This Giant Carnivore Ran on Hooves. Scientists Are Investigating Its Massive Skull and Crushing Teeth to Decipher the Beast’s True Nature

The beast’s head was huge. From the back to the front, its fossil skull measures more than 2.7 feet and preserves an impressive armament of piercing and crushing teeth. It looks like a skull that could belong to the wolfish, menacing Gmork from The Neverending Story—the cranium of a giant meat-eater unlike anything alive today.

But what was this creature, really? Paleontologists know it as Andrewsarchus mongoliensis, and after decades of mystery, they’re still working to piece together its true nature.

The animal’s impressive skull is displayed at New York City’s American Museum of Natural History (AMNH) and was found more than a century ago by Kan Chuen-pao, who was employed by the museum for a 1923 paleontology expedition to Inner Mongolia, in China. He uncovered the fossil in rocks that formed between 37 million and 47 million years ago, before modern carnivores like dogs, cats and bears began to flourish.

An accurate estimate of the creature’s size was impossible given the limited material, but if it had similar proportions to other carnivorous mammals of the time, the beast was clearly a giant. Andrewsarchus was immediately hailed as “the largest terrestrial carnivore which has thus far been discovered in any part of the world.”

From the beginning, the massive skull has been key to pinning down the mammal’s identity. Andrewsarchus is, to this day, known principally from this holotype, or the first fossil to bear its name. “The museum’s holotype skull of Andrewsarchus mongoliensis is still the best material for this genus and species,” says AMNH paleontologist John Flynn. The skull is “large, complete and well-preserved,” he adds, “a spectacular fossil!”

But it leaves many questions unanswered. The scientific consensus on what sort of carnivorous mammal Andrewsarchus was has shifted several times, with each new version altering what paleontologists expect were the mammal’s anatomy and habits.

A pig, a wolf, a whale

Andrewsarchus’ sheer size and general aspect immediately reminded researchers of other fossil creatures they had seen before. Paleontologist Walter Granger, who was the first to offer an assessment of the beast, thought the skull resembled a roughly piglike mammal called Entelodon. Sometimes known as “hell pigs,” Entelodon’s group of hoofed mammals were not really swine at all but an independent lineage of piggish omnivores with long jaws, flaring cheeks and impressive teeth suited to grabbing and crushing. Some of these so-called entelodonts—like Daeodon, which had previously been discovered in North America—had skulls roughly equal in size to Andrewsarchus’ and were about the size of a steer.

But 20th-century paleontologist William Diller Matthew came to a different conclusion. Upon examining the skull after its arrival in New York, he thought that Andrewsarchus was related to a strange group of carnivorous mammals that thrived between 33 million and 66 million years ago, called mesonychids. These roughly wolflike predators with hoof-tipped toes—sometimes called “wolves with hooves”—are not related to any living group of carnivores.

The giant-mesonychid interpretation of Andrewsarchus stuck. Then, in the later part of the 20th century, mesonychids were mistakenly thought to be close relatives of whales. Perhaps Andrewsarchus was some sort of enormous, land-dwelling protowhale, paleontologists hypothesized. Even so, lacking much more than the single skull found in 1923, experts couldn’t be sure precisely what Andrewsarchus was.

“There are many reasons why it is challenging to place Andrewsarchus on the mammalian family tree,” Flynn says. And that lack of additional fossils is still a stumbling block. Even though a few other examples have been found, he notes, they represent only the carnivore’s jaws and teeth. And while the AMNH Andrewsarchus skull looks impressively complete, it is actually “quite damaged,” says New York Institute of Technology paleontologist Jonathan Geisler. The skull’s teeth are worn down from chewing, making anatomical comparison to those of other mammals difficult. Part of the skull enclosing the inner ear, which can be very informative for comparisons, is either missing or plastered over by historical preservation methods.

On top of that, Andrewsarchus “is specialized in its own way,” says Flynn; it carries unique traits that might obscure anatomical similarities with other mammals that could help scientists pinpoint the beast’s relationships. The story is common among hoofed mammals called artiodactyls, which include entelodonts and mesonychids as well as modern deer, camels and hippos.

Andrewsarchus also ranks among these hoofed mammals, despite the fact that its feet have not yet been uncovered. Several details of the animal’s skull anatomy—such as a wrinkled appearance to the cusps of its cheek teeth, also seen on the chompers of pigs and their relatives—placed it among this group. In other words, paleontologists have enough to material to feel confident that Andrewsarchus was an artiodactyl, but whether it’s part of a known group or deserves its own offshoot on the family tree has remained unclear.

Piecing together the life of the beast

Even with the challenging dearth of fossils from the rest of its body, paleontologists are getting closer to working out what sort of creature Andrewsarchus was, assembling clues from its skull and what they know about other species.

A 2023 study on hoofed fossil mammals nudged Andrewsarchus closer to what Granger had expected a century before: In the new family tree, Andrewsarchus came out as a close relative of entelodonts, perhaps representing what an ancestral form of these hell pigs might have looked like. A revised analysis of the arrangement of the mammal’s teeth, as well as the specific details of the cusps on those teeth, placed Andrewsarchus closer to early entelodonts than to other groups of artiodactyls.

Despite their fearsome-looking skulls, entelodonts and their relatives were more opportunistic feeders than dedicated hunters. Their cheek teeth are domed, making them best suited to cracking and crushing—whether plant food or bones—rather than tearing into flesh. And while their skulls show plenty of space for large jaw muscles, their bones have a shorter ridge for those muscles to anchor onto—indicating a weaker bite than expected for a predator.

Entelodonts and their close relatives—which seem to include Andrewsarchus—were more cosmopolitan in their tastes than carnivores like cats and dogs. The ancient mammals could have easily scavenged carcasses, munched on vegetation and, when opportunity presented itself, hunted down fresh prey. In the fossil beds of North America, paleontologists have found remains of the small camel Poebrotherium with entelodont bite marks. Perhaps Andrewsarchus was an omnivore similar to the entelodonts, hunting smaller mammals when possible but rounding out its diet with plants and carrion.

Given such a history of change, Andrewsarchus might get moved across the tree of life again. While Flynn acknowledges the recent support for a close relationship to entelodonts, he adds that the available information from Andrewsarchus is so scant that alternative relationships are possible. Geisler, of the New York Institute of Technology, notes this as well, suggesting that the resemblance between Andrewsarchus and entelodonts is one not of heritage but of convergent evolution, in which two different groups evolve similar anatomy independently. Several groups of fossil mammals also evolved the large openings for jaw-closing muscles and long jaws that Andrewsarchus and entelodonts share—which hints that this particular way of living and feeding worked well for various groups and can’t immediately be taken as a sign of close relations.

Andrewsarchus has become a fossil celebrity—even making an appearance in the BBC’s Walking With Prehistoric Beasts—despite its full anatomy and nature remaining largely a mystery. The critical fossils that could fill in those blanks may still be out there, deep underground, or perhaps they’re already in museum collections, labeled as something else. Paleontologists can say that Andrewsarchus was an impressively large mammal capable of busting bones and snatching smaller beasts—but, for now, little else is certain.