Late Ordovician oceans contracted and reshaped vertebrate evolution

About 445 million years ago the Late Ordovician Mass Extinction removed roughly 85% of marine species, and a new Science Advances study reports that isolated refuges after the event allowed jawed vertebrates to diversify and later spread.

Around 445 million years ago the planet moved from a warm greenhouse state into an icehouse climate as glaciers expanded across Gondwana and shallow seas contracted. This environmental change is linked in the fossil record to the Late Ordovician Mass Extinction, which removed a large share of marine species. A team at the Okinawa Institute of Science and Technology assembled a long fossil database and reports that survivors were confined to isolated refuges. The researchers say those refuges appear to have favored jawed vertebrates, which later diversified and spread. Key points: - Timing and scale: The Late Ordovician event, about 445 million years ago, is reported to have eliminated roughly 85% of marine species. - Two-phase environmental shift: The extinction is described in the record as a rapid cooling to an icehouse state followed, millions of years later, by warming with low-oxygen, sulfur-rich waters; the ultimate cause remains debated. - Refugia and diversification: The OIST team compiled two centuries of paleontological data and found survivors concentrated in isolated refuges, where jawed vertebrate diversity rose over subsequent millions of years. - Geographic pattern: Fossils from regions now in South China include some of the earliest complete jawed fishes; these taxa appear to have remained in stable refuges before later dispersal. Summary: The study links the Late Ordovician loss of marine species to an ecological reset that created opportunities for jawed vertebrates to diversify and eventually influence modern marine lineages. Undetermined at this time.