385 million-year-old discovery hidden inside extinct coral updates history books

Inside the fossilised remains of an extinct species, scientists found the first ever sign of a symbiotic relationship between coral and algae.

Inset: a tabulata coral. Background: An aerial shot of the Great Barrier Reef.
Researchers discovered signs of symbiosis in long-extinct tabulata corals, which existed hundreds of millions of years before modern corals. Source: Simon F. Zoppe/Getty

Most people associate coral with colour. But the outer layer of the coral is actually transparent with a white skeleton, and it’s the algae living within its tissue that creates such a brilliant appearance.

As ocean temperatures warm, coral reefs appear to “bleach”, however what we’re actually seeing is the algae being expelled due to heat stress, leaving its host naked.

Right now climate change is straining this long-established relationship between coral and algae, and this week researchers have shared new evidence they came together earlier than previously thought. Inside the fossilised remains of extinct coral, they found ancient signs of them intertwining hundreds of millions of years ago.

The relationship between the two lifeforms is mutually beneficial. In shallow waters, coral gives algae protection from harsh sunlight, and the algae offers a "quid pro quo" by producing large amounts of energy through photosynthesis to help its host grow.

After ancient corals from Africa and Germany were unearthed, a research team led by the Max Planck Institute for Chemistry found evidence that symbiosis began around 385 million years ago, around 170 million years earlier than previously thought.

Their paper has been published in the journal Nature.

The team examined two types of long-lost coral species. The first was rugosa, an order that grew in a horn-like shape, and it showed no sign of having formed a relationship with algae. But it was a different story when they examined honeycomb-shaped tabulata corals.

Because algae is squishy and doesn’t form a skeleton it wasn’t immediately apparent in the fossil, but by analysing the nitrogen isotopes in the rock, scientists were able to distinguish if the corals were pulling energy from symbiosis.

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Bleached coral on the Great Barrier Reef.
Coral bleaches when the algae it contains is expelled due to stress. Source: AAP

Coral thrived during the Devonian period, but this came to an end around 374 million years ago. Changes in the climate, sea level rises, and a reduction in oxygen and carbon dioxide in the atmosphere may have all contributed to a mass extinction event that killed around three-quarters of the world's species.

That was the world’s second mass extinction event, and it occurred over 100 million years before the first dinosaurs existed. Today many experts believe the world is undergoing its sixth mass extinction, but this one is not caused by natural occurrences, but rather the impact of humankind on the environment.

A cluster of fossils from the mid Devonian period.
At the end of the Devonian period, the world’s second mass extinction event occurred, resulting in three quarters of species being wiped out. Source: Getty

Not only are land animals vanishing from the landscape – non-profit Worldwide Fund for Nature has found a 73 per cent decline in monitored wildlife populations in just 50 years – but the oceans are also rapidly changing.

This year, testing found sea surface temperatures have been at their highest in 400 years around the Great Barrier Reef. Climate change, a process primarily caused by the burning of fossil fuels, will likely result in temperatures rising by at least two degrees since industrialisation began. By the time this temperature is reached, it's expected close to 100 per cent of the world’s reefs will be impacted by coral bleaching.

While bleaching does not immediately kill corals, it puts them under increased stress and at a higher risk of death. As bleaching events become more frequent, corals will be unlikely to recover.

The first only occurred in 1998. But since then, the Great Barrier Reef has experienced mass bleaching in 2002, 2016, 2017, 2020, 2022 and 2024.

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