Thylacoleo carnifex

Thylacoleo carnifex, the largest carnivorous Australian mammal known, may have hunted other Pleistocene megafauna like the giant Diprotodon. Thylacoleo was one of the first fossil mammals described from Australia, discovered not long after European settlement. It may have been an ambush predator or scavenger, and had enormous slicing cheek teeth, large stabbing incisor teeth (replacements for the canine teeth of other carnivorous mammals) and a huge thumb claw that may have been used to disembowel its prey.

Identification

Thylacoleo carnifex is the last and largest member of the Thylacoleonidae ('marsupial lions'). Distinguishing features of thylacoleonids include enlarged cheek teeth (the third premolars) that formed long shearing blades. These were developed at the expense of posterior molar teeth, which were either reduced in size or absent.

Thylacoleo carnifex had a wide, heavy, short-snouted skull with a bony bar (postorbital bar), as in primates, behind the orbits. It has the longest shearing tooth of the thylacoleonids as well as large, serrated, canine-like upper incisors and horizontally oriented lower incisors (replacements for the canine teeth of other carnivorous mammals). Thylacoleo also had an enlarged thumb claw encased in a sheath (as in cats) that may have been used to disembowel its prey.

Limb proportions of Thylacoleo suggest that it was cursorial (adapted for running) but not swift. Forelimb proportions are similar to those of some arboreal/saltatorial animals. Its clawed forelimb may have been used to reach out and bring food towards its mouth. Its pseudo-opposable thumb suggests that Thylacoleo may have also been at least partly scansorial (adapted for climbing). Western Australian and Tasmanian specimens are smaller than eastern Australian specimens, and sexual dimorphism (where one sex is larger than the other) is reported. The weight of Thylacoleo is estimated to have ranged from 90-160 kilograms.

Habitat

Most of the sites where Thylacoleo fossils have been found are interpreted as dry, open forest habitat (e.g., the Darling Downs and the Wellington, Naracoorte and Nullarbor caves

Distribution

Thylacoleo carnifex was widely distributed across Australia during the Pleistocene. It has been found in all Australian states as well as the Northern Territory, including the Darling Downs (Queensland), Wellington Caves (New South Wales), Naracoorte Caves (South Australia) and Thylacoleo Cave on the Nullarbor Plain (Western Australia).

Feeding and diet

The diet of Thylacoleo has been the subject of much debate. Thylacoleo has been described as a carnivore, a bone crusher, a scavenger or perhaps even an herbivore. It was first described by Sir Richard Owen as 'one of the fellest and most destructive of predatory beasts' (Owen 1859). Its unusual teeth and herbivorous ancestry, however, prompted further suggestions, including crocodile eggs, carrion, meat and bone marrow. The suggested diet raising the most eyebrows, however, is that of cycad nuts or native cucumbers (dubbed the 'melon-muncher' hypothesis). Palaeontologist Robert Broom had earlier pointed out that since Thylacoleo had no grinding teeth to process plant matter, it could not have included much plant food in its diet.

Most scientists today believe that Thylacoleo was carnivorous. Its teeth seem clearly designed for slicing flesh, and its powerful forearms and large, retractable thumb claws suggest a fierce, powerful predator. A study of the skull and jaw found that Thylacoleo had a head shape typical of carnivores, similar to the skulls of other carnivores except for the reduced canines, the use of incisors as stabbing teeth, and co-opting of a premolar rather than molar as a 'carnassial' tooth (a tooth specialized for carnivory). Further study on its 'bite strength' finds that Thylacoleo had the most powerful bite of any mammalian predator, living or extinct, and that it could have taken prey much larger than itself (such as subadult Diprotodon).

Life history cycle

Little has been found in the fossil record to give us direct information about the lifestyle of Thylacoleo. However, three individuals from Moree, NSW shed some light on the raising of young. An adult female adult with a very young baby ('pouch-young') and a second, older juvenile ('young-at-foot') were found in association, almost certainly a family group. The adult is represented by a complete skeleton, the baby by an incomplete lower jaw and the juvenile by a skull, all held in the fossil collection of the Australian Museum.

Fossils description

For about 100 years after its discovery, Thylacoleo carnifex was known only from fragmentary remains (teeth, partial skulls and jaws, and some postcranial fossils). The first complete skull of Thylacoleo was described in 1956, and the first near-complete skeleton (lacking a foot and tail) was found at Moree, NSW in 1966 (the mother with young described above). Much material has been recovered from Naracoorte Caves in South Australia, and several complete individuals have been discovered in Thylacoleo Cave on the Nullarbor Plain, Western Australia.

Evolutionary relationships

Most palaeontologists think that the ancestors of thylacoleonids were herbivores, an unusual occurrence since most carnivores evolved from other carnivorous lineages. One proposal suggests that thylacoleonids evolved from a possum ancestor (Phalangeroidea) based on dental formula, the skull of the cuscus Phalanger, and on a phalangerid-like musculature. Alternatively, evidence from certain skull features may show that thylacoleonids branched off the vombatiform line, the lineage that includes wombats and koalas.

References

• Finch,M. E. 1982. The discovery and interpretation of Thylacoleo carnifex (Thylacoleonidae, Marsupialia). pp. 537-551 in Archer, M. (ed) Carnivorous Marsupials, Vol. 2. Royal Zoological Society of New South Wales, Mosman.
• Finch, M. E. and Freedman, L. 1982. An odontometric study of the species of Thylacoleo (Thylacoleonidae, Marsupialia). pp. 553-572 in Archer, M. (ed) Carnivorous Marsupials, Vol. 2. Royal Zoological Society of New South Wales, Mosman.
• Owen, R. 1858. Odontology. Teeth of mammals. Encyclopedia Britannica (8th Edition), Vol. 16, p. 447.
• Owen, R. 1859. On the fossil mammals of Australia. Part II. Description of a mutilated skull of the large marsupial carnivore (Thylacoleo carnifex Owen), from a calcareous conglomerate stratum, eighty miles S. W. of Melbourne, Victoria. Philosophical Transactions of the Royal Society 149, 309-322.
• Owen, R. 1871. On the fossil mammals of Australia. Part IV. Dentition and mandible of Thylacoleo carnifex, with remarks on arguments for its herbivory. Philosophical Transactions of the Royal Society 161, 213-266.
• Pledge, N. 1977. A new species of Thylacoleo (Marsupialia, Thylacoleonidae), with notes on the occurrence and distribution of Thylacoleonidae in South Australia. Records of the South Australian Museum 17, 277-283.
• Prideaux, G. J., Long, J. A., Ayliffe, L. K., Hellstrom, J. C., Pillans, B., Boles, W. E., Hutchinson, M. N., Roberts, R. G., Cupper, M. L., Arnold, L. J., Devine, P. D. and Warburton, N. M. 2007. An arid-adapted middle Pleistocene fauna from south-central Australia. Nature 445, 422-425.
• Wells, R. T. and Nichol, B. 1977. On the manus and pes of Thylacoleo carnifex Owen (Marsupialia), Transactions of the Royal Society of South Australia 101,139-146.
• Wells, R., Horton, D. R. and Rogers, P. 1982. Thylacoleo carnifex Owen (Thylacoleonidae): marsupial carnivore? pp. 573-585 in Archer, M. (ed) Carnivorous Marsupials, Vol. 2. Royal Zoological Society of New South Wales, Mosman.
• Woods, J. T. 1956. The skull of Thylacoleo carnifex. Memoirs of the Queensland Museum 13, 125-140.
• Wroe, S., Myers, T. J., Seebacher, F., Kear, B., Gillespie, A., Crowther, M. and Salisbury, S. 2003. An alternative method for predicting body mass: the case of the Pleistocene marsupial lion. Paleobiology 29, 404-412.
• Wroe, S., McHenry, C. and Thomason, J. 2005. Bite club: Comparative bite force in big biting mammals and the prediction of predatory behaviour in fossil taxa. Proceedings of the Royal Society B (online edition), 1-7.

Further reading

• Long, J. A. et al. 2002. Prehistoric Mammals of Australia and New Guinea: One Hundred Million Years of Evolution. Johns Hopkins University Press, Baltimore, 240 pp.