Great Ordovician Biodiversification

The Great Ordovician Biodiversification Event, or GOBE, was a long surge in the variety of marine animals during the Ordovician period, from about 485 to 445 million years ago. Over tens of millions of years, the number of different genera in the oceans rose several-fold. The change affected nearly every group, including brachiopods, mollusks, corals, sea lilies and trilobites, and reefs and floating plankton spread through the seas.

It followed the Cambrian Explosion, which created the main animal body plans but each was represented by few species. The Ordovician filled these designs out into many species and ecological roles. The seas now held filter feeders that rose above the seafloor on stalks, animals that burrowed deeper into the mud, and large predators such as giant straight-shelled cephalopods, relatives of squid that measured several meters. Plankton, tiny drifting organisms, diversified too, providing a rich food supply for the rest of the web.

Why it happened is unsettled. Proposed drivers include a cooler climate, which expanded habitat in shallow seas, more oxygen, more nutrients from volcanic activity and weathering, and the arrangement of continents, which placed large areas of warm shallow shelf. These factors probably worked together. The event ended with a major mass extinction at the close of the Ordovician, around 445 million years ago, when glaciation lowered sea levels and cooled the oceans, but the groups that survived formed the ocean communities for the next 200 million years.