Thylacoleo ("pouch lion") is an extinct genus of carnivorous marsupials that lived in Australia from the late Pliocene to the Late Pleistocene (until around 40,000 years ago), often known as marsupial lions. They were the largest and last members of the family Thylacoleonidae, occupying the position of apex predator within Australian ecosystems. The largest and last species, Thylacoleo carnifex, had an estimated average weight of , approaching the weight of a modern lioness (Panthera leo).

Taxonomy

The first Thylacoleo fossil findings were discovered by Thomas Mitchell in the 1830s in the Wellington Valley of New South Wales, though not recognised as such at the time. The generic holotype, consisting of broken teeth, jaws, and a skull, was discovered by a pastoralist, William Avery, near Lake Colungolac from which the species Thylacoleo carnifex was described by Richard Owen. It was not until 1966 that the first nearly-complete skeleton was found. The only pieces missing were a foot and the tail. Currently, the Nullarbor Plain of West Australia remains to be the greatest finding site. These fossils now reside at the Australian Museum. It was not until 1966 that the first nearly-complete skeleton was found.

  • Thylacoleo hilli lived during the Pliocene and was half the size of T. crassidentatus. It is the oldest member of the genus.

T. hilli was described by Neville Pledge in a study published in the records of the South Australia Museum in 1977. The holotype is a third premolar, discovered at a cave in Curramulka in South Australia, exhibiting the carnivorous characteristics of the genus and around half the size of T. carnifex. This tooth was collected by Alan Hill, a speleologist and founding member of the Cave Exploration Group of South Australia, while examining a site known as the "Town Cave" in 1956; the specific epithet hilli honours the collector of the first specimen. Material found amidst the fauna at Bow River in New South Wales, dated to the early Pliocene, was also referred to the species in 1982.

A fragment of an incisor, unworn and only diagnosable to the genus, was located at a site in Curramulka, close to the Town Cave site, and referred to the species for the apparent correlation in size when compared to the better known T. carnifex.

The marsupial lion is classified in the order Diprotodontia along with many other well-known marsupials such as kangaroos, possums, and the koala. It is further classified in its own family, the Thylacoleonidae, of which three genera and 11 species are recognised, all extinct. The term marsupial lion (lower case) is often applied to other members of this family. Distinct possum-like characteristics led Thylacoleo to be regarded as members of Phalangeroidea for a few decades. Though a few authors continued to hint at phalangeroid affinities for thylacoleonids as recently as the 1990s, cranial and other characteristics have generally led to their inclusion within vombatiformes, and as stem-members of the wombat lineage. Marsupial lions and other ecologically and morphologically diverse vombatiforms were once represented by over 60 species of carnivorous, herbivorous, terrestrial and arboreal forms ranging in size from 3 kg to 2.5 tonnes. Only two families represented by four herbivorous species (koalas and three species of wombat) have survived into modern times and are considered the marsupial lion's closest living relatives.

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. while Thylacoleo is thought to be terrestrial with some climbing capabilities.

Description

thumb|Size comparison of Thylacoleo carnifex compared to a human. Scale bar =

T. carnifex was a large carnivore, comparable to living big cats in size. Measurements taken from a number of specimens show they averaged in weight, although individuals as large as might not have been uncommon, and the largest weight was of . This would make it comparable to female lions and female tigers in general size. 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

thumb|Skull of Thylacoleo carnifex

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. 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. 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

left|thumb|Skeleton at Naracoorte Caves

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.

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. Owen found little support in his lifetime, despite the pointing out of Thylacoleos retractable claws, something only found in mammalian carnivores, and its lack of any ability to chew plant material.

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, As late as 1954, doubts were still being raised as to whether it was actually a hypercarnivore. 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.

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. It has been contested as to the relative importance of climatic change vs the impact of recently arrived Indigenous Australians (who arrived in Australia around 50-60,000 years ago) in the extinctions. There is limited evidence of human interaction with extinct megafauna in Australia.

References

  • New study finds no evidence for theory humans wiped out megafauna
  • Thylacoleo - Australia's Marsupial Lion
  • Thylacoleo in Pleistocene Australia
  • Steve Wroe's Web Page: Australian Megafauna
  • Western Australian Museum: Thylacoleo - a voracious hunter