A new study investigates how an extinct carnivorous marsupial, with canines so large they extended from the top of its skull, could hunt effectively despite having large eyes, much like a cow or horse.
Carnivores often have forward-facing eyes, allowing stereoscopic three-dimensional vision, a useful adaptation for judging the position of prey before attacking. On the other hand, herbivores have eyes on the sides of their heads, allowing them to cover more of the visual field if they are attacked by predators.
Scientists from the American Museum of Natural History and the Argentine Institute of Nivology, Glaciology and Environmental Sciences in Mendoza (Argentina) studied an exception to this rule. The sabertooth marsupial Thylacosmilus atrox had eyes on the sides of its head, but it was a carnivorous predator. The question is whether he could see in 3D. The results are published in the journal Communications Biology.
This animal is popularly known as the metatherium or “saber-toothed” marsupial because its extraordinarily large upper canines resemble those of the saber-toothed tiger that evolved in North America. Thylacosmilus lived in South America until its extinction about three million years ago. was a member of sparassodontaa group of highly carnivorous mammals related to living marsupials.
Although species of sparasodonts differed considerably in size (Thylacosmilus could weigh 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.
However, the orbits of Thylacosmilus, a putative hypercarnivore whose diet is estimated to be at least 70% meat, were arranged like those of an ungulate, with orbits oriented to the sides. 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?
The teeth determined the eyes of the metatherium
“You cannot understand the cranial organization of Thylacosmilus without first dealing with these huge canines,” explains lead author Charlène Gaillard. “Not only were they large, they grew infinitely, to the point where the canine roots protruded from the top of the skull. This had consequences, one of which was that there was no space available for the eye sockets in the usual carnivore position in front of the face.”
Gaillard used computed tomography and 3D virtual reconstructions to assess orbital organization in various fossils and modern mammals. Thus, he was able to determine how the visual system of Thylacosmilus compared to that of other carnivores or other mammals in general. While orbital convergence is low in some modern carnivores, Thylacosmilus was extreme in this respect: it had an orbital convergence value of just 35 degrees, compared to a typical predator such as a cat of around 65 degrees.
However, good stereoscopic vision also depends on the degree of frontality, which is a measure of how well the eyeballs are positioned within the eye sockets. “Thylacosmilus was able to compensate for having the eyes on the side of the head by making the eye sockets protrude slightly and orient almost vertically to maximize visual field overlap,” explains co-author Analia M. Forasiepi. “Although its orbits were not positioned favorably for 3D vision, it could achieve around 70% visual field overlap, evidently enough to make it a successful active predator.”
The strange orientation of the orbits in Thylacosmilus actually represents a morphological compromise between the primary 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 avoid this, some mammals, including primates, have evolved a bony structure that closes the orbits laterally. Thylacosmilus followed suit, another example of convergence between unrelated species.
That leaves us with one last question: what would be the use of growing huge, growing teeth that required redesigning the entire skull? Why did no other sparasodont, or any other carnivorous mammal, convergently evolve the same adaptation? According to Forasiepi, “Looking for pure adaptive explanations in evolutionary biology is fun but largely pointless”. One thing is certain: Thylacosmilus was no freak of nature, but in its time and place it managed, seemingly admirably, to survive as a predator and hunt its prey.
REFERENCE
Seeing through the eyes of the sabertooth Thylacosmilus atrox (Metatheria, Sparassodonta)
