Among modern-day carnivores, dogs, wolves, and their relatives (known as canids) are unique. Their snouts are long and narrow—in contrast to the short, compact faces of cats and weasels—and many of them are pack-hunting, high-endurance “pursuit predators” that run their prey to exhaustion.
But millions of years ago, the world was full of other groups of carnivores that tried out this “dog-like” look and lifestyle. Some looked and acted a lot like the canids we know today, but they evolved these features independently—much like how the recently-extinct thylacine had strikingly dog-like features despite being a marsupial, separated from canids by more than 100 million years.
These lost animal groups had features similar to those of other carnivore families, making them appear like impossible hybrids of modern predators. Many of these “not-quite dogs” were similar in size and shape to modern canids. But others were truly massive apex predators, reigning over the landscape as nightmarish versions of man’s best friend.
If you stepped out of your time machine 10 million years ago into the muggy, sparse woodlands of western Nebraska, there’s a decent chance you might happen upon a lone bear-dog trundling through the grass and underbrush. Despite the name, these animals were neither bears nor dogs. Rather, they made up a distinct family—Amphicyonidae—and were found in the northern continents from about 40 to 5 million years ago.
The earliest bear-dogs were small and fox-like, and ran on their toes the way dogs and cats do. These small species lived in the shadows of creodonts—primitive predators that started to fade out of the picture tens of millions of years later. Bear-dogs picked up the predatory slack in a lot of places, growing in size, exploiting new habitats as the planet cooled and forests shrank globally.
Later in their evolution, some species had become absolutely monstrous.
Amphicyon ingens, native to North America about 20 to 9 million years ago, was a titan, weighing well over half a ton—significantly heftier than your average polar bear. Like many later bear-dogs, it walked on its giant palms like a bear, but had a low, elongated body, a long tail like a dog, and a head more like that of a wolf’s, equipped with bone-crushing jaws.
Amphicyon looked like a mix of dog and bear, and probably lived a little like both—chasing, mauling, and tearing apart big, fast prey like horses or camels, but also eating berries and roots from time to time.
If bear-dogs were dog-like animals that behaved a bit like bears, then dog-bears were the complete opposite: bear-like animals that behaved a bit like dogs.
“Dog-bears” were a strange set of predators found across Eurasia and North America from about 30 to 5 million years ago. They made up a now-extinct grouping called Hemicyonidae—”hemi-cyon” literally meaning “half-dog”—and were very closely related to modern bears. In fact, dog-bears are often considered an extinct part of the bear family.
Dog-bears, despite their evolutionary kinship with bears, moved and hunted very much like canids. They had long legs and ran on their toes, making them exceptionally quick on their feet for their size. They likely spent a lot of their time chasing down prey. Dog-bears had short tails like bears, but their teeth were specialized for slicing flesh, so they probably had little in the way of a bear’s omnivorous inclinations.
They never reached quite the gargantuan proportions of the bear-dogs they shared their world with, but some varieties—like Hemicyon—were unsettlingly big. Hemicyon grew to at least the size of a large grizzly, but with its ability to run for long distances and hypercarnivorous diet, it would have been one of the most formidable predators of its era, especially considering that dog-bears may have been pack hunters.
Athletic, pack-hunting grizzly-wolf hybrids? Be thankful you weren’t born in the Miocene epoch.
All canids alive today belong to single subfamily: Caninae. But for most of the timespan of the evolution of dogs, wolves, and foxes, up until about 2 million years ago, there were other subfamilies. One of these extinct groups was Borophaginae—also known as “bone-crushing dogs” or “hyena-dogs.”
Hyena-dogs were so named for their skulls, which unlike other canids (and a lot like hyenas) were thick, powerful, and studded with immense teeth well-suited for cracking bones. Hyena-dogs were originally thought to be specialized scavengers based on their adaptations for bone-splitting, but it’s more likely they were top, pack-hunting predators in their ecosystems, devouring their prey entirely, unlike their more delicate wolf cousins.
For millions of years, hyena-dogs were among the most ferocious and capable predators in North America, despite many not being particularly physically imposing at a time when there were plenty of jumbo-sized mammals trekking across North America’s plains. Borophagus, for example, was about as big as a stout coyote, but was probably the dominant carnivore of its time.
Some species were outliers though. Epicyon, an early member of the group, was probably the largest canid ever. It wasn’t as adept at crushing bones and slurping marrow as its later relatives, but it was as big as a lioness.
Tens of millions of years ago, the world was crowded with dog-like predators, some of which were unworldly and horrific in their proportions. But, they all died out, and only a particular take on the “dog strategy”—the clever, but more fragile canids—survived into the modern day, illustrating being bigger and badder may not actually be better in the long run.