Beast with 360 degree vision that sheds light on the evolution of carnivores

A terrifying saber-toothed marsupial had teeth so large that its eyes sat on the side of its head, like a horse or a cow, according to new research.

The roots went straight back to his brain, creating a unique vision to catch prey by opening his eyes wide.

A study published in the journal Communications Biology found that Thylacosmilus atrox could shed new light on carnivore evolution.

The beast in the bag, about the size of a jaguar, would have been a deadly hunter, able to see almost 360 degrees without moving.

“One cannot understand cranial organization in Thylacosmilus without first confronting these massive canines,” said lead author Charlene Gaillard, a doctoral student at Argentina’s Institute of Nivology, Glaciology and Environmental Sciences (INAGLIA).

“Not only were they big, they kept growing, so big that the roots reached over their skulls. One of the consequences of this was that there was no room left for the eye sockets in the position in front of the face that is common for carnivores.”

Grazing animals must be wary to flee quickly in the event of an attack, but the phenomenon was first observed in a carnivore.

Carnivore skulls usually have forward orbits or eye sockets to allow for 3D or “stereoscopic” vision, but the thylacosmilus had widely spaced eyes more typical of a herbivore.

It was more closely related to kangaroos and koalas than Smilodons, the famous saber-toothed cat that became extinct at the end of the last Ice Age.

“Compensation appears to be key to understanding how the skull of Thylacosmilus is put together,” said American Museum of Natural History Professor Ross MacPhee, one of the study’s co-authors.

“In fact, during early skull development, the dog’s growth pattern would have moved the orbits of the front of the face, leading to the result we see in adult skulls.”

MacPhee attributes the odd orientation of the orbits in Thylacosmilus to a morphological compromise between the skull’s primary function of holding and protecting the brain and senses.

The skull likely had a secondary function unique to this species, which was to provide ample space for the massive canine teeth to develop.

The international team used CT scans and virtual 3D reconstructions to assess the orbital organization of a range of fossil and modern mammals.

Forward-facing eye sockets are important for judging the prey’s position before it leaps.

Thylacosmilus was a member of a group known as the sparassodonts, highly carnivorous mammals related to living marsupials.

Most carnivorous mammals resembled placental carnivores such as cats and dogs with forward-facing eyes.

But the Thylacosmilus, an animal with a mostly meat-based diet, had eyes like those of a herbivore, usually on the side. In this situation, the visual fields do not overlap enough for the brain to integrate them into 3D.

“Thylacosmilus was able to compensate for its eyes on the side of its head by slightly elongating its eye sockets and aligning them nearly vertically to maximize visual field overlap,” said study co-author Dr. Analia Forasiepi, also from INAGLIA.

“While its orbits weren’t well positioned for 3D vision, it was able to overlap about 70 percent of the field of view — apparently enough to make it a successful active predator.”

The lateral shift of the orbits was not the only modification Thylacosmilus developed to accommodate its fangs while preserving other functions.

The placement of the eyes on the side of the skull means they are close to the temporal muscles of mastication, which can cause distortion during eating.

To control this, some mammals, including primates, have developed a bony structure that closes the eye sockets from the side. Thylacosmilus did the same.

“Maybe it enabled incursion in some unknown way. But if so, why hasn’t any other sparassodont — or even any other mammalian carnivore — convergently evolved the same adaptation?” Gaillard said.

“The fangs of Thylacosmilus don’t wear down like the incisors of rodents. Instead, they seem to have just continued to grow at the root, eventually extending almost to the back of the skull.

Due to its size, Thylacosmilus’ teeth were larger than any other known saber-tooth, within a few millimeters of its very small braincase.

“The search for clear adaptive explanations in evolutionary biology is fun but largely futile,” says Dr. Forasiepi.

“One thing is clear. Thylacosmilus was no quirk of nature, but in his time and place he apparently managed to survive 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.”