Today when we think of Marsupials kangaroos, koalas come to mind but there once was a carnivorous marsupial with canines so large they extended across the top of its skull.
It could hunt effectively despite having wide-set eyes, like a cow or a horse. The skulls of carnivores typically have forward-facing eye sockets, or orbits, which helps enable stereoscopic (3D) vision, a useful adaptation for judging the position of prey before pouncing.
Popularly known as the “marsupial (or metatherian) sabertooth” because its extraordinarily large upper canines, Thylacosmilus lived in South America until its extinction about 3 million years ago. It was a member of Sparassodonta, a group of highly carnivorous mammals related to living marsupials.
Although sparassodont species differed considerably in size—Thylacosmilus may have weighed as much as 100 kilograms (220 pounds)—the great majority resembled placental carnivores like cats and dogs in having forward-facing eyes and, presumably, full 3D vision.
By contrast, the orbits of Thylacosmilus, a supposed hypercarnivore—an animal with a diet estimated to consist of at least 70 percent meat—were positioned like those of an ungulate, with orbits that face mostly laterally. In this situation, the visual fields do not overlap sufficiently for the brain to integrate them in 3D. Why would a hypercarnivore evolve such a peculiar adaptation? A team of researchers from Argentina and the United States set out to look for an explanation.
Charlène Gaillard, a Ph.D. student in the Instituto Argentino de Nivología, Glaciología, y Ciencias Ambientales (INAGLIA) explained:
“You can’t understand cranial organization in Thylacosmilus without first confronting those enormous canines. 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.”
Analia M. Forasiepi, INAGLIA and a researcher in CONICET, the Argentinian science and research agency, added:
“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.
“Even though its orbits were not favourably positioned for 3D vision, it could achieve about 70 percent of visual field overlap—evidently, enough to make it a successful active predator.”
Lateral displacement of the orbits was not the only cranial modification that Thylacosmilus developed to accommodate its canines while retaining other functions. Placing the eyes on the side of the skull brings them close to the temporal chewing muscles, which might result in deformation during eating. To control for this, some mammals, including primates, have developed a bony structure that closes off the eye sockets from the side. Thylacosmilus did the same thing—another example of convergence among unrelated species.
Ross D. E. MacPhee, a senior curator at the American Museum of Natural History said:
“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 actually represents a morphological compromise between the primary function of the cranium, which is to hold and protect the brain and sense organs, and a collateral function unique to this species, which was to provide enough room for the development of the enormous canines.”
Click on this link to access the article, Seeing through the eyes of the sabertooth Thylacosmilus atrox (Metatheria, Sparassodonta, published in Communications Biology
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