Investigating the vision of an extinct carnivorous marsupial with peculiar adaptations

In a recent study, researchers from the American Museum of Natural History and the Instituto Argentino de Nivología, Glaciología, y Ciencias Ambientales in Argentina investigated the hunting techniques of an extinct carnivorous marsupial relative, Thylacosmilus atrox. This creature had exceptionally large canines that extended over its skull, similar to those of the North American placental sabertooth. Despite having wide-set eyes like cows or horses, which are not typically conducive to 3D vision, Thylacosmilus was able to hunt effectively. The study, published in Communications Biology, aimed to determine whether Thylacosmilus had any 3D vision capability.

Thylacosmilus lived in South America until its extinction about three million years ago and was a member of Sparassodonta, a group of highly carnivorous mammals related to modern-day marsupials. Although sparassodont species varied in size, with Thylacosmilus potentially weighing up to 100 kilograms (220 pounds), most had forward-facing eyes and full 3D vision, similar to placental carnivores such as cats and dogs. However, Thylacosmilus had laterally-facing orbits similar to ungulates, with insufficient overlap in visual fields for the brain to integrate them in 3D. This peculiar adaptation is unusual for a hypercarnivore, an animal with a diet consisting of at least 70 percent meat, prompting the researchers to investigate the reason for its evolution.

Lead author Charlène Gaillard, a Ph.D. student at the Instituto Argentino de Nivología, Glaciología, y Ciencias Ambientales (INAGLIA), highlighted the importance of Thylacosmilus’ enormous canines in understanding its cranial organization. These canines were not only large but also ever-growing, to the extent that they extended over the tops of the skull. This meant that there was no space for the orbits to be in their usual position on the front of the face.

Gaillard employed CT scanning and 3D virtual reconstructions to study orbital organization in various fossil and modern mammals. She compared the visual system of Thylacosmilus to that of other carnivores and mammals in general. While some modern carnivores exhibit low orbital convergence, Thylacosmilus was extreme in this regard, with an orbital convergence value as low as 35 degrees, compared to a typical predator like a cat, which is around 65 degrees.

However, good stereoscopic vision also depends on the degree of frontation, which refers to the position of the eyeballs within the orbits. Co-author Analia M. Forasiepi, also in INAGLIA and a researcher in CONICET, the Argentinian science and research agency, explained that Thylacosmilus compensated for having side-facing eyes by protruding its orbits somewhat and orienting them almost vertically to increase the overlap of the visual field. Despite its unfavorable orbital position for 3D vision, Thylacosmilus could achieve about 70 percent visual field overlap, which was sufficient to make it a successful active predator.

A reconstruction of the skull of Thylacosmilus atrox. Credit: © Jorge Blanco

Lead author Charlène Gaillard, emphasized the importance of considering Thylacosmilus’s enormous canines when studying its cranial organization. She used CT scanning and 3D virtual reconstructions to compare the orbital organization of Thylacosmilus to other mammals. The results showed that Thylacosmilus had an unusually low orbital convergence, as low as 35 degrees, compared to a typical predator like a cat, which has an orbital convergence value of around 65 degrees.

However, Thylacosmilus compensated for its unique orbital organization by orienting its orbits vertically and sticking them out somewhat. This increased visual field overlap by about 70%, which was enough to make it a successful active predator despite its suboptimal 3D vision.

According to co-author Ross D. E. MacPhee, a senior curator at the American Museum of Natural History, the growth pattern of Thylacosmilus’s canines during early cranial development displaced the orbits away from the front of the face, resulting in the odd orientation of the orbits we see in adult skulls. This represents a morphological compromise between protecting the brain and sense organs and providing enough room for the development of the enormous canines.

Thylacosmilus also developed a bony structure to protect its eye sockets from deformation by the temporal chewing muscles, as seen in primates. The purpose of Thylacosmilus’s huge, ever-growing teeth remains unclear, and Gaillard and co-author Analia M. Forasiepi caution against looking for clear-cut adaptive explanations in evolutionary biology. Thylacosmilus was not a freak of nature but a successful ambush predator that managed to survive in its time and place.

Source: American Museum of Natural History

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