eventually dominate animal life on Earth.

“While we don’t know the ultimate causes of LOME, we do know that there was a clear before and after the event. The fossil record shows it,” explained Sallan. 

The extinction came in two stages. First, the planet rapidly switched from a warmer greenhouse to a much colder icehouse climate. Most of Gondwana was covered with thick ice, drying out shallow ocean habitats. A few million years later, biodiversity began to recover, but the climate flipped again. The cold-adapted marine life drowned in warm, sulfuric, and oxygen-depleted water as the ice caps melted.

During these waves of mass extinction, most vertebrate survivors were confined to refugia, or  isolated biodiversity hotspots separated by large areas of deep ocean. In these zones, surviving jawed vertebrates evidently had an advantage. 

In the new study, the team pulled years of paleontological data about the Ordovician and early Silurian paleontology to build a new database of the fossil record during this dramatic period in Earth’s history.

“That helped us reconstruct the ecosystems of the refugia,” added study co-author and Ph.D. student Wahei Hagiwara. “From this, we could quantify the genus-level diversity of the period, showing how LOME led directly to a gradual, but dramatic increase in gnathostome biodiversity. And the trend is clear – the mass extinction pulses led directly to increased speciation after several millions of years.”

Of fish and finches

With this new database, the team linked the rising jawed vertebrate biodiversity to not only this first mass extinction, but also location. They could trace the movement of species around the world and pinpoint specific refugia that played a role in helping vertebrates diversify. 

“For example, in what is now South China, we see the first full-body fossils of jawed fishes that are directly related to modern sharks,” explained Hagiwara. “They were concentrated in these stable refugia for millions of years until they had evolved the ability to cross the open ocean to other ecosystems.”

Merging the fossil record with biogeography, morphology, and ecology, can help us better understand the course of evolution. 

“Did jaws evolve in order to create a new ecological niche, or did our ancestors fill an existing niche first, and then diversify?” asks Sallan. “Our study points to the latter. In being confined to geographically small areas with lots of open slots in the ecosystem left by the dead jawless vertebrates and other animals, gnathostomes could suddenly inhabit a wide range of different niches.” 

A similar trend is seen in Darwin’s finches on the Galápagos Islands off the coast of Ecuador. These birds took advantage of new opportunities to diversify their diet to survive. Over time, their beaks evolved into different shapes to better suit their needs.

The diversity reset cycle

While jawed fishes were trapped in South China, their jawless relatives continued to evolve in parallel elsewhere. The jawless fish ruled the wider sea for the next 40 million years, diversifying into different types of reef fish. Why jawed fishes—among all other survivors—came to dominate once they spread out from the refugia remains a mystery.

According to the team, instead of wiping Earth’s ecological slate clean, the Late Ordovician mass extinction triggered a reset. Early vertebrate species stepped into the niches left behind by extinct conodonts and arthropods, rebuilding the same ecological structure, just with new animals. This pattern also repeats across the Paleozoic following other extinction events driven by similar environmental conditions. The team calls this a recurring “diversity-reset cycle,” where evolution restores ecosystems by converging on the same designs.

“This work helps explain why jaws evolved, why jawed vertebrates ultimately prevailed, and why modern marine life traces back to these survivors rather than to earlier forms like conodonts and trilobites,” said Sallan. “Revealing these long-term patterns and their underlying processes is one of the exciting aspects of evolutionary biology.”

 

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