After the dinosaur extinctions of 65 million years ago, mammals were using their spectacular adaptability to evolve and diversify at an astonishing rate. In the process, they laid the foundations for the major mammal groups we see today, including the enormous titanotheres.

But then, around 47 million years ago, a new set of problems came. The Earth’s climate changed yet again. Many places became drier, and where that happened, the forest thinned out and was replaced by low-scattered bushes and grass. In this environment, the titanotheres thrived and became the mammal monsters we speak of here.

Badlands of South Dakota
Badlands of South Dakota

And those new environments presented new challenges to animals and ushered in the age of the mammal monsters.

Scientists are finding stunning evidence of this change in the Great Plains of North America. This dramatic country in South Dakota is known as the Badlands. Streams and rivers have eroded the rocks into fantastic shapes. But 40 million years ago, these were layers of sediment laid down across an open flood plain.

Palaeontologist Clint Boyd is looking here for the fossilised remains of creatures from that ancient time. And he’s finding mammals that were giants.

Dr Clint Boyd
Dr Clint Boyd

Clint Boyd “This is part of the bone we call the femur, the upper thigh bone, and this round surface right here is for the hip socket. And so you can see it’s a very large. We’d be talking about a very large animal. And not only do we have the thigh bone but we’ve got ankle bones spread out over here and then cascading down from that spot, we’ve got some of the tailbones is coming down. So, if we add all this up together, based on the size, we’re looking at an animal that’s probably about 2 metres tall at the hips.”

Dr Sertich introduces the titanotheres

The creature is known as a Titanothere. It was a herbivore. It fed on the lush vegetation that once covered this area of the United States. A range of different specimens have been collected at the Denver Museum of Nature and Science. And they reveal that the first Titanotheres were built on a much smaller scale.

Joseph Serlich
Joseph Serlich

Dr Joseph Sertich “When Titanotheres first appeared on the scene, they look like this. This is the lower jaw of one of the first Titanotheres, and it’s one of these sheep-sized animals. In only 5 million years, members of the group go from sheep-sized… to about the size of a small horse. Within only 15 million years of their first appearance, Titanotheres look like this. Here you can see the skull of one of these Titanotheres.”

In evolutionary terms, the size increase is astonishingly quick. But what drove this remarkable change?

Another fossil could provide an explanation. It dates back to the time of the first and smallest Titanotheres, but it’s a very different type of mammal.

Titanotheres Skeleton
Titanotheres Skeleton

Joseph Sertich “This is the skull of Malfelis Badwaterensis, the ‘bad cat from bad water’. This was the largest predator at the time. This is the skull. This large crest is for the large jaw muscles which would have given a powerful shearing bite that ran these blade-like teeth, perfect for chopping up a Titanothere. And what’s interesting is that Malfelis was exactly the same size as the top herbivores of the time, like Titanotheres.”

Malfelis Badwaterensis related to the titanotheres
Malfelis Badwaterensis

The earliest Titanotheres could hide from these bad cats in dense forest environments. But as those forests began to thin out, the Titanotheres were more vulnerable to attack. One way to improve their chances was to grow bigger.

Large Herbivore

Joseph Sertich “An herbivore is much more likely to survive an encounter with a predator if it’s a little bit larger. And so there was a bit of an arms race between the predators and the prey. And animals like Titanotheres the were able to escape this predator pressure by becoming the super-sized giants we see 35 million years ago.”

Fossilised remains of Titanotheres from the Badlands of South Dakota and elsewhere across the Great Plains allow us to reconstruct its rapid growth spurt.

From modest beginnings, they increased their bulk ten times over… ’til the largest stood over 8 feet tall.

On the open grasslands that increasingly covered the Earth, many other giant mammals evolved. As these giant creatures became better known, the term Megafauna evolved.

Paraceratherium

The giant sloth was found in California. In China, I’ve come to see the remains of mammoths. And a remarkable creature that was the largest land mammal to walk this Earth. This great beast is called Paraceratherium.

It stood 5 m tall and nearly 8 m long. Those furry little mammals scampering about in the shadows had produced descendants that could stare the biggest dinosaur in the eye.

Paraceratherium Dino Rhino
Paraceratherium Mammal Monsters
Source: DinoAnimals.com

Today, the elephant is one of the few species of Megafauna to have survived. But those outsized versions have otherwise disappeared from the planet. So, what happened to them? Their eventual extinction coincides with another key event in the history of the Earth.

From around two and a half million years ago, ice sheets spread down from the North and up from the South to cover vast areas of the continents. But it was only when the ice finally retreated, just 10,000 years ago, that the Megafauna vanished.

Some have blamed that on the rise and fall of the temperature as the ice age finally came to a close. But others have sought the culprit amongst the mammals themselves. In a newly-evolved super predator – homo-sapiens

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Titanotheres the mammal monsters that began as tiny creatures