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Megalania ( Varanus priscus) is an extinct species of giant monitor lizard, part of the megafaunal assemblage that inhabited Australia during the Pleistocene. It is the largest terrestrial lizard known to have existed, but the fragmentary nature of known remains make estimates highly uncertain. Recent studies suggest that most known specimens would have reached around in body length excluding the tail, while some individuals would have been significantly larger, reaching sizes around in length.
Megalania is thought to have had a similar ecology to the living Komodo dragon ( Varanus komodoensis) which may be its closest living relative. The youngest fossil remains of giant monitor lizards in Australia date to around 50,000 years ago. The first indigenous settlers of Australia might have encountered megalania, and been a factor in megalania's extinction. While originally megalania was considered to be the only member of the titular genus " Megalania", today it is considered a member of the genus Varanus, being closely related to other Australian monitor lizards.
Megalania is included within Varanus because its morphology suggests that it is more closely related to some species of Varanus than others, so excluding V. priscus from Varanus renders the latter genus an Paraphyletic. Ralph Molnar noted in 2004 that, even if every species of the genus Varanus were divided into groups currently designated as subgenera, V. priscus would still be classified in the genus Varanus, because this is the current subgenus name, as well as genus name, for all Australian monitors. Unless other Australian monitor species were each also classified their own exclusive genera, " Megalania" would not be a valid genus name. However, Molnar noted that "megalania" is suitable for use as a vernacular, rather than scientific name, for the species Varanus priscus.
In 2003, Erickson and colleagues suggested that a large specimen with an estimated longevity of 16 years, QM F4452/3, would have belonged to an individual up to in snout-vent length based on femur length. In a book published in 2004, Ralph Molnar determined a range of potential sizes for megalania, made by scaling up from dorsal vertebrae, after he determined a relationship between dorsal vertebrae width and snout-vent length. The average snout-vent length of known specimens were around , and such individuals would have weighed up to . The largest vertebra (QM 2942) would have belonged to an individual with a snout-vent length of and weighed up to .
In 2012, Conrad and colleagues estimated the size of megalania based on comparing two known specimens with all known species of Varanus. The authors of the study suggested that the braincase (BMNH 39965) likely belonged to an individual around in precaudal length, while the largest specimen available to them (AMNH FR 6304) suggested that this individual would have reached up to in precaudal length. They also noted that it is possible for megalania to reach over in precaudal length, given that the largest specimens of modern varanid species are larger than average individuals by 151 to 225 percent.
Some scientists regard with skepticism the contention that megalania was the only, or even principal, predator of the Australian Pleistocene megafauna. They note that the marsupial lion ( Thylacoleo carnifex) has been implicated with the butchery of very large Pleistocene mammals, while megalania has not. In addition, they note that megalania fossils are extremely uncommon, in contrast to T. carnifex's wide distribution across Australian Pleistocene deposits. Quinkana, a genus of terrestrial crocodiles that grew up to 6 m and was present until around 40,000 years ago, has also been marked as another apex predator of Australian megafauna.
Komodo dragons, megalania's closest relative, are known to have evolved in Australia before spreading to their current range in Indonesia, as fossil evidence from Queensland has implied. If one were to reconstruct the ecosystems that existed before the arrival of the humans on Australia, reintroducing Komodo dragons as an ecological proxy of megalania to the continent has been suggested.
A study published in 2009 using Wroe's earlier size estimates and an analysis of 18 closely related lizard species estimated a sprinting speed of . This speed is comparable to that of the Extant taxon freshwater crocodile ( Crocodylus johnstoni).
The scales of megalania would possibly be similar to those of their extant relatives, possessing a honeycomb microstructure and both durable and resilient to water evaporation.
Confrontations between megalania and early Aboriginal Australians may have inspired tales of fearsome creatures such as the whowie.
Palaeobiology
Megalania is the largest terrestrial lizard known to have existed. Judging from its size, it would have fed mostly upon medium- to large-sized animals, including any of the giant marsupials such as Diprotodon, along with other reptiles and small mammals, as well as birds and their eggs and chicks. It had heavily built limbs and body, a large skull complete with a small crest between the eyes, and a jaw full of serrated, blade-like teeth.
Venom
Along with other Varanidae, such as the Komodo dragon and the Nile monitor, megalania belongs to the proposed clade Toxicofera, which contains all known reptile Clade possessing toxin-secreting oral glands, as well as their close venomous and nonvenomous relatives, including Iguania, Anguimorpha, and . Closely related varanids use a potent venom found in glands inside the jaw. The venom in these lizards have been shown to be a haemotoxin. The venom would act as an anticoagulant and would greatly increase the bleeding the prey received from its wounds. This would rapidly decrease the prey's blood pressure and lead to systemic shock. Being a member of Anguimorpha, megalania may have been venomous and if so, would be the largest venomous vertebrate known.
Extinction
The youngest remains of the species date to the Late Pleistocene, with the youngest remains possibly referrable to the species being a large osteoderm dating to approximately 50,000 years ago from the Mount Etna Caves National Park in central-eastern Queensland. A study examined the morphology of nine closely related extant varanid lizards and then allometry scaled and compared them to V. priscus, found that the musculature of the limbs, posture, muscular mass, and possible muscular composition of the animal would most likely have been inefficient when attempting to outrun the early human settlers who colonised Australia during that time. Considering many other species of Australian megafauna went extinct around the same time, either due to human predation or being outcompeted by them, the same can be assumed for megalania.
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