The cradle of Earth’s rich ocean life was a massive coral reef system 20 million years ago

New research published today in Science Advances reveals that the largest expansion of coral reefs in the past 100 million years happened about 20 to 10 million years ago, between Australia and Southeast Asia.

This vast reef system likely laid the foundations for the extraordinary diversity of marine life we see today.

Coral reefs are among the most diverse ecosystems on Earth. They support about a quarter of all marine species while covering less than 1% of the oceans. Yet scientists have long grappled with the question of how such immense diversity arose in the first place. Where did it begin, and what made it possible?

Our new study uncovers a turning point deep in Earth’s history – a time when reefs didn’t just grow, but expanded on a scale far beyond anything we see today. This expansion may have created the ecological space needed for modern coral reef life to flourish.

Coral reefs are major biodiversity hotspots. Ahmer Kalam/Unsplash

An enduring mystery

Biodiversity simply refers to the variety of life in a given place. On coral reefs, this diversity is staggering: thousands of species of fish, corals and other organisms coexist in tightly packed ecosystems.

However, despite decades of research, the origins of this richness have remained an enduring mystery.

Our new study reveals that changes in environmental, biological and tectonic conditions about 20 million years ago promoted the dramatic expansion of coral reefs across a region stretching between Australia and Southeast Asia.

Today, this area is known as the Indo-Australian Archipelago. It’s recognised as a global hotspot of marine biodiversity, especially in an area called the Coral Triangle.

The expansion of reefs in this area coincided with the emergence of many familiar reef organisms, including plating corals and iconic fish groups like parrotfishes.

To uncover this, we combined evidence from geological records, fossils and genetic data. Together, these independent lines of evidence allowed us to pinpoint when and where modern reef biodiversity began to take shape, without relying on any single source alone.

Results suggest reef expansion itself played a crucial role in generating biodiversity. As reefs grew larger, they likely created new habitats and ecological opportunities, allowing species to evolve and diversify.

We have now named this ancient network of reefs the Great Indo-Australian Miocene Reef System. The large reefs in this system were mostly built by corals and crustose coralline algae, an essential group of algae for holding together reef structures. These reefs also provided very important habitat for fish groups that we see on coral reefs today, such as surgeonfishes and butterflyfishes.

Remnants of an epic reef

Surprisingly, the region where this expansion occurred is not where the largest reefs are found today. Instead, reefs off northwestern Australia – including Ashmore Reef, Scott Reef, and the Rowley Shoals – may be remnants of what was once one of the largest reef systems to have ever existed.

Previous geological work has shown this ancient west Australian barrier reef rivalled the extent of the present-day Great Barrier Reef. The new findings go further, suggesting individual reefs within this system may have been far larger than any modern reef.

In fact, the roots of modern marine fish and coral biodiversity may lie in this unexpected place off Australia’s west coast. Over millions of years, biodiversity spread and accumulated elsewhere, particularly across the Indo-Pacific Ocean.

However, there are still uncertainties. Reconstructing ecosystems from millions of years ago requires combining incomplete records. Some aspects of reef size and how these ecosystems connected remain difficult to resolve, as the geological record only contains the remnants of entire reef systems.

But the overall pattern is clear. A massive expansion of reefs about 20 million years ago coincided with the rise of modern marine diversity.

The message is also simple. To understand where biodiversity is today, we need to look deep into the past. The richest ecosystems on Earth may owe their origins to places that no longer appear exceptional – hidden chapters of Earth’s history that continue to shape life in our oceans.The Conversation

Coral reefs support thousands of species in a small area. Francesco Ungaro/Unsplash

Alexandre Siqueira, ARC DECRA and Vice-Chancellor's Research Fellow, School of Science, Edith Cowan University

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Curious Kids: what was the biggest dinosaur that ever lived?

What actually was the biggest dinosaur?

– Zavier, 14, Tauranga, New Zealand.

Great question Zavier, and one that palaeontologists (scientists who study fossil animals and plants) are interested in all around the world.

And let’s face it, kids of all ages (and I include adults here) are fascinated by dinosaurs that break records for the biggest, the longest, the scariest or the fastest. It’s why, to this day, one of most famous dinosaurs is still Tyranosaurus rex, the tyrant king.

These record-breaking dinosaurs are part of the reason why the Jurassic Park movie franchise has been so successful. Just think of the scene where Dr Alan Grant (played by New Zealand actor Sam Neill) is stunned by the giant sauropod dinosaur rearing up to reach the highest leaves in the tree with its long neck.

But how do scientists work out how big and heavy a dinosaur was? And what were the biggest dinosaurs that ever lived?

Calculating dinosaur size

In an ideal world, calculating how big a dinosaur was would be easy – with a nearly complete skeleton. Standing next to the remarkable Triceratops skeleton on permanent display at Melbourne Museum makes you realise how gigantic and formidable these creatures were.

By measuring bone proportions (such as length, width or circumference) and plugging them into mathematical formulas and computer models, scientists can compare the measurements to those of living animals. They can then work out the likely size and weight of dinosaurs.

Calculating the size of dinosaurs is easy when you have near complete skeletons like this Triceratops at Melbourne Museum. Ginkgoales via Wikimedia Commons, CC BY-NC-SA

Every palaeontologist has their own favourite formula or computer model. Some are more accurate than others, which can lead to heated arguments!

In palaeontology, however, we are not always blessed with nearly complete skeletons. In a process called “taphonomy” – basically, what happens to the bones after an animal dies – dinosaur skeletons can be broken up and bones lost.

The more fragmented the remains of a dinosaur are, the more error is introduced into size and weight estimates.

Enter the titanosaurs

If we could travel back in time to South America during the Cretaceous period (about 143 million to 66 million years ago), we’d find a land ruled by a group of four-legged, long-necked and long-tailed, plant-eating sauropods. They would have towered over us, and the ground would shake with every step they took.

These were the titanosaurs. They reached their largest sizes during this period, before an asteroid crashed into what is now modern day Mexico 66 million years ago, making them extinct.

There are several contenders among the titanosaurs for the biggest dinosaur ever. Even the list below is controversial, with my palaeontology students pointing out several other possible contenders.

But based on six partial skeletons, the best estimate is for Patagotitan, which is thought to have been 31 meters long and to have weighed 50–57 tonnes.

A couple of others might have been as big or even bigger. Argentinosaurus has been calculated to be longer and heavier at 30–35 metres and 65–80 tonnes. And Puertasaurus was thought to be around 30 metres long and 50 tonnes.

But while the available bones of Argentinosaurus and Puertasaursus suggest reptiles of colossal size (the complete thigh bone of Argentinosaurus is 2.5 metres long!), there is currently not enough fossil material to be confident of those estimates.

Spinosaurus rules North Africa

An ocean away from South America’s titanosaurs, Spinosaurus lived in what is now North Africa during the Cretaceous period.

By a very small margin, Spinosaurus is currently thought to have been the largest carnivorous (meat-eating) dinosaur, weighing in at 7.4 tonnes and 14 meters long. Other Cretaceous giants are right up there, too, including Tyranosaurus rex from North America, Gigantosaurus from South America, and Carcharodontosaurus from North Africa.

Spinosaurus is unique among predatory dinosaurs in that it was semi-aquatic and had adapted to eating fish. You can see in the picture above how similar its skull shape was to a modern crocodile.

Palaeontology is now more popular than ever – maybe because of the ongoing Jurassic Park series – with a fossil “gold rush” occurring in the Southern Hemisphere.

The latest Jurassic Park movie – in cinemas from July 2025 – is about finding the biggest prehistoric species from land, sea, and air.

Members of the public (known as “fossil forecasters”) are making new discoveries all the time.

So, who knows? The next discovery might turn out to be a new record holder as the biggest or longest dinosaur to have ever lived. There can be only one!

Hello curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to curiouskids@theconversation.edu.auThe Conversation

Nic Rawlence, Associate Professor in Ancient DNA, University of Otago

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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