Meet the three million-year-old sabretooth marsupial with ever-growing TEETH

A saber-toothed marsupial that lived three million years ago had teeth so huge that they forced its eyes to the sides of its head.

Scientists at the Instituto Argentino de Nivología, Glaciología, y Ciencias Ambientales in Argentina modeled the skull of Thylacosmilus – an extinct hypercarnivore that lived in South America.

They found that he couldn’t see well in 3D because his eyes were spaced far apart to accommodate his huge fangs.

These teeth were ‘growing through’; they do not wear out, and their roots extended to the front of the creature’s skull and around to the back.

However, it could still be a successful predator because its eye sockets protruded outward, giving it a wide enough field of view.

Thylacosmilus, or Thylacosmilus atrox, were 220 lb (100 kg) marsupials; thus creatures born before they are fully developed must first be carried in their mother’s pouches.

However, they had huge upper canine teeth that more closely resembled those of “placental” animals that develop entirely in the womb, such as saber-toothed cats and tigers.

It was a member of Sparassodontia group of carnivorous mammals to which modern marsupials are related, but which resemble placental carnivores in appearance.

Fossil evidence suggests the vast majority of the Sparassodontia species had forward-facing eyes.

However, Thylacosmilus had eyes on the side of its head just like horses and cows, leading researchers to wonder what this did to its eyesight.

If the fields of view in each eye didn’t overlap enough, they would have had trouble seeing the world in three dimensions and therefore judging the prey’s position.

But their diet is thought to be at least 70 percent meat, making them a “hypercarnivore” and incredibly deadly predator.

For their study, published today in Communication Biologythe researchers wanted to discover how the marsupial could see and why it developed such a unique head.

‘You can’t understand the cranial organization in Thylacosmilus without first confronting those huge fangs,’ says PhD student and lead author Charlène Gaillard.

‘They weren’t just big; they continued to grow, to such an extent that the roots of the fangs ran over the tops of their skulls.

“This had consequences, including no room for the eye sockets in the usual carnivore position at the front of the face.”

Ms. Gaillard used CT scanning and virtual 3D reconstructions to analyze the marsupial’s skull and compare it to that of other extinct and living mammals.

Carnivores typically have forward-facing eye sockets, meaning each eye’s fields of vision overlap by about 65°, allowing them to see and hunt in 3D.

But this “orbital convergence value” turned out to be only 35° in Thylacosmilus, so it had developed another property that improved its vision.

“Thylacosmilus was able to compensate for having its eyes on the side of its head by protruding its eye sockets slightly and orienting them almost vertically, in order to maximize the visual field overlap,” says co-author Dr. .Analia Forasiepi.

“Although its orbits were not positioned favorably for 3D vision, it was able to achieve about 70 percent of the visual field overlap — apparently enough to make it a successful active predator.”

The researchers say the creature’s unusual eye location resulted from the fangs growing upward during early skull development and pushing the sockets sideways.

Co-author Ross MacPhee, a senior curator at the American Museum of Natural History, said: “The odd orientation of the orbits in Thylacosmilus actually represents a morphological compromise between the skull’s primary function, which is to hold and protect the brain and senses, and a side function that is unique to this species, which is to provide enough space for the development of the huge canine teeth.’

Thylacosmilus had also developed a bony structure on the side of its eye sockets that protected its eyes from the oft-used chewing muscles they were close to.

All of these unique skull modifications were supposed to accommodate its ever-growing teeth, and the researchers still aren’t quite sure why they were so important to the species.

“Maybe it made predation easier in some unknown way,” Ms Gaillard said.

“But if so, why didn’t any other sparassodont — or, for that matter, any other mammalian carnivore — convergently evolve the same adaptation?”

Dr. Forasiepi added: ‘Looking for clear adaptive explanations in evolutionary biology is fun but largely pointless.

‘One thing is clear: Thylacosmilus was no vagary of nature, but in his time and place he survived, apparently quite admirably, as an ambush predator.

“We can consider it an anomaly because it doesn’t fit our preconceived categories of what a true mammalian carnivore should look like, but evolution makes its own rules.”