A reconstruction of Thylacosmilus atrox

A reconstruction of Thylacosmilus atrox – JORGE BRANCO


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 sockets or sockets facing forward, which helps to allow for stereoscopic (3D) vision, a useful adaptation for judging the position of prey 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. Its results are published in the journal Communication Biology.

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

On the other hand, the orbits of Thylacosmilus, a putative hypercarnivore, an animal with an estimated diet of at least 70% meat, were positioned like those of an ungulate, with orbits looking mainly laterally. In this situation, the visual fields don’t overlap enough for the brain to integrate them in 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 these 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 grew constantly, to the point where the canine roots continued over the top of their skulls. This had consequences, one was that there was no space available for the eye sockets in the usual position of carnivores in front of the face.“.

Gaillard used CT scans and 3D virtual reconstructions to assess orbital organization in various fossils and modern mammals. He was able to determine how the visual system of Thylacosmilus would compare to that of other carnivores or other mammals in general. While low orbital convergence occurs in some modern carnivores, Thylacosmilus was extreme in this respect: 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 fit within the eye sockets. “Thylacosmilus was able to compensate by having the eyes on the side of the head slightly projecting the orbits and orienting almost vertically, to increase the visual field overlap as much as possible,” said co-author Analia M. Forasiepi, also from INAGLIA. and researcher at CONICET, the Argentine science and research agency. “Although their orbits were not positioned favorably for 3D vision, they could achieve about 70% visual field overlap, evidently, enough to make it a successful active predator.”

Compensation appears to be the key to understanding how the Thylacosmilus skull formed.” said study co-author Ross DE MacPhee, senior curator at the American Museum of Natural History. “In effect, the growth pattern of the 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 odd orientation of the orbits in Thylacosmilus really represents a compromise.” 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 huge 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 temporalis muscles of mastication, which can cause deformities when eating. To control this, some mammals, including primates, have developed a bony structure that closes the eye sockets on one side. Thylacosmilus followed suit, another example of convergence between unrelated species.

This leaves a final question: what purpose would the development of huge, ever-growing teeth have served that required re-engineering the entire skull?

“It could have facilitated predation in some unknown way,” Gaillard said, “but if so, why haven’t any other sparasodonts, or any other carnivorous mammal, evolved the same adaptation in a convergent fashion, as rodent incisors. In instead, they seem to have continued to grow at the root, and finally extended almost to the back of the skull.”