With bulging eyes and a killer smile, this sabertooth was an absolute nightmare

Forever-growing teeth led to some interesting evolutionary developments for the carnivorous Thylacosmilus.
An illustration of Thylacosmilus atrox, an extinct marsupial with giant canine teeth,
A reconstruction of Thylacosmilus atrox. Jorge Blanco

Starting around seven million years ago, the Thylacosmilus atrox, also known as the “marsupial sabertooth,” prowled South America with canine teeth so large that they reached the top of their skull. The hypercarnivore, meaning it had a diet of at least 70 percent meat, and possibly used its tongue to slurp out the innards of its prey. Thylacosmilus went extinct about 3 million years ago and was a member of Sparassodonta, a group of carnivorous mammals that related to living marsupials.

The 200 pound beast had wide-set eyes like a cow, which are considerably different compared to the forward facing eye sockets of cats.  The strange setup helped the creature see in stereoscopic vision (or 3D).

[Related: Giant wombats the size of small cars once roamed Australia.]

With eyes like this, objects do not overlap sufficiently for the brain to integrate them in three-dimensions. Scientists have long been perplexed as to why the ferocious hunter would evolve such a strange adaptation in its skull. 

Now, a study published March 21 in the journal Communications Biology answers a few questions on how this extinct animal with a unique skull could see and hunt in its ancient world. 

Scientists from Argentina and the US used CT scanning and 3D virtual reconstructions to assess how the nasal cavities were organized in both modern and fossilized mammals.  The scans and reconstructions enabled the team to compare Thylacosmilus’ visual system with other carnivores or other mammals in general and study orbital convergence. This is the way the eyes move together and point inward when they look at objects that are close by. 

Thylacosmilus had an orbital convergence value as low as 35 degrees, which is pretty extreme compared to that of a typical predator at around 65 degrees. 

Thylacosmilus was able to compensate for having its eyes on the side of its head by sticking its orbits out somewhat and orienting them almost vertically, to increase visual field overlap as much as possible,” said co-author Analia M. Forasiepi, from Instituto Argentino de Nivología, Glaciología, y Ciencias Ambientales (INAGLIA) and a researcher at the Argentinian science and research agency, in a statement. “Even though its orbits were not favorably positioned for 3D vision, it could achieve about 70 percent of visual field overlap—evidently, enough to make it a successful active predator.”

According to the team, how Thylacosmilus was able to compensate for low orbital convergence appears to be the key to understanding how this extinct marsuipial’s skull was put together. The growth patterns in their canines during early stages of development would have moved the eye sockets away from the face, resulting in the wide-set eyes seen in adults. 

“You can’t understand cranial organization in Thylacosmilus without first confronting those enormous canines,” said Charlène Gaillard, a Ph.D. student also at INAGLIA and study co-author, in a statement. “They weren’t just large; they were ever-growing, to such an extent that the roots of the canines continued over the tops of their skulls. This had consequences, one of which was that no room was available for the orbits in the usual carnivore position on the front of the face.”

[Related: Koalas use their noses to find friends and avoid enemies.]

The strange placement of the eye sockets was not the only modification that Thylacosmilus developed to accommodate its enormous canines. Since their eyes are much closer to their chewing muscles, these muscles risk getting deformed while eating. The eyes on the side of the skull also brings them closer to the mouth’s chewing muscles, possibly resulting in a deformation of the mouth muscles. Some mammals, including primates and Thylacosmilus, have developed a bony structure that closes off the eye sockets from the side as a way to control this. 

Now, a new question remains—why would the animal develop huge, constantly growing teeth that required its whole skull to be re-engineered? 

“It might have made predation easier in some unknown way,” said Gaillard. “The canines of Thylacosmilus did not wear down, like the incisors of rodents. Instead, they just seem to have continued growing at the root, eventually extending almost to the rear of the skull.”