Thylacoleo: Herbivore or Carnivore?

Without a doubt, the extinct marsupial predator Thylacoleo was one of the strangest carnivorous mammals ever to have evolved. This predator from ancient Australia did not have piercing canines but instead bit into prey with large, forward facing incisors, and it sheared flesh from its kills with huge, cleaver-like premolars. Even though it evolved from herbivorous ancestors, we now know that Thylacoleo was most certainly a carnivore.

The preferred diet of Thylacoleo has not always been so clear-cut, however. In 1859 the famed anatomist Richard Owen identified it as a carnivore, "one of the fellest and most destructive of predatory beasts", and he thought it was related to living marsupial carnivores like quolls and the Tasmanian devil. When more of the skull was found, however, Owen changed his mind about its relationships. The incisor teeth linked it more closely to diprotodonts, a group of herbivorous marsupials including giant wombat like creatures and living kangaroos (among many others), so somehow a carnivorous mammal had been derived from herbivorous ancestors.*

*[I am not sure if Owen made this connection or tried to square it with his own thoughts on evolutionary theory. I will look into the question, but I am not sure if I will turn up anything.]

William Henry Flower, another London anatomist, disagreed. In 1868 he read a paper before the Geological Society on the "probable habits of the extinct Australian marsupial", but he did not dive into discussion of Thylacoleo right away. A few years earlier the paleontologist Hugh Falconer had described the lower jaw of a fossil animal he called Plagiaulax. (This mammal is now known to belong to an extinct group called the multituberculates.) The jaw looked superficially similar to the lower jaw of Thylacoleo, but Falconer thought it belonged to a small, herbivorous marsupial like the living rat-kangaroos.

Owen, by contrast, thought that Plagiaulax was a carnivorous marsupial, perhaps based upon conclusions he had previously come to about Thylacoleo. Together Thylacoleo and Plagiaulax would represent a previously unknown form for mammalian carnivores. Flower sided with Falconer, though, and if the jaw of Plagiaulax belonged to an herbivore, was it possible that Owen had misidentified the habits of Thylacoleo, too?

Comparative anatomy held the key. While Owen made the case that the premolars of Thylacoleo had the same function as the more pointed carnissal shearing teeth of carnivores, Flower thought that the incisors and premolars of Thylacoleo more closely resembled those of rat-kangaroos. Other features, like the large areas of the skull used for jaw muscle attachment, could be chalked up to differences in comparing large animals to small ones, and all in all it seemed that Thylacoleo was a super-sized version of a rat-kangaroo or a phalanger.

Flower's reasoning was sound. In surveying marsupials and carnivorous mammals the jaws of Thylacoleo most closely resembled those of herbivores, and it seemed to be more closely related to herbivorous marsupials than carnivorous ones. The simplest explanation based upon anatomy and family relationships was that Thylacoleo was an herbivore, too. But what was it eating? This was more difficult to determine;

What was the particular form of food associated with the most singular dentition of Thylacoleo, it would be hazardous to do more than conjecture. As the flora of the country in which this strange animal existed has probably undergone as great a change as the fauna, it is not unlikely that the material upon which it subsisted has passed away with the creature itself. It may have been some kind of root or bulb ; it may have been fruit ; it may have been flesh. But the hypothesis that Thylacoleo was the destroyer of the gigantic herbivorous marsupials (many times as large as itself) with which its remains are found associated, the Diprotodons and the Nototheres, appears to me to require more proof than has yet been adduced in its favour.

Owen was not convinced and later fired back a defense of his views, but other naturalists were sympathetic to Flower's hypothesis. It was a stretch to turn Thylacoleo into a terrifying carnivore when its skull was so similar to those of marsupial herbivores, but there was one important factor that had not been given due attention. Thylacoleo lacked grinding teeth to chew up the vegetation it supposedly ate. Its teeth looked better adapted to piercing, stabbing, and shearing; it could slice and dice, but not properly pulverize the plants it consumed like other herbivores.

Thus the notion that Thylacoleo was a carnivore was revived by paleontologist Robert Broom in 1898, but his considerations were not unanimously well-received. In a communication to the American Naturalist the American paleontologist H.F. Osborn called Broom's hypothesis "quite unjustifiable", but the zoologist B.A. Bensley agreed with Broom's conclusions. The case was still open.

Most of these considerations were based upon studies of the skull. When more of Thylacoleo came to light, however, there could be little doubt that it was a predator. A bit larger than a leopard, but smaller than the average lion, Thylacoleo had long arms, semi-opposable thumbs, retractable claws, and a bite force stronger (for its size) than any other terrestrial meat-eating mammal. Isotope tests of its teeth, as well, have confirmed that it ate meat, and it certainly was as fearsome a predator as Owen surmised.

Ultimately Flower's hypothesis was rejected, but this did not mean his science was bad. He compared what little evidence was available at the time to other creatures and urged caution in trying to reconstruct the life of Thylacoleo on the basis of teeth alone. Flower's views may not longer make it into textbooks, but his exchange with Owen is a good example of how science operates.