Thylacosmilus Was Not a Saber-Tooth Predator

Tuesday, June 30, 2020

Skulls and life reconstructions of Thylacosmilus atrox (left) and Smilodon fatalis (right). Image credit: Stephan Lautenschlager.

Thylacosmilus atrox, an extinct marsupial that roamed South America between 9 and 3 million years ago (Neogene period), was not the ecological analogue of saber-tooth cats, and likely did not use its impressive canines to dispatch its prey, according to new research led by University of Bristol scientists.

Thylacosmilus atrox, also known as the ‘marsupial saber-tooth,’ is often presented as a classic case of convergent evolution, where animals appear similar in form despite having very different evolutionary relationships.

The ancient creature had huge, ever-growing canines, leading paleontologists to speculate that it was an even more vicious predator than the placental saber-tooths such as the North American species Smilodon fatalis.

The jaguar-sized Thylacosmilus atrox was around half the size (average body mass 117 kg) of Smilodon fatalis, which was lion-sized or larger (245 kg).

But was it really a fierce predator like the extinct saber-tooth cats, which seem to have been much like modern cats but with a different mode of killing their prey?

Thylacosmilus atrox has impressive canines, for sure: but if you look at the whole picture of its anatomy, lots of things simply don’t add up,” said University of Bristol’s Professor Christine Janis, first author of the study.

“For example, it just about lacks incisors, which big cats today use to get meat off the bone, and its lower jaws were not fused together.”

“In addition, the canines of Thylacosmilus atrox were different from the teeth of other saber-toothed mammals, being triangular in shape like a claw rather than flat like a blade.”

In the study, Professor Janis and colleagues compared aspects of the skull and teeth of Thylacosmilus atrox with both present-day big cats and a diversity of extinct saber-toothed cats.

“The skull superficially looks rather like that of a saber-toothed placental. But if you actually quantify things, it becomes clear that Thylacosmilus atrox’s skull was different in many details from any known carnivorous mammal, past or present,” said co-author Dr. Borja Figueirido, a researcher at the University of Málaga.

“Previous studies by other researchers have shown Thylacosmilus atrox to have had a weaker bite than Smilodon fatalis,” said University of Birmingham’s Dr. Stephan Lautenschlager, senior author of the study.

“But what we can show is there was probably a difference in behavior between the two species: Thylacosmilus atrox’s skull and canines are weaker in a stabbing action than those of Smilodon fatalis, but are stronger in a ‘pull-back’ type of action.”

“This suggests that Thylacosmilus atrox was not using its canines to kill with, but perhaps instead to open carcasses.”

Finally, the other teeth of Thylacosmilus atrox also pose problems for the interpretation of this animal as a cat-like predator, whether saber-toothed or not.

Besides the puzzling lack of incisors, the molars are small, and did not wear down along the sides as seen in an animal feeding on meat.

“The molars tend to wear flat from the top, rather like you see in a bone crusher. But if you examine the detailed microwear on tooth surfaces, it’s clear that it was eating soft food. Its wear is most similar to that of cheetahs which eat from fresh carcasses and suggests an even softer diet than fed to captive lions,” said co-author Dr. Larisa DeSantis, a researcher at Vanderbilt University.

Thylacosmilus atrox was not a bone-crusher and may have instead specialized on internal organs.”

“It’s a bit of a mystery as to what this animal was actually doing but it’s clear that it wasn’t just a marsupial version of a saber-toothed cat like Smilodon fatalis,” Professor Janis said.

“In addition to the differences in the skull and the teeth, it was also short-legged and stiff-backed, and lacked retractile claws, so it would have had difficulties in pursuing its prey, pouncing on it and holding on to it. I suspect it was some sort of specialized scavenger.”

“It may have employed those canines to open carcasses and perhaps also used a big tongue to help extract the innards: other mammals that have lost the incisors, like walruses and anteaters, also have big tongues that they use in feeding.”

The study was published in the journal PeerJ.


C. M. Janis et al. 2020. An eye for a tooth: Thylacosmilus was not a marsupial ‘saber-tooth predator.’ PeerJ 8: e9346; doi: 10.7717/peerj.9346

This article is based on text provided by the University of Bristol.