Geology shows where ancient reefs once rose from the seafloor.
Dr. Siqueira’s team calls them worn-down survivors of what they’ve named the Great Indo-Australian Miocene Reef System.
“The new findings go further, suggesting individual reefs within this system may have been far larger than any modern reef,” Siqueira said.
This vast ancient reef network had never been formally unified under a single name.
The new study gives researchers a framework to map the Great Indo-Australian Miocene Reef System and trace what remains of it today.
Australia’s northwest coast has never ranked high on the list of places reef scientists study first. The reefs there are small, remote, and often overlooked.
New research suggests those overlooked reefs may be the last surviving fragments of the largest reef system in 100 million years – and the ancient origin point of today’s extraordinary marine diversity.
A time of massive reef expansion
The study was led by Dr. Alexandre Siqueira, a research fellow at Edith Cowan University (ECU).
The researchers found that during the Miocene, roughly 20 to 10 million years ago, reefs expanded on a scale not seen in 100 million years.
Reefs hold about a quarter of all marine species but cover less than one percent of the ocean. How did that happen in the first place?
Three lines of evidence
Dr. Siqueira and his colleagues drew on three independent records: geological maps of ancient reefs, fossils, and the genetic family trees of living reef species.
Each record alone is partial. Geology shows where ancient reefs once rose from the seafloor. Fossils preserve species in patches. Genetic trees show when lineages split but not where.
“This study uncovers a turning point deep in Earth’s history – a time when reefs didn’t just grow, but magnified on a scale far beyond anything we see today,” Siqueira said.
All three lines aligned on a single window roughly 20 million years ago, when reefs across what is now the Coral Triangle grew on a scale the ocean hasn’t matched since.
A recent study tracking the region’s biodiversity over 40 million years captured the rise in species but never asked how large those reefs actually were.
The west coast surprise
What surprised the researchers most was the location. The largest reefs in this ancient network were not near Indonesia or the Philippines, but off Australia’s northwest coast.
What’s left today is a scattering of small, remote reefs – modest by Great Barrier Reef standards.
Dr. Siqueira’s team calls them worn-down survivors of what they’ve named the Great Indo-Australian Miocene Reef System.
Geologists have known for decades that a barrier reef once tracked Australia’s northwest coast, roughly matching the Great Barrier Reef in extent.
What no one had measured was how big the individual reefs within it grew.
“The new findings go further, suggesting individual reefs within this system may have been far larger than any modern reef,” Siqueira said.
The role of tectonic plates
The timing raises an obvious question: why did reefs expand so dramatically 20 million years ago? The researchers trace the answer to shifting tectonic plates.
By then, Australia’s tectonic plate was pushing north into the Southeast Asian margin, opening vast shallow, warm seas – exactly what coral needs.
An earlier paper reached the same conclusion, simulating 140 million years of plate motion to map where shallow tropical seas opened up.
What modeling alone couldn’t supply was fossil and genetic evidence. The current study reveals that the reefs in those new seas didn’t just exist – they flourished on a scale the modern ocean doesn’t match.
When fish lineages exploded
Reefs aren’t just rock and coral. They’re scaffolding for everything that lives on them. As the Indo-Australian system grew, fish family trees were branching fast.
Parrotfishes are one of the clearest examples. A separate genetic study on wrasses and parrotfish found that reef lineages diversified rapidly between 20 and 15 million years ago. Non-reef branches didn’t.
Dr. Siqueira’s analysis suggests the timing wasn’t coincidence. Larger reefs likely created more habitat types, opening ecological space that new species moved into.
Many questions still remain. Whether reefs grew first and pulled in new species, or whether it worked the other way around, is still debated.
The weight of climate, tectonics, and biology in driving the expansion isn’t settled.
“It adds one more piece of evidence to the puzzle on how coral reef systems as a whole evolved,” said Dr. Siqueira.
Living artifacts of an ancient reef
The search for marine biodiversity’s origins has changed. For decades, the question was why the Coral Triangle is so rich. It inherited its riches from a vanished neighbor.
That changes the conversation about conservation. Australia’s northwest reefs – Ashmore, Scott, and Rowley – have been treated as remote outposts.
They may be living artifacts of a system that seeded today’s tropical oceans.
This vast ancient reef network had never been formally unified under a single name. The new study gives researchers a framework to map the Great Indo-Australian Miocene Reef System and trace what remains of it today.
The study is published in the journal Science Advances.
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