Explanation to the eyes of prey of an extinct hypercarnivore

March 21 () –

A new study investigates how an extinct carnivorous marsupial relative with canines up to its skull could hunt effectively despite having wide eyes, like a cow or a horse.

Carnivore skulls often have forward-facing eye sockets or orbits, which helps enable stereoscopic (3D) vision, a useful adaptation for judging prey position before attacking. Scientists from the American Museum of Natural History and the Argentine Institute of Nivology, Glaciology and Environmental Sciences, andstudied whether the “saber-toothed marsupial” Thylacosmilus atrox could see in 3D. Their results are published in the journal Communications Biology.

Popularly known as the “marsupial (or metatherium) sabertooth” because its extraordinarily large upper canines are reminiscent of the more famous placental sabertooths that evolved in North America, Thylacosmilus lived in South America until its extinction about 3 million years ago. years. It was a member of the Sparassodonta, a group of highly carnivorous mammals related to modern marsupials.is. Although species of sparasodonts differed considerably in size — Thylacosmilus may have weighed up to 100 kilograms — the vast majority resembled placental carnivores such as cats and dogs in having forward-facing eyes and, presumably, full three-dimensional vision.

By contrast, the orbits of Thylacosmilus, a putative hypercarnivore, an animal with a diet estimated to consist of at least 70 percent meat, were positioned like those of an ungulate, with orbits looking mainly laterally. In this situation, the visual fields do not overlap enough for the brain to integrate them into 3D. Why would a hypercarnivore develop such a peculiar adaptation? A team of researchers from Argentina and the United States set out to find an explanation.

“You cannot understand the cranial organization in Thylacosmilus without first confronting those huge canines,” said lead author Charlène Gaillard, a doctoral student at the Argentine Institute of Nivology, Glaciology and Environmental Sciences (INAGLIA). “Not only were they large, but they were constantly growing, to such an extent that the roots of the canines continued over the top of their skulls. This had consequences, one of which was that there was no space available for the orbits in the usual position of carnivores in the front of the face“.

Gaillard used CT scans and 3D virtual reconstructions to assess orbital organization in various fossil and modern mammals. He was able to determine how the visual system of Thylacosmilus would have compared to that of other carnivores or other mammals in general. Although low orbital convergence occurs in some modern carnivores, Thylacosmilus was extreme in this regard: it had an orbital convergence value as low as 35 degrees, compared to that of a typical predator, such as a cat, around 65 degrees.

However, good stereoscopic vision is also based on the degree of frontalization, which is a measure of how well the eyeballs sit within the orbits. “Thylacosmilus was able to compensate for having its eyes on the side of its head by slightly protruding its orbits and oriented almost vertically, to increase visual field overlap as much as possible,” said co-author Analia M. Forasiepi, also at INAGLIA. and researcher at CONICET, the Argentine science and research agency. “Although its orbits were not favorably positioned for 3D vision, it could achieve about 70 percent visual field overlap, evidently, enough to make it a successful active predator.”

“The compensation appears to be the key to understanding how the Thylacosmilus skull came together.” said study co-author Ross DE MacPhee, senior curator at the American Museum of Natural History. “In effect, the growth pattern of canines during early cranial development would have displaced the orbits away from the front of the face, producing the result we see in adult skulls. The strange orientation of the orbits in Thylacosmilus actually represents a compromise.” between the main function of the skull, which is to support and protect the brain and sense organs, and a collateral function unique to this species, which was to provide enough space for the development of the enormous canines.”

The lateral displacement of the orbits was not the only cranial modification that Thylacosmilus developed to accommodate its canines while preserving other functions. Placing the eyes on the side of the skull brings them closer to the temporal chewing muscles, which could cause deformity when eating. To control this, some mammals, including primates, have evolved a bony structure that closes off their eye sockets on one side. Thylacosmilus followed suit, another example of convergence between unrelated species.

This leaves one final question: what purpose would have served the development of enormous and constantly growing teeth that required reengineering of the entire skull?

“It could have facilitated predation in some unknown way,” Gaillard said, “but if so, why didn’t any other sparasodonts, or any other carnivorous mammals, develop the same adaptation in a convergent fashion? Thylacosmilus canines did not They wore away, like the incisors of rodents. Instead, they seem to have continued to grow at the root, and finally extended almost to the back of the skull.”