What Happened to the Bone-Crushing Dogs That Once Hunted Across North America?

Before going extinct roughly two million years ago, canids known as borophagines took down and consumed much larger prey

Riley Black | Science Correspondent

August 4, 2025

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Long before coyotes and millions of years before wolves, bone-crushing dogs padded across North America. The prehistoric canids came in an array of sizes, from small nippers comparable to foxes to huge, skeleton-busting predators larger than the largest gray wolf. For more than 31 million years, they were among the most versatile carnivores on the continent, and remainders of their prehistoric meals indicate that they truly did eat more bones than other canids. But by 1.8 million years ago, they were all gone.

Paleontologists know bone-crushing dogs by their technical name, borophagines. The short, deep muzzles of the canids have often reminded paleontologists of spotted hyenas, carnivores that are more closely related to cats than dogs. Borophagines evolved jaw specializations to crack open and consume more bone than predators like wolves do. But as experts have continued to study borophagines, they’ve found that the predators were not simply dogs acting like hyenas. The canids hunted during a span when grasslands populated by immense herbivorous mammals like camels, rhinos, horses and elephants were spreading with them, and borophagines had the ability to bust large carcasses into splinters.

The earliest borophagines evolved in North America about 34 million years ago. The canids evolved from earlier dogs that were lankier and more civet-like. Some remained relatively small and were more omnivorous, similar to foxes and coyotes today. But many evolved to be some of the largest carnivores in their ancient habitats. Borophagus itself, whose name means “gluttonous eater,” was about the same size as a large coyote. Epicyon was even larger, the biggest dog of all time, standing three feet tall at the shoulder and weighing more than 300 pounds. The tall, short-snouted skulls of these large, bone-crushing forms would have made the dogs look more cat-like than wolf-like. Much like cats and hyenas, the borophagines evolved to prioritize jaw crushing power over faster bites.

Borophagus, Epicyon and their relatives first came to the attention of paleontologists in the late 19th century, and they were interpreted as dogs with powerful bites very early on. Robust, worn-down cheek teeth found in the jaws of the canids left little doubt that they were frequently eating bones. But it wasn’t until 2018 that paleontologists finally found better proof: fossil scat packed with bone shards. In rocks roughly six million years old, paleontologists found a latrine of fossil feces left behind by Borophagus, their bones found in the same area. When Natural History Museum of Los Angeles County paleontologist Xiaoming Wang and colleagues examined the contents of the feces fossils, they found bone fragments that had not been fully digested. The shards also suggested that Borophagus preyed on herbivores larger than itself, animals that were between 77 and 220 pounds. That would be the equivalent of a coyote hunting mule deer.

“No modern North American carnivore is capable of crushing bones like borophagines could,” Wang says. At the same time, he notes, the dog wasn’t exactly like spotted hyenas, either.

Some of the key differences can be seen in which part of the jaws does the bone crushing. In hyenas, University of California, Berkeley, paleontologist Jack Tseng says, the molars are reduced, and most of the bone-crushing is done with premolars. Some of those premolars are specialized into a distinctive pyramid shape, while other premolar teeth have been specialized for cutting. Among borophagines, he notes, the premolar teeth were important in cracking bone. The molars behind those premolars, like in wolves, were important for further crushing and grinding. Where borophagines went all-in on cracking and crushing, hyenas can both crack bone and efficiently slice flesh.

The fact that so many bone shards were found inside the Borophagus feces fossils also indicates that the dogs couldn’t digest bones as completely as hyenas can. Borophagines were neither just like wolves nor were they quite like hyenas, occupying a unique niche no longer seen on the continent they used to roam.

The limbs of the borophagines are strange, too. “There is ongoing research that suggests neither cats nor wolves are appropriate analogs for borophagines,” Tseng says. The limbs of the dogs seem almost bear- or badger-like. The anatomy, which Tseng describes as a “head scratcher,” may indicate that the way borophagines moved and used their limbs might have been significantly different from other dogs, living and fossil.

Whatever borophagines were doing, they were successful carnivores through half of the Cenozoic Era. After tens of millions of years of being top dogs in North America, however, the borophagines vanished. Borophagus was among the last of these dogs, and it disappeared about 1.8 million years ago. The question facing paleontologists is why these dogs fell into extinction only to be replaced by carnivores that lacked the same bone-crushing abilities.

“The exact reason is hard to say,” Wang says, especially because borophagines were specialized predators. If the preferred prey of borophagines became rarer or even slipped into extinction, then the bone-crushing dogs would have dwindled with them.

Research into borophagine extinction, University of California, Berkeley, paleontologist Emily Bōgner says, needs to look at how modern carnivores in North America became established. Borophagines evolved as roughly hyena-like carnivores in North America, when hyenas already existed in Africa and Eurasia. The similarities may have prevented borophagines from dispersing to Eurasia. The inability of the borophagines to wander elsewhere to spin off new forms, as hyenas and other carnivorans did, may have eventually led to their demise as Earth’s ecosystems continued to shift. The dogs were experts at breaking down carcasses, but, in a changing world, even such powerful predators sometimes got squeezed out.

Riley Black | | Read More

Riley Black is the author of The Last Days of the Dinosaurs and many other books. She is a science correspondent for Smithsonian magazine covering fossils and natural history, and she writes about the prehistoric past for a variety of publications.