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How the ‘marsupial’ thylacine saw its world

Reconstruction of Thylacosmilus atrox. Credit: © Jorge Blanco

A new study looks at how an extinct carnivorous marsupial relative with canines so large they extend across the top of its skull hunts effectively despite having wide 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 swooping. Scientists from the American Museum of Natural History and the Argentine Institute of Physiology, Glaciología, y Ciencias Ambientales in Mendoza, Argentina, have studied whether a “marsupial tooth” Thylacosmilus atrox It can be seen in 3D at all. their results Posted today in the journal Communication biology.

Popularly known as the “marsupial (or metatherian) sabertooth” because its unusually large upper canines are reminiscent of those more famous placental canines that evolved in North America, Thylacosmilus It lived in South America until its extinction about 3 million years ago. It was a member of the Sparassodonta, a group of highly carnivorous mammals related to the living marsupials. Although sparsodont species differ greatly in size –Thylacosmilus They could weigh up to 100 kilograms (220 lb) – and the vast majority had cat- and dog-like placental carnivores in their frontal eyes, and possibly full 3D vision. By contrast, the orbits of Thylacosmilus, is supposed to be a hypertrophied carnivore—an animal with a diet estimated to consist of at least 70 percent meat—placed like those of ungulates, with orbits mostly facing laterally. In this case, the visual fields do not overlap enough for the brain to combine them into a 3D image. Why would a hypereating animal evolve such a strange adaptation? A team of researchers from Argentina and the United States set out to find an explanation.

“You can’t understand the skull’s organization Thylacosmilus Without encountering those huge tusks first, “they were constantly growing, so much so that the roots of the tusks continued over the tops of their skulls. This had consequences, one of them being that there was no room available for the orbits in the usual position of carnivores at the front of the face.”

Gaillard used computed tomography and virtual 3D reconstructions to assess orbital organization in a number of fossil and modern mammals. She was able to determine how the visual system of the Thylacosmilus compared to those found in other carnivores or other mammals in general. Although low orbital convergence occurs in some modern carnivores, Thylacosmilus It was extreme in this regard: it had an orbital affinity value as low as 35°, compared to a typical predator, such as a cat, at about 65°.

However, good stereoscopic vision also depends on the degree of presbyopia, which is a measure of how well the eyeballs are located within the orbits. “Thylacosmilus She was able to compensate for having her eyes on the side of her head by closing their orbits somewhat and orienting them almost vertically, to maximize visual field overlap as much as possible,” said co-author Analia M., a researcher at CONICET, the Argentine science and research agency. Their orbits weren’t well-placed for 3D vision, yet they could achieve about 70 percent visual field overlap — clearly enough to make them a successful active predator.”

Reconstruction of the skull of Thylacosmilus atrox. Credit: © Jorge Blanco

“Compensation seems to be the key to understanding how the skull works Thylacosmilus said study co-author Ross D. McPhee, senior curator of the American Museum of Natural History. Thylacosmilus It actually represents a morphological compromise between the primary function of the skull, which is to hold and protect the brain and sensory organs, and the side function unique to this species, which was to provide enough room for the development of enormous canines. ”

Lateral displacement of the orbits was not the only cranial modification Thylacosmilus It was developed to accommodate its canines while retaining other functions. The position of the eyes on the side of the skull brings them close to the temporal muscles of mastication, which can lead to deformity during eating. To control this, some mammals, including primates, have evolved a bony structure that closes the eye sockets from the side. Thylacosmilus Do the same thing – another example of the convergence of unrelated species.

This leaves one last question: what purpose would have been served by developing huge, ever-growing teeth that would require re-engineering the entire skull?

It may have made predation easier in some unknown way, Gillard said, but, if that were the case, why didn’t any other sparassodont — or, for that matter, any other carnivorous mammal — evolve the same adaptation so convergently? Thylacosmilus It didn’t wear out like rodent cutters. Instead, they seem to have continued to grow at the root, eventually extending almost to the back of the skull.”

Forasiepi emphasized this point, saying, “The search for clear adaptive explanations in evolutionary biology is fun but largely futile. One thing is clear: Thylacosmilus He was not a freak by nature, but in his time and place he managed, it seems, admirably to survive as an ambush predator. We might consider it an anomaly because it doesn’t fit our preconceived categories of what a mammalian carnivore should look like, but evolution makes its own rules.”

more information:
Seeing through the eyes of the sabertooth Thylacosmilus atrox (Metatheria, Sparassodonta, Communication biology (2023). DOI: 10.1038/s42003-023-04624-5

Journal information:
Communication biology

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