Thylacoleo

Evolution

The ancestors of thylacoleonids are believed to have been herbivores, something unusual for carnivores. They are members of the Vombatiformes, an almost entirely herbivorous order of marsupials, the only extant representatives of which are koalas and wombats, as well as extinct members such as the diprotodontids and palorchestids. The group first appeared in the Late Oligocene. The earliest thylacoleonids like Microleo were small possum-like animals, with the group increasing in size during the Miocene, with representatives like the leopard-sized Wakaleo. The genus Thylacoleo first appeared during the Pliocene, and represented the only extant genus of the family from that time until the end of the Pleistocene. The youngest representative of Thylacoleo and the thylacoleonids, T. carnifex, is the largest known member of the family. The earliest thylacoleonids are thought to have been arboreal (tree dwelling) animals, while Thylacoleo is thought to be terrestrial with some climbing capabilities.

Description

T. carnifex was a large carnivore, comparable to living big cats in size. Measurements taken from a number of specimens show they averaged 101 to 130 kg (223 to 287 lb) in weight, although individuals as large as 124–160 kg (273–353 lb) might not have been uncommon, and the largest weight was of 128–164 kg (282–362 lb). This would make it comparable to female lions and female tigers in general size. Estimates of the size of T. carnifex based on dental remains are typically dubious, in contrast to estimates based on proximal limb bone circumference. It is speculated that the species may have had some form of striped pattern based on rock art found in Arnhem Land which may depict the species, although this is uncertain.

Skull

Like other thylacoleonids, Thylacoleo had blade-like third premolar teeth in the upper and lower jaws, that functioned as the carnassial teeth, with these teeth being present much further forwards in the jaw than in other mammals. Compared to earlier thyacoleonids, the third premolars were considerably enlarged. Thylacoleo also had a proportionally large pair of first incisors in the upper and lower jaws, which functioned analogously to other carnivores' canine teeth. They also had true canines but they served little purpose as they were stubby and not very sharp. Compared to earlier thylacoleonids, the number of molar teeth was reduced.

Pound for pound, T. carnifex had the strongest bite of any mammal species, living or extinct; a T. carnifex weighing 101 kg (223 lb) had a bite comparable to that of a 250 kg African lion, and research suggests that Thylacoleo could hunt and take prey much larger than itself. Larger animals that were likely prey include Diprotodon spp. and giant kangaroos. It seems improbable that Thylacoleo could achieve as high a bite force as a modern-day lion; however, this might have been possible when taking into consideration the size of its brain and skull. Carnivores usually have rather large brains when compared to herbivorous marsupials, which lessens the amount of bone that can be devoted to enhancing bite force. Thylacoleo however, is thought to have had substantially stronger muscle attachments and therefore a smaller brain. Some later studies questioned the ability of the canine teeth to deliver a killing bite.

Using 3D modeling based on X-ray computed tomography scans, marsupial lions were found to be unable to use the prolonged, suffocating bite typical of living big cats. They instead had an extremely efficient and unique bite; the incisors would have been used to stab at and pierce the flesh of their prey while the more specialised carnassials crushed the windpipe, severed the spinal cord, and lacerated the major blood vessels such as the carotid artery and jugular vein. Compared to an African lion which may take 15 minutes to kill a large catch, the marsupial lion could kill a large animal in less than a minute. The skull was so specialized for big game that it was very inefficient at catching smaller animals, which possibly contributed to its extinction.

Postcranium

Thylacoleo had highly mobile and powerful forelimbs used to grapple prey, with each manus having a single very large retractable hooked claw set on large semi-opposable thumbs, which are suggested to have been used deal a killing blow.

The hind feet had four functional toes, the first digit being much reduced in size, but possessing a roughened pad similar to that of possums, which may have assisted with climbing. The discovery in 2005 of a specimen which included complete hind feet provided evidence that the marsupial lion exhibited syndactyly (fused second and third toes) like other diprotodonts.

Its strong forelimbs and retracting claws mean that Thylacoleo possibly climbed trees and perhaps carried carcasses to keep the kill for itself (similar to the leopard today). The climbing ability would have also helped them climb out of caves, which could therefore have been used as dens to rear their young. Specialised tail bones called chevrons strengthened the tail, likely allowing the animal to use it to prop itself up while rearing on its hind legs, which may have been done when climbing or attacking prey.

The lumbar region is relatively rigid and straight, and suggests that the lower back was relatively inflexible.

Ecology

When Thylacoleo was first described by Richard Owen, he considered it to be a carnivore, based on the morphology of its skull and teeth. However other anatomists, such as William Henry Flower disagreed. Flower was the first to place Thylacoleo with the Diprotodonts, noting its skull and teeth to be laid out more like those of the koala and the wombat, and suggested that it was more likely a herbivore. Owen did not disagree with Flower's placement of Thylacoleo with the Diprotodonts, but still maintained that it was a carnivore, despite its herbivorous ancestry. Owen found little support in his lifetime, despite the pointing out of Thylacoleo's retractable claws, something only found in mammalian carnivores, and its lack of any ability to chew plant material. In 1911, a study by Spencer and Walcott claimed that certain marks on the bones of megafauna had been made by Thylacoleo, but according to Horton (1979) they were not sufficiently rigorous, resulting in their arguments being strongly challenged by later scholars, such as Anderson (1929), and later Gill (1951, 1952, 1954), thereby leaving the issue unresolved.

Besides the most common hypothesis that it was an active predator, a variety of other theories existed in the late 19th to early 20th centuries as to the diet and feeding of Thylacoleo, with hypotheses of it being a scavenger filling the ecological niche of hyenas, being a specialist of crocodile eggs, or even a melon-eater. As late as 1954, doubts were still being raised as to whether it was actually a hypercarnivore.

In 1981, another paper was published arguing that certain cuts to bones of large marsupials had been caused by Thylacoleo. This paper by Horton and Wright was able to counter earlier arguments that such marks were the result of humans, largely by pointing out the presence of similar marks on the opposite side of many bones. They concluded that humans were extremely unlikely to have made the marks in question, but that if so "they had set out to produce only marks consistent with what Thylacoleo would produce". Since then, the academic consensus has emerged that Thylacoleo was a predator and a hypercarnivore.

The marsupial lion's limb proportions and muscle mass distribution indicate that, although it was a powerful animal, it was not a particularly fast runner. Paleontologists conjecture that it was an ambush predator, possibly using leaping. Incisions on bones of the extinct large kangaroo Macropus titan, and the general morphology of Thylacoleo suggests that it fed in a similar manner to modern cheetahs, by using their sharp teeth to slice open the ribcage of their prey, thereby accessing the internal organs. They may have killed by using their front claws as either stabbing weapons or as a way to grab their prey with strangulation or suffocation. Other large bodied prey may have included the large kangaroos Sthenurus, Procoptodon, Protemnodon, Macropus and Osphranter as well as possibly the largest Australian marsupial, the rhinoceros-sized vombatoid Diprotodon. The animal appears to have coexisted with the thylacine; however, it did not compete with it because their dietary niches did not overlap.

Trace fossils in the form of claw marks and bones from caves in Western Australia analyzed by Samuel Arman and Gavin Prideaux indicate marsupial lions could also climb rock faces, and likely reared their young in such caves as a way of protecting them from potential predators. The large proportion of juveniles in the caves suggests communal living and gregariousness in T. carnifex.

Like many predators, it was probably also an opportunistic scavenger, feeding on carrion and driving off less powerful predators from their kills. It also may have shared behaviours exhibited by recent diprotodont marsupials such as kangaroos, like digging shallow holes under trees to reduce body temperature during the day.

Analysis of finds on the Nullarbor Plain suggests that Thylacoleo carnifex inhabited open, arid environments similar to those found across much of Australia today.

Extinction

Thylacoleo is thought to have become extinct around 40,000 years ago as part of the Late Pleistocene megafauna extinctions, essentially simultanteously with the vast majority of Australian megafauna. The relative importance of climatic change versus the impact of recently arrived Indigenous Australians (who arrived in Australia around 50-60,000 years ago) in the extinctions has been debated. There is limited evidence of human interaction with extinct megafauna in Australia.