Thylacoleo

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Thylacoleo
Temporal range: late Pliocene—late Pleistocene
Skeletal diagram of T. carnifex (top) and restored musculature based on living marsupials (bottom)
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Infraclass: Marsupialia
Order: Diprotodontia
Family: Thylacoleonidae
Genus: Thylacoleo
Owen, 1859
Type species
Thylacoleo carnifex
Species
  • T. carnifex
  • T. crassidentatus
  • T. hilli

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 approached the weight of a lioness. The estimated average weight for the species ranges from 101 to 130 kg (223 to 287 lb).[1]

Taxonomy

The genus was first published in 1859, erected to describe the type species Thylacoleo carnifex. The new taxon was established in examination of fossil specimens provided to Richard Owen. The familial alliance takes its name from this description, the so-called marsupial lions of Thylacoleonidae.

The colloquial name "marsupial lion" alludes to the genus name, which was named after its superficial resemblance to the placental lion and its ecological niche as a large predator.

Genus: Thylacoleo (Thylacopardus) – Australia's marsupial lions, that lived from about 2 million years ago, during the Late Pliocene Epoch and became extinct about 40,000 years ago, during the Late Pleistocene Epoch. Three species are known:

  • Thylacoleo carnifex The holotype cranium was collected from Lake Colongulac in 1843 by pastoralist William Adeney. A partial rostrum collected by Adeney in 1876 from the same locality would later be found to belong to the same individual.[2]
  • Thylacoleo crassidentatus lived during the Pliocene, around 5 million years ago, and was about the size of a large dog. Its fossils have been found in southeastern Queensland.[3][4]
  • Thylacoleo hilli lived during the Pliocene and was half the size of T. crassidentatus. It is the oldest member of the genus.[5]

Fossils of other representatives of Thylacoleonidae, such as Microleo and Wakaleo, date back to the Late Oligocene Epoch, some 24 million years ago.[6]

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.[5] 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.[7] 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.[8]

The marsupial lion is classified in the order Diprotodontia along with many other well-known marsupials such as kangaroos,

herbivorous species (koalas and three species of wombat) have survived into modern times and are considered the marsupial lion's closest living relatives.[10]

Evolution

The ancestors of

palorchestids.[11] The group first appeared in the Late Oligocene. The earliest thylacoleonids like Microleo were small possum-like animals,[12] with the group increasing in size during the Miocene, with representatives like the leopoard 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.[11] The earliest thylacoleonids are thought to have been arboreal (tree dwelling) animals,[13] while Thylacoleo is thought to be terrestrial with some climbing capabilities.[14]

Description

T. carnifex is the

largest carnivorous mammal known to have ever existed in Australia, and one of the largest metatherian carnivores known (comparable to Thylacosmilus and Borhyaena species, but smaller than Proborhyaena gigantea). Individuals ranged up to around 75 cm (30 in) high at the shoulder and about 150 cm (59 in) from head to tail. 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).[14] This would make it comparable to female lions and female tigers
in general size.

Skull

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.[15][16] Compared to earlier thyacoleonids, the third premolars were considerably enlarged.[17] Thylacoleo also had a proportionally large pair of first incisors in the upper and lower jaws, which functioned analogously to other carnivores canine teeth.[15][18] They also had true canines but they served little purpose as they were stubby and not very sharp.[19] Compared to earlier thylacoleonids, the number of molar teeth was reduced.[17]

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.[16] 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.[20] Some later studies questioned the ability of the canine teeth to deliver a killing bite.[21]

Using

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.[22][23]

Postcranium

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.[21]

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.[24]

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

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.[26]

The lumbar region is relatively rigid and straight, and suggests that the lower back was relatively inflexible.[26]

Behaviour and diet

Diet

When Thylacoleo was first described by Richard Owen, he considered it to be a carnivore, based on the morphology of its skull and teeth.[27] 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.[28] Owen found little support in his lifetime, despite the pointing out of Thylacoleo's retractable claws, something only found in mammalian carnivores,[29] and its lack of any ability to chew plant material.[28] 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.[30] 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".[30] Since then, the academic consensus has emerged that Thylacoleo was a predator and a hypercarnivore.[21]

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,[31] being a specialist of crocodile eggs,[25] or even a melon-eater.[32] As late as 1954, doubts were still being raised as to whether it was actually a hypercarnivore.[21]

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

diprotodont marsupials such as kangaroos, like digging shallow holes under trees to reduce body temperature during the day.[36]

Feeding

The feeding behaviour of Thylacoleo remains a topic of academic debate, largely due the lack of any living analogue.[21] While considered a powerful hunter, and a fierce predator, it has been theorized that due to its physiology Thylacoleo was, in fact, a slow runner, limiting its ability to chase prey. Analysis of its scapula suggests "walking and trotting, rather than climbing ... the pelvis similarly agrees with that of ambulators and cursors [walkers and runners]". These bones indicate that Thylacoleo was a slow to medium-paced runner, which is likely to mean it was an ambush predator. That fits with the stripes: camouflage of the kind one would need for stalking and hiding in a largely forested habitat (like tigers or leopards) rather than chasing across open spaces (like lions)."

Tasmanian devil.[26] New evidence also suggests that it may have been arboreal, and was at the very least capable of climbing trees.[25]

Incisions on bones of 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.[30]

Palaeoecology

Life restoration of Thylacoleo carnifex

Numerous fossil discoveries indicate the marsupial lion was distributed across much of the

scrub and woodland punctuated by waterholes and water courses.[citation needed
]

It would have coexisted with many of the so-called

Wakaleo alcootaensis, and may have occupied habitat to the exclusion of that carnivore.[39]

Australia's

apex mammalian predators have replaced the marsupial lions after their disappearance.[41]

Extinction

Thylacoleo is thought to have become extinct around 40,000 years ago as part of the

Late Pleistocene 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.[42]

Discoveries

Drawing of T. carnifex skull fragments, 1859

The first specimens of Thylacoleo were collected in the early 1830s from the Wellington Valley region of New South Wales by Major (later Sir) Thomas Mitchell, however they were not recognised as Thylacoleo at the time.[19]

The species was first described by

Victoria[27]

In 2002, eight remarkably complete skeletons of T. carnifex were discovered in a limestone cave under Nullarbor Plain, where the animals fell through a narrow opening in the plain above. Based on the placement of their skeletons, at least some survived the fall, only to die of thirst and starvation.[43][44]

In 2008, rock art depicting what some speculate to be Thylacoleo was discovered on the northwestern coast of the Kimberley. However, the thylacine, a marsupial that had a striped coat like depicted in the rock art, has been argued to be the more likely subject of the work.[45] The drawing represented only the second example of megafauna depicted by the indigenous inhabitants of Australia. The image contains details that would otherwise have remained only conjecture; the tail is depicted with a tufted tip, it has pointed ears rather than rounded, and the coat is striped. The prominence of the eye, a feature rarely shown in other animal images of the region, raises the possibility that the creature may have been a nocturnal hunter.[46] In 2009, a second image was found that depicts a Thylacoleo interacting with a hunter who is in the act of spearing or fending the animal off with a multiple-barbed spear. Much smaller and less detailed than the 2008 find, it may depict a thylacine, but the comparative size indicates a Thylacoleo is more likely, meaning that it is possible that Thylacoleo was extant until more recently than previously thought.[47]

In 2016,

trace fossils in Tight Entrance Cave were identified as being the scratch marks of a Thylacoleo.[25]

Fossils

The first Thylacoleo fossil findings, discovered by Thomas Mitchell were found 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.[27] 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.[48][29]

It was reported that in 2012, an accumulation of vertebrate trace and body fossils were found in the Victorian Volcanic Plains in southeastern Australia. It was determined that Thylacoleo was the only taxon that represented three divergent fossil records: skeletal, footprints, and bite marks. What this suggests is that these large carnivores had behavioral characteristics that could have increased their likelihood of their presence being detected within a fossil fauna.[49]

A characteristic seen in the remains of skull fragments is a set of carnassial teeth, suggesting the carnivorous habits of Thylacoleo. Tooth fossils of the Thylacoleo exhibit specific degrees of erosion that are credited to the utility of the carnassial teeth remains as they were used for hunting and consuming prey in a prehistoric Australia teeming with other megafauna. The specialisation found in the dental history of the marsupial indicates its status in the predatory hierarchy in which it existed.[50]

References

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