445 million years ago, life on our planet was forever changed. During a geological blink of an eye, glaciers formed over the supercontinent Gondwana, drying out many of the vast, shallow seas like a sponge and giving us an 'icehouse climate' that, together with radically changed ocean chemistry, ultimately caused the extinction of about 85% of all marine species - the majority of life on Earth.
In a new Science Advances study, researchers from the Okinawa Institute of Science and Technology (OIST) have now proved that from this biological havoc, known as the Late Ordovician Mass Extinction (LOME), came an unprecedented richness of vertebrate life. During the upheaval, one group came to dominate all others, putting life on the path to what we know it as today: jawed vertebrates. "We have demonstrated that jawed fishes only became dominant because this event happened," says senior author Professor Lauren Sallan of the Macroevolution Unit at OIST. "And fundamentally, we have nuanced our understanding of evolution by drawing a line between the fossil record, ecology, and biogeography."
A fuller picture of life at the Ordovician sunset
The Ordovician period, spanning from roughly 486 to 443 million years ago, was a time when Earth looked very different. The southern supercontinent, Gondwana, dominated the planet, surrounded by vast, shallow seas. The poles were ice-free, and the water was pleasantly warm thanks to a greenhouse climate. And while the coasts were slowly being colonized by liverwort-like plants and many-legged arthropods, the surrounding basins were teeming with diverse - and bizarre - forms of life. Large-eyed, lamprey-like conodonts snaked around towering sea sponges, trilobites scuttled among swarms of shelled mollusks, while human-sized sea scorpions and giant nautiloids with pointy shells up to five meters in length patrolled the waters in search of prey. Few and far in between these strange creatures were the humble ancestors of gnathostomes, or jawed vertebrates, which would later come to dominate animal life on the planet.
"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," says Prof. Sallan. The extinction came in two pulses: First, the planet rapidly switched from a greenhouse to an icehouse climate, covering most of Gondwana with glaciers that dried out the shallow ocean habitats. Then, a few million years later, just as biodiversity was beginning to recover, the climate flipped again, melting the icecaps and drowning the now cold-adapted marine life with warm, sulfuric, and oxygen-depleted water.
During and after these waves of death, most of the vertebrate survivors were confined to refugia - isolated biodiversity hotspots separated by unsurmountable swaths of deep ocean - where surviving gnathostomes evidently had an advantage. "We pulled together 200 years of late Ordovician and early Silurian paleontology," adds first author Wahei Hagiwara, former research intern in the Macroevolution Unit who is now an OIST PhD student, "creating a new database of the fossil record that helped us reconstruct the ecosystems of the refugia. 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."
From toothed "worms" to Darwin's finches
In constructing this comprehensive database of fossils from across the world, the researchers were able to link the rising gnathostome biodiversity to not only LOME, but also location. "This is the first time that we've been able to quantitatively examine the biogeography before and after a mass extinction event. We could trace the movement of species across the globe - and it's how we've been able to identify specific refugia, which we now know played a significant role in the subsequent diversification of all vertebrates," explains Prof. Sallan. Hagiwara adds: "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. 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."
By integrating the fossil record with biogeography, morphology, and ecology, these findings have helped nuance our understanding 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 Prof. 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 clear with Darwin's finches on the Galápagos Islands, which took advantage of new opportunities to diversify their diet to survive - and over time, their beaks evolved different shapes to better suit the niche they came to occupy.
While the jawed fishes were trapped in South China, their jawless relatives continued to evolve in parallel elsewhere, ruling the wider seas for the next 40 million years. These diversified into many different forms of reef fishes, some of which had alternative mouth structures. But why jawed fishes, among all other survivors, came to dominate later once they spread from the refugia remains mysterious.
The researchers found that rather than wiping the slate clean, LOME triggered an ecological reset. Early vertebrates stepped into the niches left vacant by conodonts and arthropods, rebuilding the same ecological structure but with new species. This pattern repeats across the Paleozoic following extinction events driven by similar environmental conditions, forming what the team calls a recurring 'diversity-reset cycle' in which evolution restores ecosystems by converging on the same functional designs.
Prof. Sallan summarizes: "By integrating location, morphology, ecology, and biodiversity, we can finally see how early vertebrate ecosystems rebuilt themselves after major environmental disruptions. 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. Revealing these long-term patterns and their underlying processes is one of the exciting aspects of evolutionary biology."
