Prehistoric South America’s sabre-toothed marsupial hunter, Thylacosmilus atrox, developed vision unique in carnivores – to make space for its massive canines.
Canine teeth are common in mammalian predators. Even us humans have our little incisors.
The most famous sabre-toothed animal was, of course, the sabre-toothed cat also known as Smilodon fatalis. Though smaller in stature, the muscular Smilodon would have weighed more than lions, getting up to 300 kilograms. They roamed the Americas until as recently as around 10,000 years ago.
Gorgonopsids in the Permian period more than 250 million years ago were the first mammal ancestors to evolve fangs.
Even prehistoric Australia had a fanged top predator called Thylacoleo – a jaguar sized “marsupial lion” which died out around 46,000 years ago.
But Thylacosmilus stands out for its massive canines which are so long they extended across the animal’s skull.
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Carnivores typically have forward-facing eyes which helps enable stereoscopic, or 3D, vision. This depth perception is useful in judging the position of prey before pouncing.
Though a hypercarnivore with a diet estimated to be at least 70 percent meat, Thylacosmilus breaks this mould with eye orbits positioned on its skull more like those of ungulates – hoofed prey animals. The positioning of ungulate eyes lessens their 3D vision but increases their field of view in case predators are lurking off to their side.
Thylacosmilus belongs to a group of highly carnivorous mammals related to today’s marsupials called Sparassodonta. Sparassodonts largely resembled cats and dogs and had forward-facing eyes.
Why was Thylacosmilus different?
“You can’t understand cranial organization in Thylacosmilus without first confronting those enormous canines,” says Charlène Gaillard, a Ph.D. student in the Instituto Argentino de Nivología, Glaciología, y Ciencias Ambientales (INAGLIA) in Argentina.
“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.”
Gaillard used CT scanning and 3D reconstructions of Thylacosmilus skulls as well as the skulls of modern and other fossil animals.
She determined that typical orbital convergence (the amount of overlap between each eye’s sight which enhances 3D vision) in typical predators is around 65 degrees. In Thylacoleo, the convergence is as low as 35 degrees.
“Thylacosmilus 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,” says Analia M. Forasiepi, also in INAGLIA. “Even though its orbits were not favourably positioned for 3D vision, it could achieve about 70 percent 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 was put together,” says Ross D. E. MacPhee, senior curator at the American Museum of Natural History. “The odd orientation of the orbits in Thylacosmilus 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.”
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Thylacosmilus lived in South America before going extinct around three million years ago after the land bridge connecting North and South America was established. It is likely that this continental convergence saw the arrival of the larger, more powerful Smilodon which out-competed the marsupial sabretooth.
Exactly why Thylacosmilus evolved its ever-growing massive canines is unclear, the researchers say. But one thing is clear – it was clearly a well-adapted hunter, if a little odd.
The study is published in the journal Communications Biology.