A terrifying saber-toothed marsupial had such huge fangs its eyes were at the side of its head – like a horse or cow, according to a recent research in Communications Biology.
Its eye roots went right back to its braincase – producing unique vision for catching prey by pushing its peepers out wide.
About the size of a jaguar, the pouched marsupial beast would have been a deadly hunter – capable of seeing nearly 360 degrees without moving. This killing machine Thylacosmilus atrox sheds fresh light on the evolution of meat-eaters, says scientists.
Lead author Charlene Gaillard, a Ph.D. student at the Argentine Institute of Nivology, Glaciology and Environmental Sciences (INAGLIA), said: “You can’t understand cranial organization in Thylacosmilus without first confronting those enormous canines”.
“They weren’t just large, they were ever-growing, to such an extent the roots 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.”
Grazing animals need to check all around so they can flee rapidly in case of attack but this is the first time the phenomenon has been identified in a carnivore.
Thylacosmilus also had wide-set eyes more typical of a herbivore.
It was more closely related to kangaroos and koalas than Smilodon – the famous saber-toothed cat that died out at the end of the last Ice Age.
Co-author Professor Ross MacPhee, of the American Museum of Natural History, New York, said: “Compensation appears to be the key to understanding how the skull of Thylacosmilus was put together.
“In effect, the growth pattern of the 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 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.”
The international team used CT scans and 3D virtual reconstructions to assess orbital organization in a number of fossil and modern mammals.
Skulls of carnivores normally have forward-facing eye sockets, or orbits, to enable 3D or “stereoscopic” vision.
The adaptation is important for judging the position of prey before pouncing.
Thylacosmilus was a member of a group known as sparassodonts – highly carnivorous mammals related to living marsupials.
Most resembled placental carnivores like cats and dogs in having forward-facing eyes, but those of Thylacosmilus – an animal with a mainly meat-based diet – were positioned like those of an ungulate, facing mostly laterally.
In this situation, visual fields do not overlap sufficiently for the brain to integrate them in 3D.
Co-author Dr. Analia Forasiepi, also from INAGLIA, said: “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.
“Even though its orbits were not favorably positioned for 3D vision, it could achieve about 70 percent of visual field overlap—evidently, enough to make it a successful active predator.”
Lateral displacement of the orbits was not the only modification Thylacosmilus developed to accommodate its canines while retaining other functions.
Placing the eyes on the side of the skull brings them close to the temporal chewing muscles, which might result in deformation during eating.
To control for this, some mammals, including primates, have developed a bony structure that closes off the eye sockets from the side. Thylacosmilus did the same thing.
Ms. Gaillard said: “It might have made predation easier in some unknown way, but if so, why didn’t any other sparassodont or for that matter, any other mammalian carnivore develop the same adaptation convergently?
“The canines of Thylacosmilus did not wear down, like the incisors of rodents. Instead, they just seem to have continued growing at the root, eventually extending almost to the rear of the skull.”
For its size, Thylacosmilus’ teeth were larger than those of any other known sabretooth – going back to within millimeters of its very small brain case.
Dr. Forasiepi added : “To look for clear-cut adaptive explanations in evolutionary biology is fun but largely futile.
“One thing is clear, thylacosmilus was not a freak of nature, but in its time and place it managed, apparently quite admirably, to survive as an ambush predator.
“We may view it as an anomaly because it doesn’t fit within our preconceived categories of what a proper mammalian carnivore should look like, but evolution makes its own rules.”
Produced in association with SWNS Talker
Edited by Deborah .C. Amirize and Joseph Hammond
