FEMALE NARRATOR: When you think of the world's most fierce, numerous, and dominant meat-eating mammals, you probably think of animals like the big cats or wolves or bears.

But long before any of those animals existed, there was another predator that prowled North America.

And it was neither cat nor dog nor bear, but it was their distant relative.

It had the tapering snout and long tail of a dog, but it walked on the soles of its feet and had a large body like a bear.

Its name was amphicyon, and it was one of the largest predatory mammals ever to walk the Earth.

And it was just one of a very diverse group of ancient meat-eaters that ruled the Northern Hemisphere for over 30 million years.

Some of these creatures were fox-like forest dwellers.

Others were fast, long-legged predators.

And others still, like amphicyon, were nearly as big as polar bears.

Scientists refer to this group of extinct beasts as the amphicyonids.

But because of their strange combination of bear-like and dog-like traits, they're sometimes confusingly called the beardogs.

And even though you've never met one of these animals, the beardogs are key to understanding the history of an important branch of the mammal family tree because at the height of their reign, the beardogs played a pivotal role in out-competing another group of early dominant predators, a group we've told you about before, which has also since gone extinct.

And by getting rid of them, the beardogs paved the way for their own relatives, the carnivores we know today.

Natural history, like all history, is full of overlooked characters, figures that quietly shaped the world around them.

And if you ask me, the saga of the carnivores could never be complete without the forgotten story of the beardogs.

Now, most of the meat eating mammals that you know today, like cats, dogs, and bears, are members of the same group, the order known as Carnivora.

And it might sound like to be a member of this group, you just have to eat meat.

But Carnivora isn't synonymous with carnivore.

Carnivora includes animals with diverse diets, not just meat-eaters.

They range from the bamboo-chomping giant pandas to more omnivorous animals, like raccoons and even walruses.

So a carnivore is any animal that eats other animals.

But carnivorans are a specific taxonomic group of placental mammals.

And some of the earliest members of this group were the beardogs.

Beardogs first appear in the fossil record around 40 million years ago during the Eocene Epoch.

They probably originated in North America and spread into Europe before the two continents slowly became separated.

Because of that, fossils show that two distinct lineages of beardogs wound up evolving in isolation for millions of years.

One of the oldest known species is actually a tiny creature from North America called the Daphoenus demilo.

It lived in the late Eocene from Wyoming to Saskatchewan, where it survived on a diet of plants and small animals in the dense forest that covered the continent.

And it, like all beardogs, belonged to a group of mammals known as the Caniformia, the same group that includes today's bears and dogs, along with other familiar mammals like seals and skunks.

Scientists think that beardogs diverged from other caniforms early on, forming their own branch of the mammal family tree.

Both bears and dogs evolved later.

But today, they are the beardogs' closest living relatives.

And because of that, beardogs share a lot of similarities with both animals.

For example, beardogs tended to have longer, pointer snouts like dogs and bears do.

And many beardogs were large-bodied and walked with their feet flat on the ground like bears, but also had a long dog-like tail.

Now, after the North American beardogs became separated from the Eurasian species, the two groups found themselves on very different evolutionary paths.

In North America, beardogs diverged into two sub-families, each of which played a big role in shaping the predator/prey dynamics of the continent.

The first group appears in the fossil record about 40 million years ago in places like Texas and Wyoming.

Early species of this group were well-adapted for hunting in dense forests, like our friend daphoenus, which was probably about the size of a swift fox or maybe as big as a coyote.

But when these first beardogs came about, there were bigger dogs on the scene.

Though, they weren't actually dogs either.

The fiercest apex predators in Eocene North America were the hyaenodonts-- bigger, stronger carnivores, which we've talked about before.

Hyaenodonts had big, powerful jaws filled with blade-like teeth.

Plus, we can tell from their heel bones that some of them could make powerful leaps, suggesting they probably used an ambush hunting strategy, hiding in the forest undergrowth and tackling their prey with brute force.

Now, those first little beardogs might not have seemed like much of a threat to the hyena dogs.

But circumstances soon swung in their favor.

As the Oligocene Epoch began about 33 million years ago, North America was beginning to go through a major cooling and drying phase.

And this opened up the landscape, turning forests into savannas.

This new open environment spelled trouble for the specialized hunting style of the hyaenodonts.

And they had another problem to deal with too-- their teeth.

Hyaenodont teeth were all adapted for ripping and tearing flesh.

Great when you're only eating meat, but less so when your food of choice suddenly declines.

Beardogs, on the other hand, had more varied teeth.

Their front teeth were used for tearing, but their molars had different shapes and sizes.

Some could be adapted for crushing, puncturing, or holding prey, depending on what was selected for.

And those differentiated teeth proved to be a key advantage for the beardogs.

Plus, by the start of the Oligocene, some beardog species had gotten big-- big enough to compete with the hyaenodonts for the same food while maintaining more of this adaptability.

The combination of direct competition from beardogs and an unforgiving new environment spelled the end for the hyaenodonts.

By about 30 million years ago, they had vanished from North America.

Soon beardogs were on the rise.

By about 23 million years ago, several large beardogs appeared with long legs that gave them a more efficient stride, perfect for chasing down prey like three-toed horses an early camelids.

But the North American beardogs were about to face much stiffer competition, and from their own kind.

About 18 million years ago, a new threat crossed the Bering Land Bridge, even larger beardogs, the ones that had evolved separately on Eurasia.

And over there, the beardog story had played out differently than it had in North America.

At first, the Arabian beardogs struggled to thrive, in part because they also faced competition from the hyaenodonts.

But when those bigger carnivores started to decline, the bigger dogs had their moment.

They quickly spread across the continent and began to fill the niches that the hyaenodonts had left empty.

But rather than adapting to becoming sleek and long-legged like their American relatives, Eurasian beardogs became heavier and more robust.

They were also probably more opportunistic and adaptable, hunting or scavenging on large prey like hyenas do today.

In fact, we have direct evidence of this.

In 2006, researchers described a rhino jaw from Portugal that had bite marks matching the teeth of a Eurasian beardog called Amphicyon gigantias.

So when the large beardogs arrived in North America, it spelled trouble for the locals.

And they were just one part of a whole wave of Eurasian carnivores that came over, shaking up the dynamics of whole ecosystems.

These new carnivores out-competed the North American beardogs.

And by about 17 million years ago, they were extinct.

The Eurasian beardogs, however, thrived in their new habitat.

They grew even larger, reaching sizes that put them among the biggest mammalian land predators of all time.

Two species of these giants were ambush predators the size of polar bears that roamed Western North America.

They had powerful forelimbs, massive clawed feet, and muscular jaws that could deliver a crushing bite.

But these beardogs were about to meet the same double threat as the hyaenodonts that came before them-- new competition and climate change.

However, this time, the situation was reversed.

Beardogs had out-competed the hyaenodonts, in part by being adaptable generalists, able to eat more kinds of food when conditions changed.

But then, those beardogs were replaced by more specialized hunters, members of their own family-- the modern carnivorans.

Some of these new carnivorans included hyenas in Eurasia and Borophagines in North America.

You may know them as the bone-crushing dogs.

Both of these animals could shatter bones with their teeth, something that beardogs couldn't do, which made them more efficient scavengers.

Plus, there was also the arrival of the cats, which began to compete for the niche of hyper-carnivore, with their razor-sharp teeth and retractable claws.

Meanwhile, bears-- actual real bears-- had diverged and spread, with teeth that were adapted to a more omnivorous lifestyle.

All of these new carnivore friends were doing everything that beardogs could do, but better, and in more targeted ways.

Clearly, the days of the beardogs were numbered.

The North American beardogs had already been out-competed by the giant Eurasian beardogs.

And with only huge ambush predators left on both continents, they couldn't stand up to the more specialized cats, dogs, and bears.

Some of the last species we find in the fossil record belong to the genus of the mighty Amphicyon.

By about 9 million years ago, beardogs on both continents were gone.

But we shouldn't forget the role they played in shaping predator dynamics in the age of mammals.

Their rise helped the carnivorans outdo their competitors, the hyaenodonts.

And their fall was accompanied by the rise of the modern cats, dogs, and bears we know today.

When the last of them vanished, the beardogs had formed a kind of bridge between old and new, a missing puzzle piece in the 45-ish-million-year saga of the meat-eating mammals.