Approximately 445 million years ago, a mass extinction event dramatically altered life on Earth. During this brief geological period, glaciers formed over the supercontinent Gondwana, drying up many of the vast, shallow seas and creating an ‘icehouse climate.’ This, combined with significant changes in ocean chemistry, led to the extinction of about 85% of all marine species, which constituted the majority of life at the time.
In a recent study published in Science Advances, researchers from the Okinawa Institute of Science and Technology (OIST) have demonstrated that this catastrophic event, known as the Late Ordovician Mass Extinction (LOME), paved the way for a new era of vertebrate life. The upheaval allowed jawed vertebrates to rise to dominance, fundamentally shaping the course of evolution. “We have demonstrated that jawed fishes only became dominant because this event happened,” stated Professor Lauren Sallan, senior author of the study and a member 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 characterized by a vastly different Earth. The southern supercontinent Gondwana was surrounded by extensive shallow seas, and the poles were ice-free, maintaining a warm greenhouse climate. Coastal areas were slowly being colonized by early plants and arthropods, while the oceans teemed with diverse and bizarre life forms. Among these were large-eyed, lamprey-like conodonts, trilobites, and human-sized sea scorpions.
Few and far between these creatures were the ancestors of jawed vertebrates, or gnathostomes, who would later dominate animal life. “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 Prof. Sallan. The extinction unfolded in two main phases: an initial switch from a greenhouse to an icehouse climate that dried out shallow ocean habitats, followed by a return to warmer conditions that devastated cold-adapted marine life.
Survival and Speciation in Isolated Refugia
During and after these extinction pulses, surviving vertebrates were confined to isolated refugia—biodiversity hotspots separated by vast oceans. Here, gnathostomes had a distinct advantage. “We pulled together 200 years of late Ordovician and early Silurian paleontology,” noted Wahei Hagiwara, the study’s first author and an OIST PhD student. “Creating a new database of the fossil record helped us reconstruct the ecosystems of the refugia, showing how LOME led directly to a gradual, but dramatic increase in gnathostome biodiversity.”
This comprehensive fossil database allowed researchers to link the rise in gnathostome diversity not only to LOME but also to specific locations. “This is the first time that we’ve been able to quantitatively examine the biogeography before and after a mass extinction event,” Prof. Sallan explained. For instance, in present-day South China, the first full-body fossils of jawed fishes, related to modern sharks, were concentrated in stable refugia for millions of years.
From Toothed “Worms” to Darwin’s Finches
By integrating the fossil record with biogeography, morphology, and ecology, the study provides new insights into evolutionary processes. “Did jaws evolve in order to create a new ecological niche, or did our ancestors fill an existing niche first, and then diversify?” asked Prof. Sallan. “Our study points to the latter.” This pattern mirrors the evolutionary trajectory of Darwin’s finches on the Galápagos Islands, which diversified their diet and evolved different beak shapes to suit new ecological niches.
While jawed fishes were confined to refugia, their jawless relatives continued to evolve in other regions, dominating the seas for the next 40 million years. These jawless vertebrates diversified into various forms, some developing alternative mouth structures. However, the eventual dominance of jawed fishes remains a mystery.
The Ecological Reset and Evolutionary Patterns
Rather than erasing life, LOME triggered an ecological reset. Early vertebrates occupied the niches left by extinct species, rebuilding ecosystems with new species. This recurring ‘diversity-reset cycle’ is a pattern seen throughout the Paleozoic era, where evolution restores ecosystems by converging on similar functional designs.
Prof. Sallan summarized the findings: “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.”
These revelations shed light on the long-term patterns and processes underlying evolution, offering exciting insights into the history of life on Earth.
