A new study shows that the Late Ordovician Mass Extinction around 445 million years ago not only wiped out 85% of marine species but also paved the way for jawed vertebrates to thrive. Researchers from the Okinawa Institute of Science and Technology analyzed fossil data to demonstrate how isolated refuges allowed these early fishes to diversify after the catastrophe. This event fundamentally reshaped Earth's ecosystems, influencing modern marine life.
Around 445 million years ago, during the Ordovician period, Earth shifted dramatically from a warm greenhouse climate to a cold icehouse state as glaciers expanded across the southern supercontinent Gondwana. This change dried up shallow seas, altered ocean chemistry, and triggered the Late Ordovician Mass Extinction (LOME), which eliminated roughly 85% of marine species in two phases. The first phase involved habitat destruction from glaciation, while the second, several million years later, brought warmer, sulfur-rich, low-oxygen waters from melting icecaps that overwhelmed recovering ecosystems.
Despite the devastation, LOME created opportunities for survivors. Jawed vertebrates, or gnathostomes, which were rare before the event, gained an edge by retreating to isolated marine refuges separated by deep ocean barriers. A study published in Science Advances by scientists at the Okinawa Institute of Science and Technology (OIST) used a comprehensive fossil database spanning 200 years of paleontology to track changes in genus-level diversity. "We pulled together 200 years of late Ordovician and early Silurian paleontology," said first author Wahei Hagiwara, now a PhD student at OIST.
The analysis revealed a clear trend: mass extinction pulses directly led to increased speciation in jawed vertebrates over millions of years. "We have demonstrated that jawed fishes only became dominant because this event happened," noted senior author Professor Lauren Sallan of OIST's Macroevolution Unit. By examining biogeography, the team identified key refuges, such as regions now in South China, where early jawed fishes related to modern sharks persisted and later spread.
Jawed vertebrates filled ecological niches vacated by extinct jawless species and others, rather than evolving jaws to create new roles. "Our study points to the latter," Sallan explained regarding whether jaws enabled niche creation or exploitation. This ecological reset, rather than a total wipeout, allowed jawed groups to outcompete others eventually, though jawless vertebrates dominated open oceans for another 40 million years. The findings highlight recurring "diversity-reset cycles" in evolution, explaining why modern marine life descends from these ancient survivors instead of earlier forms like conodonts and trilobites.
