Around 445 million years ago, Earth underwent a dramatic transformation that reshaped the future of life. In a remarkably short geological period, massive glaciers spread across the southern supercontinent Gondwana. As ice locked up water, vast shallow seas dried out, triggering an "icehouse climate" and radically altering ocean chemistry. The result was catastrophic. Roughly 85% of all marine species vanished, wiping out most life on the planet.

Yet from this devastation emerged an unexpected outcome. According to a new study published in Science Advances, scientists from the Okinawa Institute of Science and Technology (OIST) have shown that this event, known as the Late Ordovician Mass Extinction (LOME), set the stage for an explosion of vertebrate diversity. During the chaos, one group gained a lasting advantage and ultimately reshaped life on Earth: 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."

Earth Before the Great Die-Off

The Ordovician period, which lasted from about 486 to 443 million years ago, looked nothing like today's world. Gondwana dominated the Southern Hemisphere and was surrounded by warm, shallow seas. With no ice at the poles, the planet experienced a greenhouse climate that supported rich marine ecosystems. Early land was just beginning to host simple plants similar to liverworts, along with many-legged arthropods creeping along coastlines.

The oceans, however, were already bursting with strange and diverse life. Large-eyed, lamprey-like conodonts moved through forests of towering sea sponges. Trilobites scurried across the seafloor among dense clusters of shelled mollusks. Human-sized sea scorpions and enormous nautiloids with pointed shells stretching up to five meters hunted through the water. Among this alien cast were the early ancestors of gnathostomes, or jawed vertebrates, which were still rare and unremarkable at the time.

Two Waves of Extinction

Although scientists still debate what ultimately caused LOME, the fossil record clearly shows a sharp dividing line before and after the event. "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 unfolded in two distinct phases. First, Earth rapidly shifted from a warm greenhouse state to a cold icehouse climate. Glaciers expanded across Gondwana, draining shallow seas and destroying key marine habitats. Several million years later, just as ecosystems began to recover, the climate reversed again. Melting icecaps flooded the oceans with warmer water that was rich in sulfur and low in oxygen, overwhelming species that had adapted to colder conditions.