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.
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…
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