A new analysis of early vertebrate fossils shows that this biological catastrophe set the stage for the later dominance of jawed fishes, or gnathostomes, in marine ecosystems. Researchers at the Okinawa Institute of Science and Technology (OIST) compiled a global database covering 200 years of paleontological work on late Ordovician and early Silurian vertebrates to reconstruct the structure and diversity of these ancient communities.
The team focused on how vertebrate life responded as Earth shifted from a warm greenhouse climate to a cool icehouse state and back again. In the first extinction pulse, expanding ice sheets over Gondwana drained shallow continental seas and destroyed key habitats, while a second pulse several million years later saw melting icecaps flood recovering ecosystems with warm, oxygen-poor, sulfur-rich waters.
Before this upheaval, Ordovician oceans were dominated by a wide range of invertebrates and early vertebrates. Large-eyed, lamprey-like conodonts swam among giant sponges, trilobites, shelled mollusks, human-sized sea scorpions, and nautiloids with shells up to five meters long. Amid this diversity lived relatively inconspicuous early vertebrates, including jawless fishes such as Sacabambaspis with armored heads and forward-facing eyes, and the first ancestors of jawed fishes.
Fossils indicate that no animals resembling Sacabambaspis survived past the Late Ordovician extinction, underscoring how severely the crisis pruned early vertebrate lineages. In contrast, jawed vertebrates that managed to persist through the event later diversified into the major fish groups that dominate modern seas, including lineages related to sharks.
The study shows that during and after LOME, most vertebrate survivors were restricted to separated refugia, or biodiversity hotspots isolated by deep ocean barriers. Within these refugia, gnathostomes had a consistent advantage and gradually increased in genus-level diversity, particularly over the several million years following each extinction pulse.
By integrating fossil occurrences with biogeography, morphology, and ecology, the researchers could track how vertebrate groups shifted across the globe as conditions changed. In what is now South China, they identified the first complete body fossils of jawed fishes directly related to modern sharks, which remained confined to relatively stable refugia for millions of years before evolving traits that allowed them to cross open oceans and colonize new regions.
The work suggests that jawed fishes did not initially evolve to create entirely new ecological roles, but instead moved into ecological niches left vacant by extinct jawless vertebrates and other animals. In small, geographically restricted refugia with many empty positions in food webs, gnathostomes were able to experiment with a wide variety of lifestyles and feeding strategies.
This pattern resembles the diversification of Darwin's finches on the Galapagos Islands, where birds spread into open niches and, over time, evolved different beak shapes suited to specific diets. In the Ordovician and Silurian seas, early jawed fishes filled roles once held by conodonts and arthropods, gradually rebuilding complex ecosystems with new species but similar functional structures.
Jawless fishes did not disappear immediately after the crisis. While jawed fishes remained trapped in places such as South China, jawless relatives continued to evolve in other regions and dominated many marine environments for roughly 40 million years. These groups produced numerous reef forms and alternative feeding structures before jawed vertebrates ultimately became the most successful fishes.
The researchers found that LOME did not completely reset marine life, but instead triggered a recurring "diversity-reset cycle" in which extinctions driven by environmental change cleared ecological space that surviving vertebrates later refilled. Across the Paleozoic era, similar crises repeatedly produced ecosystems that converged on comparable functional designs, even as the specific species changed.
By linking fossil diversity patterns with geography and ecology, the study clarifies why modern marine vertebrates trace their origins to gnathostomes that survived the Late Ordovician crisis rather than to earlier groups like conodonts and trilobites. It also helps explain why jaws, once established, became a key innovation underpinning the success of vertebrates in the oceans.
The findings are published in the journal Science Advances and highlight how mass extinctions can act as engines of long-term evolutionary change, shaping which groups rise to prominence. The authors argue that understanding these deep-time "reset cycles" can provide broader insights into how present-day environmental disruptions may reorganize ecosystems and influence the future trajectory of biodiversity.
Research Report:Mass Extinction Triggered the Early Radiations of Jawed Vertebrates and Their Jawless Relatives (Gnathostomes)
Related Links
Okinawa Institute of Science and Technology Graduate University
Explore The Early Earth at TerraDaily.com
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