How Did Dinosaurs Become the Dominant Animals on Earth?

How Did Dinosaurs Become the Dominant Animals on Earth?

A skeleton and model of an early dinosaur, Herrerasaurus ischigualastensis, in the Field Museum of Natural History, Chicago, Illinois, USA.
Image Credit: AGF Srl / Alamy Stock Photo

When we think of dinosaurs, your mind may well immediately go to massive, iconic creatures such as Diplodocus, Stegosaurus or Tyrannosaurus rex. Indeed, these remarkable creatures of the Jurassic and Cretaceous periods have come to epitomise a world that was once dominated by dinosaurs.

But what’s just as fascinating – if not more so – is the story of how dinosaurs rose to prominence. How this particular group of animals became so dominant for millions of years. It’s a story that includes mass extinction events, giant apex predator crocodiles and mysteries that palaeontologists are still trying to figure out to this day.

So, when and how did the dinosaurs emerge and what was the first dinosaur species?

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The Permian extinction

To tell the story of the rise of the dinosaurs, we need to go back to their origin story. This takes us back some 252 million years, to the period before the Triassic: the Permian period.

The Permian period was a time when the world consisted of one huge supercontinent called Pangaea. The climate was hot and dry. It was a tough, unforgiving environment. But nevertheless, many plants and animals adapted and thrived during it. Among these animals, for instance, were the ancestors of mammals.

Permian amphibians: Actinodon, Ceraterpeton, Archegosaurus, Dolichosoma, and Loxomma. By Joseph Smit, 1910.

Image Credit: via Wikimedia Commons / Public Domain

But c. 252 million years ago, disaster struck these Permian ecosystems. Indeed, disaster is putting it mildly. It was a major catastrophic event, the biggest episode of mass death in the history of Earth.

Mega volcanoes erupted in modern-day Russia. Magma flowed out of these volcanoes for millions of years. When the magma finally ceased, lava had covered thousands of square miles across Pangaea. This sounds bad enough for those living in the Permian world, but worse was to follow. Alongside the lava, lots of gases came up above ground. This in turn led to serious global warming, which caused the Permian ecosystems to change so fast that it caused a mass extinction event. Roughly 95% of all Permian species died out. As palaeontologist Dr Steve Brusatte explained:

“It was the closest life has ever come to being completely erased.”

But life was not completely erased. Life had already persevered through several preceding extinction events in the world’s history, and it did so again through the Permian extinction event. Some species did survive this catastrophe: the lucky 5%.

The survivors were a whole range of animal and plant types, including the ancestors of the dinosaurs, ‘dinosaurmorphs’. These dinosaur ancestors were small reptiles – extremely fast and very agile – that quickly took advantage of the new world that followed in the wake of the Permian extinction, known as the early Triassic period. We know this because palaeontologists have found footprint and handprint fossils of tiny dinosaurmorphs that date to within a million years of the mega volcano eruptions.

From the ashes of the great Permian extinction event, the ancestors of the dinosaurs emerged. This great catastrophe would ultimately pave the way for the dawn of the dinosaurs and their eventual rise. But that rise would take time. Several million years, in fact.

The first true dinosaurs

The earliest-found fossils of creatures that palaeontologists have labelled as true dinosaurs date to c. 230 million years ago. For palaeontologists today, classifying whether an animal was a dinosaur or not centres around whether they had certain bone features, particularly around the thigh and pelvis. Consequently, the earliest known true dinosaurs date to the mid-Triassic, c. 20 million years after the great extinction event and the first dinosaurmorphs.

A key location where palaeontologists have discovered many of the earliest dinosaur fossils is in Argentina, in the Ischigualasto-Villa Union Basin. Examples of early dinosaurs found here include the sauropod ancestor Eoraptor and the early therapod Herrerasaurus.

It’s important to stress here, however, that these are the oldest true dinosaur fossils that palaeontologists know of. There are almost certainly older dinosaur fossils out there, yet to be discovered. With that in mind, the first true dinosaurs may well have emerged between 240 and 235 million years ago.

A Herrerasaurus ischigualastensis dinosaur fossil in a museum. Image shot 2010. Exact date unknown.

In the shadow of pseudosuchians

During most, if not all, of the Triassic period, dinosaurs weren’t the dominant species. They weren’t the most diverse animals, nor were they the most abundant. They were not at the top of the food chain, according to Dr Steve Brusatte:

“Dinosaurs were role players during most, if not all, the Triassic.”

The title of dominant animal belonged elsewhere during the Triassic. In the rivers and lakes, it belonged to giant salamanders, which were enormous amphibians that would have preyed on any dinosaurs that ventured too close to the waterline.

On land, the dominant animals were the pseudosuchians, huge crocodile-like beasts. During the Triassic, the pseudosuchians diversified with enormous success. Some of these ‘ancient crocs’ had beaks, while others, such as the famous Postosuchus, were apex predators. As Dr Steve Brusatte says:

“(There was) a rich menagerie of ancient crocs and they were the ones that really controlled the food webs on land. They were the top predators in most ecosystems… The dinosaurs really slotted into what was a croc-dominated world.”

End of the Triassic

Eclipsed by the much larger pseudosuchians, the dinosaurs remained small with limited diversity throughout the Triassic period. But this wouldn’t last forever.

An illustration of the Triassic period.

Image Credit: Science History Images / Alamy Stock Photo

The Triassic period continued for c. 50 million years, until another great extinction event occurred. Around 200 million years ago, the supercontinent of Pangaea began to break apart. The Earth bled lava, with massive volcanic eruptions once again occurring and lasting c. 600,000 years. Once again, this in turn led to global warming, which once again triggered a mass extinction event.

This time, however, the great victims of this extinction event were the pseudosuchians and the big amphibians. A few species of each did survive, but most died out. The great survivors, however, were the dinosaurs. Why the dinosaurs spectacularly endured the end-Triassic catastrophe and adapted so well to the rapidly changing ecosystems that ensued is a mystery, and palaeontologists are yet to find a concrete answer.

Nevertheless, whatever the reason for their extraordinary resilience at this catastrophic time, the dinosaurs survived, paving the way for their rise to prominence in the new, multi-continent world that came after the Triassic: the Jurassic period. Over the millions of years that followed, dinosaurs would grow larger. They would diversity to incredible degrees and spread across the globe. The dawn of the Jurassic period had arrived. The ‘golden age’ of the dinosaurs had begun.

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Tristan Hughes