There was a time in South America about 3 million years ago when there lived a strange carnivorous marsupial called Thylacosmilus, with large teeth that made it resemble a Saber-toothed tiger.
It was a member of the Sparassodonta, a group of highly carnivorous mammals related to living marsupials. Although sparasodont species differed considerably in size, Thylacosmilus it may have weighed up to 100 kilograms (220 lb), the vast majority resembled placental carnivores such as cats and dogs in having forward-facing eyes and, presumably, full 3D vision.
For this reason, recent studies have tried to elucidate a great mystery: how did it manage to hunt with its eyes so far apart?
Carnivore skulls often have forward-facing eye sockets, or orbits, which help enable stereoscopic (3D) vision, a useful adaptation for judging prey’s position before pouncing. Scientists from the American Museum of Natural History and the Argentine Institute of Nivology, Glaciology and Environmental Sciences in Mendoza, Argentina, studied whether the “sabertooth marsupial” Thylacosmilus atrox could see in 3D. Your results will published today in the magazine Communications Biology.
“Cannot understand cranial organization in Thylacosmilus without first confronting those huge canines,” said lead author Charlène Gaillard, a doctoral student at the Argentine Institute of Nivology, Glaciology and Environmental Sciences (INAGLIA). “They weren’t just big; They were constantly growing, to such an extent that the roots of the canines continued over the top of their skulls. This had consequences, one of which was that there was no space available for the orbits in the usual carnivore position in the front of the face.”
Gaillard used CT scans and 3D virtual reconstructions to assess orbital organization in various fossil and modern mammals. She was able to determine how the visual system of Thylacosmilus it would have been compared to those of 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 convergence value as low as 35 degrees, compared to that of a typical predator, such as a cat, at around 65 degrees.
However, good stereoscopic vision is also based on the degree of frontation, which is a measure of how well the eyeballs are situated within the orbits. «Thylacosmilus he was able to compensate for having his eyes on the side of his head by protruding their sockets a bit and orienting them almost vertically, to increase visual field overlap as much as possible,” said co-author Analia M. Forasiepi, also at INAGLIA and a researcher at CONICET, the argentine science and research agency. “Although its orbits were not favorably positioned for 3D vision, it could achieve around 70 percent visual field overlap, evidently enough to make it a successful active predator.”
“Compensation seems to be the key to understanding how the skull of Thylacosmilussaid study co-author Ross DE MacPhee, senior curator at the American Museum of Natural History. “Indeed, the growth pattern of 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 strange orientation of the orbits in Thylacosmilus it actually represents a morphological compromise between the primary function of the skull, which is to support and protect the brain and sensory organs, and a collateral function unique to this species, which was to provide enough space for the development of the enormous canines.”
“Looking for neat adaptive explanations in evolutionary biology is fun but largely pointless. One thing is clear: Thylacosmilus it was not a freak of nature, but in its time and place it managed, apparently admirably, to survive as an ambush predator. We can see it as an anomaly because it doesn’t fit into our preconceived categories of what a proper mammalian carnivore should look like, but evolution makes its own rules,” added Dr Forasiepi.
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