Paleontologists from the American Museum of Natural History, the Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales and CONICET investigated how Thylacosmilus atrox, an extinct, carnivorous sparassodont famously called the ‘sabertooth marsupial,’ could hunt effectively despite having wide-set eyes, like a cow or a horse.
Life reconstruction of Thylacosmilus atrox. Image credit: Jorge Blanco.
Vision is part of a complex neurobehavioral sensory system that is critically important in most terrestrial vertebrates.
Among mammals, primates and most predators have visual systems evolutionarily designed for stereoscopy, or the perception of depth.
This also applies to the sparassodonts — an extinct group of hypercarnivorous, non-marsupial metatherians that lived in South America through most of the Cenozoic until their extinction in the mid-Pliocene — with one exception: Thylacosmilus atrox.
Famously called the ‘sabertooth marsupial,’ the ancient predator was around half the size (average body mass 117 kg) of the saber-toothed cat Smilodon fatalis, which was lion-sized or larger (245 kg).
Its eye sockets, or orbits, 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? Paleotologist Charlène Gaillard and colleagues set out to look for an explanation.
“You can’t understand cranial organization in Thylacosmilus atrox without first confronting those enormous canines,” Gaillard said.
“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.”
The study authors used CT scanning and 3D virtual reconstructions to assess orbital organization in a number of fossil and modern mammals.
They were able to determine how the visual system of Thylacosmilus atrox would have compared to those in other carnivores or other mammals in general.
Although low orbital convergence occurs in some modern carnivores, Thylacosmilus atrox was extreme in this regard: it had an orbital convergence value as low as 35 degrees, compared to that of a typical predator, like a cat, at around 65 degrees.
However, good stereoscopic vision also relies on the degree of frontation, which is a measure of how the eyeballs are situated within the orbits.
“Thylacosmilus atrox 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,” said Dr. Analia Forasiepi, a researcher at the Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales and CONICET.
“Even though its orbits were not favorably positioned for 3D vision, it could achieve about 70% of visual field overlap — evidently, enough to make it a successful active predator.”
“Compensation appears to be the key to understanding how the skull of Thylacosmilus atrox was put together,” said Dr. Ross MacPhee, a 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 atrox 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.”
Lateral displacement of the orbits was not the only cranial modification that Thylacosmilus atrox 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 atrox did the same thing — another example of convergence among unrelated species.
This leaves a final question: what purpose would have been served by developing huge, ever-growing teeth that required re-engineering of the whole skull?
“It might have made predation easier in some unknown way. But, if so, why didn’t any other sparassodont — or for that matter, any other mammalian carnivore — develop the same adaptation convergently?” Gaillard said.
“The canines of Thylacosmilus atrox did not wear down, like the incisors of rodents.”
“Instead, they just seem to have continued growing at the root, eventually extending almost to the rear of the skull.”
The study was published in the journal Communications Biology.
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C. Gaillard et al. 2023. Seeing through the eyes of the sabertooth Thylacosmilus atrox (Metatheria, Sparassodonta). Commun Biol 6, 257; doi: 10.1038/s42003-023-04624-5
