Thylacoleo, also called "pouch lion," was a group of meat-eating mammals that lived in Australia from the late Pliocene to the Late Pleistocene, which was about 40,000 years ago. These animals are often called marsupial lions, though they were not related to modern lions. They were the largest and final members of their family, Thylacoleonidae, and served as the top predator in Australian ecosystems. The biggest and last species, Thylacoleo carnifex, weighed about 101 to 130 kilograms (223 to 287 pounds), which is similar to the weight of a modern lioness (Panthera leo).

Taxonomy

The first Thylacoleo fossils were found by Thomas Mitchell in the 1830s in the Wellington Valley of New South Wales, but they were not recognized as Thylacoleo at the time. The first complete set of fossils, including broken teeth, jaws, and a skull, was discovered by a farmer named William Avery near Lake Colungolac. These fossils were used to describe the species Thylacoleo carnifex by Richard Owen. It was not until 1966 that the first nearly-complete skeleton of Thylacoleo was found. Only the foot and tail were missing. Today, the Nullarbor Plain in Western Australia is the richest area for finding Thylacoleo fossils. These fossils are now displayed at the Australian Museum.

The genus Thylacoleo was first described in 1859 to classify the species Thylacoleo carnifex. Richard Owen created this name after studying fossil samples he received. The family name Thylacoleonidae comes from this classification, which includes animals known as "marsupial lions."

The name "marsupial lion" refers to the genus Thylacoleo, which was named because it looked somewhat like a placental lion and filled a similar role as a top predator in its environment.

Genus: Thylacoleo (Thylacopardus) – These animals, called marsupial lions, lived about 2 million years ago during the Late Pliocene Epoch and went extinct around 40,000 years ago during the Late Pleistocene Epoch. Three species are known:

• Thylacoleo carnifex: The first skull of this species was collected from Lake Colongulac in 1843 by a farmer named William Adeney. A partial jaw found by Adeney in 1876 from the same area was later identified as belonging to the same individual. The first nearly-complete skeleton was discovered in 1966.
• Thylacoleo crassidentatus: This species lived during the Pliocene Epoch, about 5 million years ago, and was about the size of a large dog. Fossils of this species have been found in southeastern Queensland.
• Thylacoleo hilli: This species lived during the Pliocene and was half the size of T. crassidentatus. It is the oldest known member of the Thylacoleo genus.

Fossils of other members of the family Thylacoleonidae, such as Microleo and Wakaleo, are dated to the Late Oligocene Epoch, about 24 million years ago.

The species Thylacoleo hilli was described by Neville Pledge in a study published in 1977. The holotype is a tooth discovered in a cave in Curramulka, South Australia, by Alan Hill, a cave explorer. This tooth showed features typical of the Thylacoleo genus and was about half the size of T. carnifex. The specific name hilli honors Alan Hill, the person who found the first specimen. Fossils from the Bow River in New South Wales, dated to the early Pliocene, were also linked to T. hilli in 1982. A fragment of an unworn incisor found near the "Town Cave" in Curramulka was also identified as belonging to T. hilli due to its size similarity to T. carnifex.

The marsupial lion is classified in the order Diprotodontia, which includes other well-known marsupials like kangaroos, possums, and koalas. It belongs to its own family, Thylacoleonidae, which includes three genera and 11 species, all now extinct. The term "marsupial lion" is sometimes used to refer to other members of this family. For many years, Thylacoleo was mistakenly grouped with possums due to its similar features. However, recent studies based on skull and other characteristics have placed Thylacoleo in the group vombatiformes, which includes the ancestors of wombats. Marsupial lions and other vombatiforms were once represented by over 60 species, including carnivores, herbivores, and animals that lived on the ground or in trees. These species ranged in size from 3 kilograms to 2.5 tonnes. Only two families, including koalas and wombats, have survived to the present day and are considered the closest living relatives of the marsupial lion.

Evolution

The ancestors of thylacoleonids are believed to have been herbivores, which is unusual for carnivores. They are part of the Vombatiformes, an order of marsupials that mostly eat plants. Living members of this order today include koalas and wombats, while extinct members include animals like diprotodontids and palorchestids. Thylacoleonids first appeared during the Late Oligocene. Early thylacoleonids, such as Microleo, were small, possum-like animals. Over time, the group grew larger, with species like the leopard-sized Wakaleo appearing during the Miocene. The genus Thylacoleo appeared during the Pliocene and was the only living genus in the family during that time until the end of the Pleistocene. The last known member of the thylacoleonids, T. carnifex, was the largest species in the family. Early thylacoleonids likely lived in trees, while Thylacoleo lived on the ground but could climb.

Description

T. carnifex was a large carnivore, similar in size to living big cats. Measurements from many fossils show that it weighed between 101 and 130 kg (223 to 287 lb) on average, though some individuals may have weighed as much as 124–160 kg (273–353 lb). The largest recorded weight was 128–164 kg (282–362 lb). This would make it about the same size as female lions and tigers. Bite force estimates based on teeth are often unreliable, while estimates from measurements of leg bones are more accurate.

Like other thylacoleonids, Thylacoleo had blade-like third premolar teeth in its upper and lower jaws, which acted as carnassial teeth. These teeth were positioned much farther forward in the jaw than in other mammals. Compared to earlier thylacoleonids, these teeth were much larger. Thylacoleo also had large first incisors in its upper and lower jaws, which worked like the canine teeth of other carnivores. It had true canines, but they were small and not sharp. Compared to earlier thylacoleonids, Thylacoleo had fewer molar teeth.

Per pound of body weight, T. carnifex had the strongest bite force of any mammal, living or extinct. A T. carnifex weighing 101 kg (223 lb) had a bite force similar to that of a 250 kg African lion. Research suggests Thylacoleo could hunt prey much larger than itself. Larger prey likely included Diprotodon spp. and giant kangaroos. It is unlikely Thylacoleo could match the bite force of a modern lion, but its brain and skull size might have allowed for greater force. Carnivores usually have larger brains than herbivorous marsupials, which limits the amount of bone available for strong bites. Thylacoleo had stronger muscle attachments and a smaller brain. Some later studies questioned whether its canine teeth could deliver a killing bite.

Using 3D models from X-ray scans, scientists found that marsupial lions could not use the long, suffocating bite typical of big cats. Instead, they had a unique and efficient bite. Their incisors would stab and pierce prey, while their specialized carnassial teeth crushed the windpipe, severed the spinal cord, and cut major blood vessels like the carotid artery and jugular vein. Compared to an African lion, which may take 15 minutes to kill a large animal, the marsupial lion could kill a large prey in less than a minute. Its skull was so specialized for hunting large prey that it was inefficient at catching smaller animals, which may have contributed to its extinction.

Thylacoleo had strong and flexible front legs with large, retractable claws on thumbs that could move somewhat like opposable thumbs. These claws were likely used to deliver a killing blow. Its hind feet had four toes, with the first one much smaller. A rough pad on the foot, similar to that of possums, may have helped with climbing. A 2005 discovery of a complete hind foot showed that Thylacoleo had fused second and third toes, like other diprotodonts.

Its strong front legs and retractable claws suggest Thylacoleo could climb trees and possibly carry prey to keep for itself, like modern leopards. Climbing ability would also help it escape caves, which may have been used as dens to raise young. Tail bones called chevrons strengthened the tail, likely helping the animal support itself while standing on its hind legs during climbing or attacking prey.

The lower back was stiff and straight, indicating the lower spine was not very flexible.

Ecology

When Thylacoleo was first described by Richard Owen, he believed it was a carnivore, based on the shape of its skull and teeth. However, other scientists, such as William Henry Flower, disagreed. Flower was the first to group Thylacoleo with the Diprotodonts, noting that its skull and teeth were more similar to those of the koala and the wombat, and suggested it might have been a herbivore. Owen did not argue against Flower's classification of Thylacoleo as a Diprotodont, but he still believed it was a carnivore, even though it shared ancestors with herbivores. Owen received little support during his lifetime, despite pointing out that Thylacoleo had retractable claws, a feature found only in carnivorous mammals, and lacked the ability to chew plant material. In 1911, a study by Spencer and Walcott claimed that marks on the bones of large extinct animals were made by Thylacoleo. However, Horton (1979) later said their research was not thorough enough, leading later scientists like Anderson (1929) and Gill (1951, 1952, 1954) to challenge their findings. This left the question of Thylacoleo's diet unresolved.

Besides the most common idea that Thylacoleo was an active predator, many other theories existed in the late 19th to early 20th centuries about its diet and feeding habits. These included ideas that it was a scavenger filling the role of hyenas, a specialist that ate crocodile eggs, or even an animal that ate melons. As late as 1954, scientists still debated whether Thylacoleo was a hypercarnivore.

In 1981, a paper by Horton and Wright argued that cuts on the bones of large marsupials were caused by Thylacoleo. This study countered earlier claims that such marks were made by humans, by showing that similar marks appeared on both sides of many bones. They concluded that humans were unlikely to have made the marks, but if they did, "they had only made marks that matched what Thylacoleo would produce." Since then, scientists generally agree that Thylacoleo was a predator and a hypercarnivore.

The marsupial lion's body structure suggests it was a strong animal but not a fast runner. Scientists believe it was an ambush predator, possibly using jumps to attack prey. Cuts on bones of the extinct large kangaroo Macropus titan and the overall shape of Thylacoleo suggest it fed like modern cheetahs, using sharp teeth to slice open the ribcages of prey to access internal organs. It may have killed prey by stabbing with its front claws or grabbing and suffocating them. Other large prey might have included kangaroos such as Sthenurus, Procoptodon, Protemnodon, Macropus, and Osphranter, as well as the giant vombat Diprotodon. Thylacoleo lived alongside the thylacine, but they did not compete because their diets were different.

Claw marks and bones found in caves in Western Australia, studied by Samuel Arman and Gavin Prideaux, suggest that marsupial lions could climb rock faces and may have raised their young in caves to protect them from predators. The high number of juvenile bones in these caves indicates that Thylacoleo carnifex may have lived in groups.

Like many predators, Thylacoleo likely scavenged, eating dead animals and driving away weaker predators from their kills. It may have also shared behaviors with modern kangaroos, such as digging shallow holes under trees to stay cool during the day.

Studies of remains found on the Nullarbor Plain suggest that Thylacoleo carnifex lived in open, dry environments similar to many parts of Australia today.

Extinction

Thylacoleo is believed to have died out about 40,000 years ago during the Late Pleistocene megafauna extinctions, which happened around the same time as most large animals in Australia. Scientists are still discussing whether changes in the climate or the arrival of humans had a bigger role in these extinctions. People who first came to Australia about 50,000 to 60,000 years ago may have influenced these events, but there is little evidence showing direct interaction between humans and the extinct megafauna.