Thylacoleo: Herbivore or Carnivore?

[Author’s Note: Yesterday I wrote about a new study that casts the thylacine – Australia’s extinct “Tasmanian tiger” – as an ambush hunter rather than a pursuit predator. The image change reminded me of a 19th century debate about the dining habits of an even older marsupial carnivore – Thylacoleo carnifex. What follows is a revised version of a post I wrote in 2009 about that argument between anatomists Richard Owen and William Henry Flower.]

Without a doubt, the extinct marsupial predator Thylacoleo was one of the strangest carnivorous mammals to have ever evolved. Instead of piercing the hide of its prey with large canines, Thylacoleo 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 the fossil mammal as a carnivore – “one of the fellest and most destructive of predatory beasts” – and on the basis of a partial skull he proposed that Thylacoleo was closely related to living marsupial carnivores like quolls and the Tasmanian devil. When more of the animal’s 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 wombats and living kangaroos (among many others). A carnivorous mammal had somehow been derived from herbivorous ancestors. (Owen rejected evolution by natural selection, but he was still an evolutionist. I have not been able to find any clues as to whether he gave special consideration to the origins of Thylacoleo, though.)

A restoration of the skull of Thylacoleo. From The Ancient Life History of the Earth.

William Henry Flower, another skilled English anatomist, disagreed. In 1868 he read a paper before the Geological Society on the “probable habits of the extinct Australian marsupial” that criticized Owen’s reconstruction. Flower did not dive into discussion of Thylacoleo right away, though. A few years earlier the paleontologist Hugh Falconer had described the lower jaw of a fossil animal he called Plagiaulax. 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 thought differently. To him, Plagiaulax was a carnivorous marsupial, and together Plagiaulax and Thylacoleo would seem to represent a previously unknown bauplan for meat-eating marsupials. Flower criticized Owen on this point. He upheld Falconer’s original determination that Plagiaulax was an herbivore, and the resemblance of this animal’s jaw to that of Thylacoleo therefore cast doubt on Owen’s interpretation. (Both naturalists were wrong, as it turned out. Plagiaulax is now known to belong to an entirely extinct group of archaic mammals called the multituberculates.)

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 the fact that Thylacoleo was a larger animal, and, in all, it seemed that Thylacoleo was a super-sized version of a rat-kangaroo or a phalanger.

The skull of a rat-kangaroo. From The Quarterly Journal of the Geological Society of London.

Flower’s reasoning was sound. The jaws of Thylacoleo most closely resembled those of herbivorous marsupials – not other carnivores – and the anatomical similarity of its skull to that of plant-eaters like wombats made it seem unlikely that Thylacoleo was slicing into carcasses. The simplest explanation, based upon anatomy and family relationships, was that Thylacoleo was an herbivore. But what was it eating? This was more difficult to determine. Flower explained:

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.

Restoration of the skull of Thylacoleo. From The Ancient Life History of the Earth.

Owen was not convinced. He 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. Thylacoleo could slice and dice, but, unlike other herbivores, the mammal could not have pulverize the vegetation it sheared off.

These features led paleontologist Robert Broom to revive the killer wombat hypothesis in 1898, but his considerations were not unanimously welcomed. In a communication to the American Naturalist the American paleontologist H.F. Osborn called Broom’s hypothesis “quite unjustifiable”, while 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 was extracted from Australia’s Pleistocene cave deposits, however, there could be little doubt that the beast 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 reconstructed bite force stronger (for its size) than any other terrestrial meat-eating mammal. Studies of the chemical composition of Thylacoleo teeth have added supplementary evidence to what has been determined on the basis of anatomy. Thylacoleo 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 was wrong, but for the right reasons.

Top Image: A reconstructed skeleton of Thylacoleo in Australia’s Victoria Fossil Cave. Image from Wikipedia.

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