An event that seemed to condemn marine life 445 million years ago actually played a decisive role in the rise of the animals most familiar to us. While a massive glaciation wiped out the majority of ocean species, a small group of animals with a major innovation – the jaw – found in this disaster a unique opportunity. Starting from this planetary crisis, evolution took a new direction.
The planet was very different before this upheaval. The Ordovician period was characterized by warm, shallow oceans, teeming with a great diversity of living beings. Trilobites crawled on the seafloor, while huge sea scorpions and nautiloids with pointed shells dominated the waters. The earliest ancestors of jawed vertebrates already existed, but they remained inconspicuous and few in number amidst this lush fauna.
A fossil specimen of Sacabambaspis, a jawless fish 14 inches (35 cm) long with an armored head. Such animals disappeared after the extinction.
Image Wikimedia This crisis unfolded in two major stages. First, the Earth cooled rapidly, with glaciers covering the supercontinent Gondwana. Shallow seas dried up, causing the first wave of extinctions. Subsequently, several million years later, the ice melted. The rapid return of warm, oxygen-poor waters finished off many species that had adapted to the cold. These drastic changes reshaped ocean chemistry and marine habitats.
To survive, some populations became trapped in isolated areas, called refugia. These pockets of biodiversity, protected by hard-to-cross barriers, served as sanctuaries. A recent study published in
Science Advances indicates that jawed vertebrates particularly benefited from this situation. Confined to restricted spaces like the region corresponding to present-day South China, they were able to develop sheltered from direct competition.
Their success rests on the ecological opportunities left vacant. With the disappearance of many jawless animals and other marine groups, new places in the ecosystem became available. Jawed vertebrates, already present, were well-positioned to occupy these niches. This situation favored rapid diversification, as each population could specialize in using particular resources, much like Galápagos finches with different beak shapes.
For nearly 40 million years, jawless vertebrates remained dominant in most open oceans. Jawed fish, meanwhile, continued their evolutionary radiation mainly from their Asian refugia. Their descendants eventually recolonized the world's seas much later, gradually replacing the old dominant groups.
The process observed here appears to be a recurring model in the history of life. After a major disruption, ecosystems do not start from scratch. They rebuild by reusing functional blueprints, but with new actors. Jawed vertebrates thus inherited the ecological roles once held by animals now extinct, like conodonts or certain arthropods.
This discovery has several facets. It shows how catastrophic events can pave the way for major evolutionary innovations. It also explains why current marine life is primarily derived from this group of survivors rather than older life forms. Understanding these cycles of diversification helps grasp the long-term mechanisms that shape biodiversity on our planet.