Wakaleo

Wakaleo, Wakaleo vanderleuri was a dog-sized thylacoleonid ('marsupial lion') and one of the largest predators in Australia during the Miocene. Although larger and more powerfully built than living marsupial carnivores like the Tasmanian Devil, Wakaleo was much smaller than its Pleistocene relative, the formidable Thylacoleo. Like other thylacoleonids, Wakaleo had teeth that were modified for stabbing and cutting.

Identification

Waklaleo vanderleuri was a dog-sized, robust thylacoleonid. The skull of Wakaleo is identified as thylacoleonid by resemblance to the skull of Thylacoleo. Shared features include similarities in the ear region, palate, base of the skull and dentition. Thylacoleonid molars are triangular, or tribosphenic, with two large upper and lower stabbing incisors and an enlarged third premolar (P3) adapted for slicing flesh.

Wakaleo was more primitive than Thylacoleo in the size and proportions of the skull (Wakaleo having a longer, narrower skull), the relationships of component skull bones and sutures, proportions and structure of the ear region, and tooth size and development. Wakaleo lacks a postorbital bar around the orbit (eye socket), a distinctive feature of the skull of Thylacoleo. The dental formula differs in that Wakaleo has an upper canine (but no lower canine), the first premolar is lost (an advanced character not seen in Thylacoleo), there is a large second premolar on upper and lower jaws, the large cutting tooth (P3) is not as enlarged as it is in Thylacoleo, and there are two molars in both jaws (for a dental formula of I1-3/1;C1/0; P2-3/2-3; M2-4/2-4). Molar reduction is extreme in Thylacoleo, which has retained the first premolar lost in Wakaleo, and canines are exceptionally reduced (the enlarged incisors taking the role of stabbing teeth).

These differences are significant, and Wakaleo and Thylacoleo have been put into separate subfamilies (Wakaleoninae and Thylacoleoninae respectively). In some features (for instance, sutural relations between component bones like the squamosal and aliosphenoid), Thylacoleo is closer to vombatoids (wombats and koalas), while Wakaleo is more like phalangeroids (possums).

Habitat

Riversleigh during the early to middle Miocene was mainly forested, with more open areas near the forest edges and freshwater streams or lakes in a karst (limestone) environment. Bullock Creek, the most northerly Cainozoic vertebrate fossil site on the Australian continent, was an area of freshwater streams and lakes during the middle to late Miocene. The climate was marked by high temperatures and strong seasonality, with long periods of low rainfall (one of the earliest fossil sites to show increasing aridity in Australia).

Distribution

Wakaleo vanderleuri is known from two sites: Bullock Creek, southeast of Camfield Homestead in north-central Northern Territory; and the Riversleigh World Heritage Fossil Site in northwestern Queensland.

Feeding and diet

As a mid-sized marsupial carnivore from the Miocene, Wakaleo vanderleuri probably ate small vertebrates like possums, birds and perhaps reptiles. Its bite force (BFQ) was substantially less than in either the smaller Priscileo or much larger Thylacoleo, suggesting that Wakaleo did not take prey that were especially large in relation to its size.

Life history cycle

As a marsupial (regardless of whether its ancestor was a possum or vombatiform), Wakaleo would have borne tiny, undeveloped young that developed further in a pouch. It probably hunted during the evening, night or early mornings, as marsupial carnivores do today, although the fossil record gives us no clues about such behaviour. The skeleton of Wakaleo is unknown, and there are likewise no clues as to whether it might have been able to climb trees in search of prey or if it was limited to hunting on the ground.

Fossils description

Wakaleo vanderleuri, first described in 1974 by Clemens and Plane (1974), is known only from skulls and lower jaws. Fossil material includes a complete skull and lower jaw material from Bullock Creek, Northern Territory, and cranial material from Riversleigh, Queensland. Of the two other species of Wakaleo, W. oldfieldi is known from a complete lower jaw and Wakaleo alcootaensis from a fragmentary maxilla.

Evolutionary relationships

There is much debate over which marsupial group gave rise to thylacoleonids. Candidates include phalangeroids (brushtail possums), vombatiforms (wombats, koalas and their extinct relatives) or possibly an unknown group of stem diprotodontians. Phalangeroid characters seen in thylacoleonids include the structure of the base of the skull, neurocranium and palate. Vombatiform characters in Thylacoleo include shared structures of the side wall of the braincase in the region of the alisphenoid and squamosal (from Murray et al. 1987).

Wakaleo does not appear to have any of these wombat-like characters, unlike Thylacoleo, and does not seem to have any relationship to vombatiform marsupials. It may be that the genus Wakaleo (although not W. vanderleuri) evolved from a phalangeroid ancestor and is ancestral to Thylacoleo, and that Thylacoleo's vombatid-like skull features evolved independently from true vombatiforms. Alternatively, vombatid-like features might be found in the postcranial skeleton of Wakaleo if discovered.

Wakaleo is related to Thylacoleo at the family level, with many shared features in common, it is distinct at the level of subfamily and does not appear to have been a direct ancestor of Thylacoleo. It may instead represent a separate lineage of thylacoleonids evolving independently since at least the Miocene (the view of Murray et al. 1987).

References

• Archer, M. 1984. The Australian marsupial radiation. Pp. 634-808 in Archer, M. and G. Clayton, G. (eds) Vertebrate Zoogeography and Evolution in Australasia. Hesperion Press, Perth.
• Clemens, W. A. and Plane, M. 1974. Mid-Tertiary Thylacoleonidae (Marsupialia, Mammalia). Journal of Paleontology 48, 652-660.
• Megirian, D. 1986. The dentary of Wakaleo vanderleuri (Thylacoleonidae, Marsupialia). The Beagle, Occasional Papers of the Northern Territory Museum.
• Murray, P., Wells, R. and Plane, M. 1987. The cranium of the Miocene thylacoleonid, Wakaleo vanderleuri: Click go the shears - a fresh bite at thylacoleonid systematics. Pp. 433-466 in Archer, M. (ed) Possums and Opossums: Studies in Evolution, Vol. 2. Surrey Beatty & Sons Pty Limited in association with The Royal Zoological Society of New South Wales, Chipping Norton.
• Wroe, S., McHenry, C. and Thompson, 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 doi:10.1098/rspb.2004.2986.

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.