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A recent study suggests that an animal that used to live long ago and ate mainly meat, known as a ‘hypercarnivore’, had special abilities that allowed it to see in 3D even though its eyes were spaced wide apart like an herbivore’s.
Scientists wanted to find out why a long-extinct marsupial, “Thylacosmilus atrox,” had giant teeth extending across the top of its skull but oddly placed eyes – like cows or horses. Usually for predators like these, their eyes face forward in order to help them track and catch prey. The scientists studied whether the animals could still see in 3D with these eye placements, and their results were published online today (March 21).
Thylacosmilus was an ancient animal, often referred to as the “marsupial sabertooth”, who lived about 3 million years ago in South America. It belonged to a group of mammals that were all very meat-eating, and is related to animals like kangaroos who you may be familiar with today. What is special about Thylacosmilus is its huge upper teeth that reminded people of the better-known sabertooth who lived in North America long ago.
Animals called sparassodonts varied in size, with some as big as 100 kilograms (220 pounds). They looked a lot like other cats and dogs that hunt animals. But the one called Thylacosmilus had its eyes positioned like an ungulate’s so it couldn’t see in three dimensions. This means that their brain was not able to process the sight enough to be able to tell what was near and far.
A group of scientists from Argentina and the US wanted to figure out why a hypercarnivore had evolved such a peculiar behaviour. So, they decided to investigate.
Charlène Gaillard, a scientist from the Instituto Argentino de Nivología, Glaciología, y Ciencias Ambientales (INAGLIA), believes that it is impossible to understand cranial organization in Thylacosmilus without looking closely at its exceptionally large teeth. These teeth were not just big but they grew continuously so much so that their roots extended over the top of its skull. This also caused problems as there was no space available for its eyes, which usually sit on the front of the face.
Gaillard used special scanning technology and 3D building models to figure out how the eyes of different mammals, both extinct and living, are organized. She looked into Thylacosmilus in particular and found that it was very different from other predators like cats – while cats typically have an orbital convergence value of around 65 degrees, Thylacosmilus had one as low as 35 degrees!
Thylacosmilus, a predator, had its eyes located on the sides of its head. Because of this, it pushed out the orbits (eye sockets) in order to increase how much it could see and overlap its vision. Even though it wasn’t an ideal setup for 3D vision, Thylacosmilus was able to get 70 percent of overlapping vision which made it very proficient in hunting.
Ross D. E. MacPhee, a curator from the American Museum of Natural History, said that figuring out how the skull of Thylacosmilus was built seems to depend on compensation. To put it simply, when the skull was growing and developing, its canines needed a lot of space which pushed and tilted the eye sockets away from their normal places in front of the face. This is why adult Thylacosmilus skulls look so strange and odd. In conclusion, these jaws had enormous canines requiring an unusual orientation to make room for them while protecting the brain and senses at the same time.
Thylacosmilus developed some changes on its skull to help fit its big canines in. Its eyes got moved to the sides of its head, which would make it hard for it to eat. To prevent this, some animals like primates made a special bony structure that protected their eyes from the side while they ate. Thylacosmilus also did this – even though it’s unrelated to these other animals – an example of how sometimes different species find similar solutions!
This brings us to the last question: Why would anyone create big, never-ending teeth that take a lot of reworking of the skull?
Gaillard asked why the canines of Thylacosmilus, which was a type of meat-eating animal, weren’t wearing down like other animals’ incisors. Instead they continued to grow and extended almost to the back of its skull – something no other carnivore had ever done!
Forasiepi said that Thylacosmilus was a powerful predator that managed to stay alive in its environment. This animal might seem strange because it doesn’t look like other mammal carnivores, but evolution operates without going by predetermined rules. It’s hard to know why things happen the way they do in nature, but we can still be impressed by how Thylacosmilus survives.
Reference: “Seeing through the eyes of the sabertooth Thylacosmilus atrox (Metatheria, Sparassodonta” by Charlène Gaillard, Ross D. E. MacPhee and Analía M. Forasiepi, 21 March 2023, Communications Biology.
DOI: 10.1038/s42003-023-04624-5
