The northwestern coast of Australia has historically been overlooked by coral reef scientists. The reefs there are minor, challenging to reach, and often neglected.
New investigations suggest that these unexamined reefs might represent the last remnants of the largest reef system spanning the last 100 million years, as well as the ancient source for today’s remarkable marine diversity. The study was spearheaded by Alexander Sequeira from Edith Cowan University (ECU), with findings published in the journal Science Advances.
The researchers determined that during the Miocene epoch, roughly 20 to 10 million years ago, reefs expanded to a magnitude unseen in the preceding 100 million years.
Reefs host about a quarter of all marine species yet cover less than one percent of the ocean. How did such a concentration occur?
Sequeira and his team synthesized evidence from three independent sources: geological maps of ancient reefs, fossil records, and genetic analysis of extant reef species.
Each single data stream is inherently incomplete on its own. Geology reveals the former locations where ancient reefs rose from the seabed. Fossils preserve species in isolated pockets. Genetic trees indicate when lineages diverged, but not the precise spatial context.
“This research illuminates a pivotal moment in Earth’s deep history—a time when reefs didn’t just grow, but expanded on a scale far surpassing anything visible today,” stated Sequeira.
Around 20 million years ago, all three lines of evidence converged on a single timeline, showing reefs throughout the area now known as the Coral Triangle flourishing on a scale unsurpassed in the ocean since that time.
A recent study tracing biodiversity in that region over 40 million years documented an increase in species numbers but failed to address the actual scale these reefs attained.
The most surprising revelation for the researchers was the location: the largest reefs within this ancient network were situated not off the coasts of Indonesia or the Philippines, but along Australia’s northwest shelf.
Today, only scattered, small, hard-to-access reefs remain there—modest features compared to the Great Barrier Reef. Sequeira’s team refers to them as eroded survivors of what they term the Great Indo-Australian Miocene Reef System.
Geologists have long known that a barrier reef system, comparable in extent to the Great Barrier Reef, once stretched along the northwestern Australian coast. However, the sheer size reached by individual reefs within that system had never been quantified.
“The new data suggests that individual reefs within this system might have dwarfs any modern reefs, becoming much larger,” Sequeira noted.
The timing of these events raises an obvious question: why did reefs expand so dramatically 20 million years ago? The researchers attribute the answer to plate tectonic shifts.
By that period, the Australian tectonic plate was moving northward toward the margin of Southeast Asia, revealing extensive shallow, warm marine areas—the perfect ingredients for coral establishment.
Earlier work had reached a similar conclusion: maps modeling tectonic plate movements over 140 million years had been created to locate the formation of shallow tropical seas.
Modeling alone proved insufficient, as it lacked context from fossil and genetic data. The current study demonstrates that reefs in these nascent seas didn’t just persist—they thrived at magnitudes unachievable in the contemporary ocean.
Reefs are far more than inert rock and coral; they form the structure supporting all the life found there. As the Indo-Australian ecosystem evolved, fish lineages branched rapidly.
Parrotfish offer one of the clearest examples. A separate genetic deep dive into wrasses and parrotfish revealed rapid diversification of reef-associated lineages between 20 and 15 million years ago. Lineages not tied to reef habitats did not undergo this rapid splitting.
Dr. Sequeira’s analysis proposes that the temporal coincidence was not arbitrary. Larger reef structures likely fostered a greater variety of habitats, thereby liberating ecological niches into which new species could move.
Numerous questions remain unresolved. Debate continues over whether the reefs arose first, subsequently drawing in new species, or if the causation worked in reverse.
The interplay between climate, tectonics, and biotic factors in driving range expansion is still not fully elucidated.
“This contributes yet another piece to the puzzle of how coral reef systems have evolved overall,” remarked Sequeira.
The search for the origins of marine biodiversity has shifted its focus. For decades, the question centered on why the Coral Triangle is so rich. It appears to have inherited its wealth from a vanished neighbor.
This necessitates a change in how conservation priorities are discussed. Australia’s northwestern reefs—Ashmore, Scott, and Rowley—have long been viewed as remote outposts.
They may, in fact, be living artifacts of the system that established the foundation for modern tropical oceans.
This vast, ancient reef network was never formally unified under a single designation. The new research provides scientists with the framework to map the Great Indo-Australian Miocene Reef System and trace what remains of it today.
About the Author
