In its day, it was quite literally top dog.

By some estimates, it could grow as big as a brown bear.

And with its powerful jaws and stout teeth, it was not only a skilled hunter -- it could also crack open the bones of its prey.

It's known today as Epicyon, and it stalked North America from sixteen million to seven million years ago, during the Miocene epoch.

The largest of these creatures were the most massive dogs that ever lived.

But they weren't like the dogs that we know today.

Epicyon hailed from a lineage known as the Borophaginae, often known by their more common - - and way more metal -- nickname, the "bone-crushing dogs."

A huge and diverse subfamily of dogs, the bone-crushers patrolled North America for more than thirty million years, before they disappeared in the not-too-distant past.

So what happened to the biggest dogs that ever lived?

Part of what happened to them was ... dogs as we know them.

Our dogs.

And another thing that happened to them?

Cats.

The only important thing that Epicyon has in common with your golden retriever or whatever is that they're both canids.

Dogs, wolves, foxes, and all their kin belong to the family Canidae.

Today there are 34 species of canids, from the leggy maned wolf to the big-eared fennec fox.

Now, some experts think the earliest canid was a small, weasel-like creature called Prohesperocyon, which first appeared about 36 million years ago in southern Texas.

Not everyone's convinced that Prohesperocyon was a canid, though.

It may have been part of a different group of mammals, called the Miacidae, which shares a common ancestor with modern carnivores.

Either way, every canid - from the giant bone-crushers to the pup that's probably watching this with you right now - all share some key traits.

They all eat meat, though there are some that eat plants and invertebrates once in a while.

And other distinguishing trait can be found in their ears.

Canids have hollow bony structures toward the back of their skulls called auditory bullae that protect the delicate bones of the middle ear.

Lots of other mammals have them too.

But in canids, they're especially big, and it's thought that these extra large spaces help dogs and wolves hear low-frequency sounds.

Now, tens of millions of years ago, some ancestral canid, whether it was Prohesperocyon or someone else, was the predecessor to the first of the three great subfamilies of canids.

And only one of these subfamilies survives today.

The earliest group was the Hesperocyoninae.

These were small, nimble carnivores that were adapted to the warm, forested world of the Late Eocene.

And the founding member of this group was Hesperocyon, which appears in the fossil record around 37 million years ago in the great plains of North America.

Who's the cutest little ancestral dog?

Aren't you?

Yes you are!

It probably ate smaller mammals, and some species may have climbed trees.

Because, just like cats, they had fully retractable claws, a trait that canids eventually lost.

As the Eocene transitioned to the Oligocene, the climate cooled.

The woodlands of North America started to gave way to grasslands.

And large herbivores moved into this new environment, evolving traits that helped them eat grass and run long distances.

And as the prey species grew, some of the hesperocyonines did as well.

In short order, this splinter group left the forests and began hunting the new prey on the new grasslands.

For example, one of Hesperocyon's descendants was a little critter called Archaeocyon.

It appears in the fossil record around 30 million years ago and may be the earliest member of the second great subfamily, the Borophaginae, the bone-crushers.

Unlike its ancestors, Archaeocyon had shorter jaws and thicker premolars.

But, it wasn't quite ready to actually crush bone.

Instead, Archaeocyon and most of the early borophagines were small, opportunistic omnivores, kinda like raccoons.

It wasn't until the mid-Miocene that new species appeared that ate meat almost exclusively and were big enough to start competing with the largest of that first wave of dogs, the The hesperocyonines.

And that's where mighty Epicyon comes in.

One species in this genus-Epicyon haydeni-was the biggest of the big, thought to be the largest canid of all time.

According to one estimate, Epicyon could've tipped the scales at 170 kilograms, making it more than twice as massive as the heaviest grey wolf on record.

But we talked to an expert in bone-crushers - - Dr. Xiaoming Wang at the Natural History Museum of Los Angeles County.

And he said that 170 kilos was probably a low estimate and that the biggest Epicyons might've been "substantially larger."

In any case, all the Epicyon species looked very different from the dogs and wolves we know today.

In addition to their distinctive domed foreheads, they had wide palates and massive cheek teeth.

These features allowed them to perform the feat that would eventually give them their full-metal nickname: They could crunch through solid bone.

They did this to get to the nutritious, calorie-dense marrow of the bone.

And we know this because, in most of the big bone-crushers, their cheek teeth show distinctive marks -- the same marks that modern hyenas get by gnawing on bones.

And some samples of fossilized poop from Epicyon have even been found to contain bits of bone.

Now, for years, scientists thought that these were signs that borophagines were scavengers.

But the more recent thinking is that at least some bone-crushers actively hunted prey that were as large -- or even larger -- than they were.

Maybe even in packs.

After all, large modern predators like wolves tend to do the same thing.

So there's no reason to think the borophagines acted differently.

And because they were powerful, but not built for speed, many experts think that bone-crushers were probably what are known as pounce-pursuit predators.

Like coyotes, they probably chased their prey for short distances, and then wrestled them to the ground.

But, whatever they were doing back then, they were doing something right.

Because, at the peak of their success, six to twelve million years ago, there were about fifteen different species of bone-crushing dogs.

In addition to the giant Epicyon, for instance, there was Cynarctus, about the size of a coyote and just as much of an opportunist.

Judging by its teeth, most of its diet consisted of insects and plants.

But, by contrast, there was also a lineage within the genus Aelurodon that became increasingly carnivorous over time.

As the Miocene epoch was drawing to a close, bone-crushing dogs roamed North America from Maryland to California and from Montana to Mexico.

Then their fortunes took a downward turn.

One of the culprits in their decline was the third and final subfamily of canids: The Caninae, the only group of dogs that would be left standing.

Canines first appear a little over 30 million years ago.

And there's a debate over whether they arose from small Hesperocyonines or from small bone-crushers.

It's just another of the many fascinating things that paleontologists are still fighting about.

But we do know that one of the first canines on record was Leptocyon.

Which, again, isn't it super cute?

I just want to...boop!

It made its debut in the early Oligocene and was about the size of a fox.

Like other early canines, it had a long snout with thinner teeth.

So it couldn't bring down big game like horses or camels, but it was adept at catching small, fast prey.

But the most noteworthy thing about these new, early canines was their legs.

While Epicyon and other bone-crushers were getting bigger and heavier, canines slowly developed into cross-country marathon runners.

The trend started way back with Hesperocyon, which had pretty long legs.

But by the time Leptocyon showed up, they were even longer, allowing it to make longer strides.

And by the late Miocene, yet another streamlining trait appeared: the reduction of the "big toe" on each foot.

Through natural selection, this fifth toe shrank away, becoming little more than a tiny nub in some species and disappearing altogether in others.

These shrinking toes helped make canines' feet and legs lighter.

And that, combined with their longer stride, allowed them to adopt a totally different hunting strategy.

Instead of pouncing on their prey like bone crushers did, canines could run their victims down for hours, until they dropped from exhaustion.

If you've ever seen a wolf hunt, you know this is the method they still use today.

And this strategy might also explain why canines are the only dogs that still exist: Because they were best equipped to go up against the newest and fiercest competitors in North America.

You could say they're dogs' oldest foes: cats.

And I make this face when I say cats because i'm not 100% a cat person Cats first evolved in Eurasia some 33 million years ago.

But about 14 million years later, they migrated across the Bering Land Bridge and quickly spread south.

And some experts think it was competition with cats that ultimately did in the bone-crushing dogs.

Large new cat species, like Pseudaelurus, were ambush predators that probably competed with the bone-crushers for the same prey.

And simply put, the cats were just better at it: More efficient, with retractable claws, they had a much easier time wrangling their prey.

So while the canines went on with their own set of prey and long-distance hunting strategies, the bone-crushers, once the most dominant of the canids, found themselves struggling for survival.

The last of the bone-crushing dogs, a genus known as Borophagus, vanished about 2 million years ago.

And that first subfamily, the hesperocyonines, had already died out about 13 million years earlier, unable to compete with both and bone-crushers and the arrival of the cats.

So two out of the three canid subfamilies are dead and gone.

And there's no definitive proof that any Hesperocyonines or bone-crushers ever left North America.

But the canines spread well beyond the continent.

They crossed Panama and entered South America, which now has its own native canine species, like the maned wolf.

Further west, canines made their way across Eurasia and into Africa.

And with a little help from seafaring humans, the forerunners of the iconic "dingo" dog landed in Australia 4,000 years ago.

So, if you're inclined to, you can read the story of the bone crushing dogs as something of a cautionary tale.

It reminds us that being "top dog" isn't all it's cracked up to be.

We tend to think of big, powerful predators as being the ones that rule their ecosystems.

But their position is actually one of the most precarious: When the local environment changes and competition appears, it's the large, specialized carnivores that often struggle to adapt.

Or if you want, you can just blame everything on the cats.