The dominance of fish and all terrestrial vertebrates (including humans) began with a massive catastrophe. Authors of a new study in the journal Science Advances concluded that the Late Ordovician extinction, 445–443 million years ago, played a key role in the “reign of fish.”
Before this extinction, the world’s oceans were different. They were dominated by jawless cyclostomes (ancestors of modern lampreys), not fish as we understand them today. They were complemented by giant trilobites, human-sized sea scorpions, and predatory nautiloids with shells up to five meters long. Jawed vertebrates (future fish and all terrestrial animals) were rare and not very diverse.
The extinction occurred in two waves, caused by sharp climatic fluctuations:
The first wave (445 million years ago) – a severe cooling and glaciation devastated shallow seas, reducing the biodiversity of all vertebrates.
The second wave (443 million years ago) – finally destroyed the unified global ecosystem.
It was after this, during the recovery period, that an evolutionary explosion occurred. Diverse groups of jawed vertebrates, including ancient sharks, began to appear in isolated marine basins. Geographic isolation allowed them to evolve independently, creating immense species diversity.
Although cyclostomes also survived the extinction, jawed vertebrates proved to be evolutionarily more plastic. They were able to occupy new ecological niches in the devastated world. The trend towards their dominance, which ultimately led to the emergence of all terrestrial animals, began around 440 million years ago.
An interesting fact: one of the possible causes of the extinction itself could have been “white nights” caused by asteroid debris in the atmosphere, which disrupted the planet’s climate balance.
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