A reconstruction of Thylacosmilus atrox. Credit: © Jorge Blanco
How the “Marsupial Sabertooth” Thylacosmilus Saw Its World
Study explains how extinct hypercarnivore likely achieved 3D vision regardless of wide-set eyes more characteristic of an herbivore than a predator.
The skulls of predators usually have forward-facing eye sockets, or orbits, which helps enable stereoscopic (3D) vision, an useful adjustment for evaluating the position of prey before striking. Researchers from the American Museum of Natural History and the Instituto Argentino de Nivología, Glaciología, y Ciencias Ambientales in Mendoza, Argentina, studied whether the “marsupial sabertooth” Thylacosmilus atrox could see in 3D at all.
Widely understood as the “marsupial (or metatherian) sabertooth” because its extremely large upper dogs recall those of the more popular placental sabertooth that progressed in North America, Thylacosmilus resided in South America up until its extinction about 3 million years back. It was a member of Sparassodonta, a group of extremely meat-eating mammals connected to living marsupials.
Although sparassodont types differed significantly in size– Thylacosmilus might have weighed as much as 100 kilograms (220 pounds)– the terrific bulk looked like placental predators like cats and canines in having forward-facing eyes and, probably, full 3D vision. By contrast, the orbits of Thylacosmilus, a supposed hypercarnivore– an animal with a diet plan approximated to consist of a minimum of 70 percent meat– 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 progress such a strange adjustment? A group of researchers from Argentina and the United States set out to try to find an explanation.
A reconstruction of the skull of Thylacosmilus atrox. Credit: © Jorge Blanco
” You cant comprehend cranial organization in Thylacosmilus without first confronting those massive canines,” stated lead author Charlène Gaillard, a Ph.D. trainee in the Instituto Argentino de Nivología, Glaciología, y Ciencias Ambientales (INAGLIA). “They werent simply big; they were ever-growing, to such an extent that the roots of the canines continued over the tops of their skulls. This had effects, one of which was that no room was offered for the orbits in the usual predator position on the front of the face.”
Gaillard utilized CT scanning and 3D virtual reconstructions to assess orbital organization in a number of fossil and contemporary mammals. She was able to identify how the visual system of Thylacosmilus would have compared to those in other carnivores or other mammals in basic. Although low orbital merging occurs in some modern-day carnivores, Thylacosmilus 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, excellent stereoscopic vision also counts on the degree of frontation, which is a procedure of how the eyeballs are positioned within the orbits. “Thylacosmilus had the ability to make up for having its eyes on the side of its head by sticking its orbits out somewhat and orienting them nearly vertically, to increase visual field overlap as much as possible,” stated co-author Analia M. Forasiepi, likewise in INAGLIA and a scientist in CONICET, the Argentinian science and research firm. “Even though its orbits were not positively positioned for 3D vision, it could attain about 70 percent of visual field overlap– obviously, enough to make it an effective active predator.”
” Compensation appears to be the key to understanding how the skull of Thylacosmilus was put together,” stated research study co-author Ross D. E. MacPhee, a senior curator at the American Museum of Natural History. “In result, the growth pattern of the dogs throughout early cranial advancement 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 in fact represents a morphological compromise between the primary function of the cranium, which is to protect the brain and hold and sense organs, and a collateral function unique to this species, which was to provide sufficient space for the advancement of the massive canines.”
Lateral displacement of the orbits was not the only cranial adjustment that Thylacosmilus established to accommodate its canines while keeping other functions. Thylacosmilus did the very same thing– another example of convergence amongst unrelated types.
This leaves a last concern: What purpose would have been served by establishing big, ever-growing teeth that needed re-engineering of the entire skull?
” It might have made predation simpler in some unknown method,” said Gaillard, “But, if so, why didnt any other sparassodont– or for that matter, any other mammalian predator– develop the very same adjustment convergently? The canines of Thylacosmilus did not use down, like the incisors of rodents. Rather, they simply appear to have continued growing at the root, eventually extending almost to the rear of the skull.”
Forasiepi highlighted this point, stating, “To look for precise adaptive descriptions in evolutionary biology is fun but mostly futile. Something is clear: Thylacosmilus was not a freak of nature, but in its time and place it managed, obviously rather admirably, to endure as an ambush predator. We may see it as an abnormality because it doesnt fit within our preconceived classifications of what a correct mammalian carnivore must appear like, but development makes its own rules.”
Referral: “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.
By contrast, the orbits of Thylacosmilus, an expected hypercarnivore– an animal with a diet approximated to consist of at least 70 percent meat– were positioned like those of an ungulate, with orbits that face mainly laterally. “Thylacosmilus was able to compensate for having its eyes on the side of its head by sticking its orbits out rather and orienting them almost vertically, to increase visual field overlap as much as possible,” stated co-author Analia M. Forasiepi, also in INAGLIA and a scientist in CONICET, the Argentinian science and research study firm.” Compensation appears to be the key to understanding how the skull of Thylacosmilus was put together,” stated study co-author Ross D. E. MacPhee, a senior manager at the American Museum of Natural History. The odd orientation of the orbits in Thylacosmilus really represents a morphological compromise in between the main function of the cranium, which is to safeguard the brain and hold and sense organs, and a security function unique to this types, which was to provide adequate room for the advancement of the huge canines.”
Lateral displacement of the orbits was not the only cranial adjustment that Thylacosmilus established to accommodate its canines while retaining other functions.