Science

Ocean life surged 500 million years ago due to cooler sea temperatures

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Sea life in the Ordovician Period

Esteban De Armas/Alamy

A period of global cooling around 500 million years ago may have triggered Earth’s largest surge in marine biodiversity.

The Great Ordovician Biodiversification Event marked a dramatic explosion in marine biodiversity between approximately 490 and 440 million years ago, resulting in the emergence of several new orders, families and genera of life that we still see today, such as sea stars and sea urchins.

“[It] is one of the key diversifications of life in Earth history, often referred to as ‘diversity’s big bang’,” says David Harper at Durham University, UK, who wasn’t involved in the study.

Proposed ideas for what sparked the event include sea level rises induced by continental drift, an increase in atmospheric oxygen and even meteors.

Another possible explanation is the global cooling that occurred during the period, which earlier studies found reduced tropical sea surface temperatures from around 40°C to 30°C (104°F to 86°F). Now, Daniel Ontiveros at the University of Lille in France and his colleagues have found that this was probably the case.

The team combined a climate model, based on fossil data and our knowledge of land configuration at the time, with an ecological model that mapped biodiversity patterns in the oceans.

The combined model suggests that, as the oceans cooled, they became more conducive to life, leading to the rapid emergence of new species. Existing species also moved from waters around the north and south poles towards the equator.

“At the start of the Ordovician, the ocean was very warm,” says Ontiveros. “Only a few species could withstand the high temperatures.” That is why aquatic life was mostly constrained to the colder, higher latitudes at the beginning of the Ordovician, he says. As the climate cooled, tropical waters became more habitable.

Researcher Gregory Beaugrand, also at the University of Lille, says the team’s climate-ecological model could be used to predict an effect of climate change, as rising sea surface temperatures may lead to fewer marine species at the equator and more at the less-extreme poles.

“The results go a long way to explain why this diversification was so rapid and why new genera or species evolved rapidly across a large region,” says Aaron Hunter at the University of Cambridge, who wasn’t involved in the study. “However, we should be cautious about the results presented as they may influence my sampling bias, as much of the fossil data comes from these high latitudes.”

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