The Rise and Fall of the Bone-Crushing Dogs

Thanks to WIX for supporting PBS Digital Studios. In his day, he was literally the best. By some estimates, it could grow to the size of a brown bear. And with his powerful jaws and strong teeth, he was not only a skilled hunter: he could also break the

bone

s of his prey. It is known today as Epicyon and stalked North America sixteen million years ago, during the Miocene Epoch. The largest of these creatures were the largest

dogs

that ever lived. But they were not like the

dogs

we know today. Epicyon came from a lineage known as the Borophaginae, often known by their more common, and much more metallic, nickname, the "

bone

-

crushing

dogs." Bonecrushers, a huge and diverse subfamily of dogs, patrolled North America for more than thirty million years, before disappearing 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 something else that happened to them? Cats. The only major thing Epicyon has in common with your golden retriever or whatever is that they're both canids. Dogs, wolves, foxes and all their relatives belong to the Canidae family. Today there are 34 species of canids, from the long-legged maned wolf to the big-eared fennec fox. Now, some experts believe that the first canid was a small weasel-like creature called Prohesperocyon, which first appeared about 36 million years ago in southern Texas.

However, not everyone is convinced that Prohesperocyon was a canid. It may have been part of a different group of mammals, called Miacidae, that shares a common ancestor with modern carnivores. Either way, all canids, from the bone-crunching giants to the pup who's probably watching this with you right now, share some key traits. They all eat meat, although there are some that eat plants and invertebrates from time to time. And another distinctive feature can be found in their ears. Canids have hollow bony structures at the back of the skull called auditory ampullae that protect the delicate bones of the middle ear.

Many other mammals have them too. But in canids, they are especially large, and these extra-large spaces are thought to help dogs and wolves hear low-frequency sounds. Now, tens of millions of years ago, some ancestral canid, whether Prohesperocyon or another, was the predecessor of the first of the three great subfamilies of canids. And today only one of these subfamilies survives. The oldest group was the Hesperocyoninae. They were small, agile carnivores that 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 about 37 million years ago on the great plains of North America.

Who is the cutest ancestral dog? It is not like this? Yes you are! It probably preyed on smaller mammals and some species may have climbed trees. Because, like cats, they had fully retractable claws, a trait that canids eventually lost. As the Eocene transitioned into the Oligocene, the climate cooled. The forests of North America began to give way to grasslands. And large herbivores moved into this new environment and developed traits that helped them eat grass and run long distances. And as the prey species grew, so did some of the hesperocyonins. Before long, this splinter group abandoned the forests and began hunting new prey in the new grasslands.

For example, one of Hesperocyon's descendants was a small creature called Archaeocyon. It appears in the fossil record about 30 million years ago and may be the oldest member of the second large subfamily, the Borophaginae, the bone crushers. Unlike its ancestors, Archaeocyon had shorter jaws and thicker premolars. But I wasn't quite ready to crush bones. Instead, Archaeocyon and most early borophagines were small, opportunistic omnivores, somewhat like raccoons. It was not until the mid-Miocene that new species appeared that ate meat almost exclusively and were large enough to begin to compete with the largest of that first wave of dogs, the hesperocyonines.

And that's where the mighty Epicyon comes into play. One species in this genus, Epicyon haydeni, was the largest of the large and was thought to be the largest canid of all time. By one estimate, Epicyon could have tipped the scales at 170 kilograms, making it more than twice as massive as the heaviest gray wolf on record. But we spoke to an expert on bone crushers: Dr. Xiaoming Wang of the Los Angeles County Museum of Natural History. And he said 170 kilos was probably a low estimate and that the largest Epicyons could have been “substantially larger.” In any case, all 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 enormous molars. These features allowed them to perform the feat that would eventually give them their full metal nickname: they could crush solid bone. They did this to reach the nutritious and calorie-rich bone marrow. And we know this because, on most large bone-crushers, their cheek teeth show distinctive markings—the same markings that modern hyenas get when gnawing on bones. And some samples of fossilized Epicyon excrement have even been found to contain pieces of bone. Now, for years, scientists thought these were signs that borophagines were scavengers.

But the more recent idea is that at least some bonecrushers actively hunted prey that was as big (or even bigger) than them. Maybe even in packs. After all, modern large predators, like wolves, tend to do the same thing. So there is no reason to think that the borophagines acted differently. And because they were powerful, but not built for speed, many experts think bone crushers were probably what are known as pursuit predators. Like coyotes, they probably chased their prey for short distances and then knocked them to the ground. But whatever they were doing back then, they were doing something right.

Because at the time of greatest success, between six and twelve million years ago, there were about fifteen different species of bone-

crushing

dogs. In addition to the giant Epicyon, for example, there was Cynarctus, coyote-sized and equally opportunistic. Judging by their teeth, most of their diet consisted of insects and plants. But, conversely, there was also a lineage within the genus Aelurodon that became increasingly carnivorous over time. As the Miocene Epoch drew to a close, bone-crushing dogs roamed North America, from Maryland to California and from Montana to Mexico. Then his luck took a downward turn. One of the culprits of its decline was the third and last subfamily of canids: the Caninae, the only group of dogs that would remain standing.

Canines first appeared just over 30 million years ago. And there is debate about whether they arose from small Hesperocyonins or small bone grinders. It's just another of the many fascinating things that paleontologists still strive for. But we do know that one of the first recorded canines 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 primitive canines, it had a long snout with thinner teeth. Therefore, he could not bring down large game animals such as horses or camels, but he was adept at capturing small, fast prey.

But the most notable thing about these new and first canines were their paws. As Epicyon and other bone crushers grew larger and heavier, canines slowly became long-distance marathon runners. The trend started a long time ago with Hesperocyon, which had quite long legs. But when Leptocyon appeared, they were even longer, allowing him to take longer steps. And at the end of the Miocene another rationalizing feature appeared: the reduction of the "big toe" of each foot. Through natural selection, this fifth toe shrank, becoming little more than a small bump in some species and disappearing completely in others. These shrinking toes helped make the canines' feet and legs lighter.

And that, combined with their longer stride, allowed them to adopt an entirely different hunting strategy. Instead of pouncing on their prey as bone crushers did, the canines could run over their victims for hours, until they fell from exhaustion. If you have ever seen a wolf hunt, you will know that this is the method they still use today. And this strategy could also explain why canines are the only dogs still around: because they were better equipped to take on North America's newer, fiercer competitors. You could say that they are the oldest enemies of dogs: cats. And I make this face when I say cats because I am not 100% a cat lover.

Cats first evolved in Eurasia about 33 million years ago. But about 14 million years later, they migrated across the Bering Land Bridge and quickly spread south. And some experts believe it was competition with cats that finally killed off bone-crunching dogs. New species of big cats, such as Pseudaelurus, were ambush predators that likely competed with bonecrushers for the same prey. And simply put, cats were better at it: more efficient, with retractable claws, they found it much easier to control their prey. So, while the canines continued with their own set of prey and long-distance hunting strategies, the bonecrushers, once the most dominant canids, found themselves fighting for survival.

The last of the bone-crushing dogs, a genus known as Borophagus, disappeared about 2 million years ago. And that first subfamily, the hesperocyonines, had already gone extinct about 13 million years earlier, unable to compete with both the bone-crushers and the arrival of cats. So, two of the three canid subfamilies are dead and gone. And there is no definitive evidence that any hesperocyonin or bone crusher ever left North America. But canines spread far beyond the continent. They crossed Panama and entered South America, which now has its own native canine species, such as the maned wolf. Further west, canines crossed Eurasia and Africa.

And with a little help from seafaring humans, the precursors of the iconic "dingo" dog landed in Australia 4,000 years ago. So if you want, you can read the story of bone-crushing dogs as a kind of warning. It reminds us that being “top dog” isn't all it's cracked up to be. We tend to think that large, powerful predators are the ones that rule their ecosystems. But their situation is actually one of the most precarious: when the local environment changes and competition appears, it is the large specialized carnivores that often struggle to adapt. Or if you want, you can blame everything on the cats.

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