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Megalania

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Megalania
Temporal range: Pleistocene, 1.5–0.05 Ma
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Megalania skeletal reconstruction on Melbourne Museum steps
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Squamata
Family: Varanidae
Genus: Varanus
Species:
†V. priscus
Binomial name
†Varanus priscus
Owen, 1859[1]
Synonyms
  • Megalania prisca (Owen, 1859)
  • Notiosaurus dentatus Owen, 1884
  • Varanus dirus de Vis, 1889
  • Varanus warburtonensis Zeitz, 1899

Megalania (Varanus priscus) is an extinct species of giant monitor lizard,[1] 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 2–3 m (6.6–9.8 ft) in body length excluding the tail, while some individuals would have been significantly larger, reaching sizes around 4.5–7 m (15–23 ft) 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[2]. The youngest fossil remains of giant monitor lizards in Australia date to around 50,000 years ago.[3] The first indigenous settlers of Australia might have encountered megalania,[4] and been a factor in megalania's extinction.[5][3][6] 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.

Taxonomy

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Illustration of the original dorsal and cervical vertebrae, 1859

Sir Richard Owen described the first known remains of megalania in 1859, from three vertebrae amongst a collection of primarily marsupial bones purchased by the British Museum, collected from the bed of a tributary of the Condamine River, west of Moreton Bay in eastern Australia. The name "Megalania prisca" was coined in the paper by Owen to mean "ancient great roamer"; the name was chosen "in reference to the terrestrial nature of the great Saurian".[1] Owen used a modification of the Greek word ἠλαίνω ēlainō ("I roam"). The close similarity to the Latin word: lania (feminine form of "butcher") has resulted in numerous taxonomic and popular descriptions of "Megalania" mistranslating the name as "ancient giant butcher." "Megalania" is no longer considered a valid genus, with many authors preferring to consider it a junior synonym of Varanus,[7][8] which encompasses all living monitor lizards. The genera "Megalania" and Varanus are respectively feminine and masculine in grammatical gender and their specific names need to match them: prisca (feminine) and priscus (masculine).[9]

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 unnatural grouping. 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.[9]

Phylogeny

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[2]

Several studies have attempted to establish the phylogenetic position of megalania within the Varanidae. An affinity with the perentie (Varanus giganteus), Australia's largest living lizard, has been suggested based on skull-roof morphology.[10] The most recent comprehensive study[2] proposes a sister-taxon relationship with the large Komodo dragon (Varanus komodoensis) based on neurocranial similarities, with the lace monitor (Varanus varius) as the closest living Australian relative. Conversely, the perentie is considered more closely related to Gould's monitor and the Argus monitor.

Size

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Two size estimations of megalania compared to extant monitor lizards and a human

The lack of complete or nearly complete fossil skeletons has made it difficult to determine the exact dimensions of megalania.[8] Early estimates placed the length of the largest individuals at 7 m (23 ft), with a maximum weight of approximately 600–620 kg (1,320–1,370 lb).[11] In 2002, Stephen Wroe considerably downsized megalania, suggesting a maximum total length of 4.5 m (15 ft) and a weight of 331 kg (730 lb) with average total lengths of 3.45 m (11.3 ft) and 97–158 kg (214–348 lb),[12] decrying the earlier maximum length estimate of 7 m (23 ft) as exaggerations based on flawed methods. In 2009, however, Wroe joined other researchers in raising the estimate to at least 5.5 m (18 ft) and 575 kg (1,268 lb).[13]

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 2.19 m (7.2 ft) in snout-vent length based on femoral length.[14] 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 2.2–2.4 m (7.2–7.9 ft), and such individuals would have weighed up to 320 kg (710 lb). The largest vertebra (QM 2942) would have belonged to an individual with a snout-vent length of 3.8 m (12 ft) and weighed up to 1,940 kg (4,280 lb).[8]

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 1.78–1.9 m (5.8–6.2 ft) in precaudal length, while the largest specimen available to them (AMNH FR 6304) suggested that this individual would have reached up to 2–2.17 m (6.6–7.1 ft) in precaudal length. They also noted that it is possible for megalania to reach over 3 m (9.8 ft) in precaudal length, given that the largest specimens of modern varanid species are larger than average individuals by 151 to 225 percent.[15]

Palaeobiology

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Life restoration

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.[citation needed] 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.[9]

Some scientists regard with skepticism the contention that megalania was the only, or even principal, predator of the Australian Pleistocene megafauna.[16] They note that the marsupial lion (Thylacoleo carnifex) has been implicated with the butchery of very large Pleistocene mammals, while megalania has not.[citation needed] 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.[17][18] 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.[19]

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 2.6–3 m/s (9.4–10.8 km/h). This speed is comparable to that of the extant freshwater crocodile (Crocodylus johnstoni).[20]

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.[21]

Venom

[edit]
Hypothetical megalania skull, at the Museum of Science, Boston

Along with other varanid lizards, such as the Komodo dragon and the Nile monitor, megalania belongs to the proposed clade Toxicofera, which contains all known reptile clades possessing toxin-secreting oral glands, as well as their close venomous and nonvenomous relatives, including Iguania, Anguimorpha, and snakes.[22][23][24] 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.[13]

Extinction

[edit]

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.[3] A study examined the morphology of nine closely related extant varanid lizards and then allometrically 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.[5] 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.

Confrontations between megalania and early Aboriginal Australians may have inspired tales of fearsome creatures such as the whowie.[8]

References

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  1. ^ a b c Owen R. (1859). "Description of Some Remains of a Gigantic Land-Lizard (Megalania Prisca, Owen) from Australia". Philosophical Transactions of the Royal Society of London. 149: 43–48. doi:10.1098/rstl.1859.0002. JSTOR 108688.
  2. ^ a b c Head, JJ.; Barrett, PM.; Rayfield, EJ. (2009). "Neurocranial osteology and systematic relationships of Varanus (Megalania) prisca Owen, 1859 (Squamata: Varanidae)". Zoological Journal of the Linnean Society. 155 (2): 445–457. doi:10.1111/j.1096-3642.2008.00448.x.
  3. ^ a b c Price, Gilbert J.; Louys, Julien; Cramb, Jonathan; Feng, Yue-xing; Zhao, Jian-xin; Hocknull, Scott A.; Webb, Gregory E.; Nguyen, Ai Duc; Joannes-Boyau, Renaud (2015-10-01). "Temporal overlap of humans and giant lizards (Varanidae; Squamata) in Pleistocene Australia". Quaternary Science Reviews. 125: 98–105. Bibcode:2015QSRv..125...98P. doi:10.1016/j.quascirev.2015.08.013.
  4. ^ Hideaki Kato (2021). 図解大事典 絶滅動物. 新星出版社. p. 229. ISBN 9784405073432.
  5. ^ a b Dick, Taylor J. M.; Clemente, Christofer J. (2016-02-18). "How to build your dragon: scaling of muscle architecture from the world's smallest to the world's largest monitor lizard". Frontiers in Zoology. 13: 8. doi:10.1186/s12983-016-0141-5. ISSN 1742-9994. PMC 4758084. PMID 26893606.
  6. ^ "Wildfacts - Megalania, giant ripper lizard". BBC. 2008. Archived from the original on 2011-12-27. Retrieved 2012-03-22.
  7. ^ Lydekker R. (1888). Catalog of the fossil Reptilia in the British Museum (Natural History) Cromwell Road S.W. Pt. 1: The Orders Ornithosauria, Crocodilia, Squamata, Rhynchocephalia, and Proterosauria. London: The Trustees. Cited in Molnar RE (2004). "The long and honorable history of monitors and their kin". In King, Ruth Allen; Pianka, Eric R.; King, Dennis (eds.). Varanoid lizards of the world. Bloomington: Indiana University Press. p. 45. ISBN 978-0-253-34366-6.)
  8. ^ a b c d Molnar, Ralph E. (2004). Dragons in the dust: the paleobiology of the giant monitor lizard Megalania. Bloomington: Indiana University Press. ISBN 978-0-253-34374-1.
  9. ^ a b c Molnar RE (2004). "History of monitors and their kin". In King, Ruth Allen; Pianka, Eric R.; King, Dennis (eds.). Varanoid lizards of the world. Bloomington: Indiana University Press. p. 588. ISBN 978-0-253-34366-6.
  10. ^ Lee MSY (1996). "Possible affinities between Varanus giganteus and Megalania prisca". Memoirs of the Queensland Museum. 39: 232.
  11. ^ Hecht, M. (1975). "The morphology and relationships of the largest known terrestrial lizard, Megalania prisca Owen, from the Pleistocene of Australia". Proceedings of the Royal Society of Victoria. 87: 239–250.
  12. ^ Wroe, S. (2002). "A review of terrestrial mammalian and reptilian carnivore ecology in Australian fossil faunas, and factors influencing their diversity: the myth of reptilian domination and its broader ramifications". Australian Journal of Zoology. 50: 1–24. doi:10.1071/zo01053. Archived from the original on 2013-01-03. Retrieved 2012-03-22.
  13. ^ a b Fry, B.; Wroe, S.; Teeuwisse, W.; Van Osch, M. J. P.; Moreno, K.; Ingle, J.; McHenry, C.; Ferrara, T.; Clausen, P.; Scheib, H.; Winter, K. L.; Greisman, L.; Roelants, K.; Van Der Weerd, L.; Clemente, C. J.; Giannakis, E.; Hodgson, W. C.; Luz, S.; Martelli, P.; Krishnasamy, K.; Kochva, E.; Kwok, H. F.; Scanlon, D.; Karas, J.; Citron, D. M.; Goldstein, E. J. C.; McNaughtan, J. E.; Norman, J. A.; et al. (2009). "A central role for venom in predation by Varanus komodoensis (Komodo Dragon) and the extinct giant Varanus (Megalania) priscus". PNAS. 106 (22): 8969–74. Bibcode:2009PNAS..106.8969F. doi:10.1073/pnas.0810883106. PMC 2690028. PMID 19451641.
  14. ^ Erickson, G.M.; De Ricqles, A.; De Buffrénil, V.; Molnar, R.E.; Bayless, M.K. (2003). "Vermiform bones and the evolution of gigantism in Megalania—How a reptilian fox became a lion". Journal of Vertebrate Paleontology. 23 (4): 966–970. Bibcode:2003JVPal..23..966E. doi:10.1671/23.
  15. ^ Conrad JL, Balcarcel AM, Mehling CM (2012). "Earliest example of a giant monitor lizard (Varanus, Varanidae, Squamata)". PLOS ONE. 7 (8): e41767. Bibcode:2012PLoSO...741767C. doi:10.1371/journal.pone.0041767. PMC 3416840. PMID 22900001.
  16. ^ Wroe S, Myers TJ, Wells RT & Gillespie, A (1999). "Estimating the weight of the Pleistocene marsupial lion, Thylacoleo carnifex (Thylacoleonidae : Marsupialia): implications for the ecomorphology of a marsupial super-predator and hypotheses of impoverishment of Australian marsupial carnivore faunas". Australian Journal of Zoology. 47 (5): 489–498. doi:10.1071/ZO99006.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  17. ^ Hocknull SA, Piper PJ, van den Bergh GD, Due RA, Morwood MJ, Kurniawan I (2009). "Dragon's Paradise Lost: Palaeobiogeography, Evolution and Extinction of the Largest-Ever Terrestrial Lizards (Varanidae)". PLOS ONE. 4 (9): e7241. Bibcode:2009PLoSO...4.7241H. doi:10.1371/journal.pone.0007241. PMC 2748693. PMID 19789642.
  18. ^ "Australia was 'hothouse' for killer lizards". Australian Broadcasting Corporation. 30 September 2009. Retrieved 30 September 2009.
  19. ^ Flannery T (1994). The Future Eaters. Grove Press. pp. 384–5. ISBN 0-8021-3943-4.
  20. ^ Clemente CJ, Thompson GG, Withers PC (2009). "Evolutionary relationships of sprint speed in Australian varanid lizards". Journal of Zoology. 278 (4): 270–280. doi:10.1111/j.1469-7998.2009.00559.x.
  21. ^ Bucklitsch, Yannick; Böhme, Wolfgang; Koch, André (2016-08-17). "Scale Morphology and Micro-Structure of Monitor Lizards (Squamata: Varanidae: Varanus spp.) and their Allies: Implications for Systematics, Ecology, and Conservation". Zootaxa. 4153 (1): 1–192. doi:10.11646/zootaxa.4153.1.1. ISSN 1175-5334. PMID 27615821.
  22. ^ Vidal, Nicholas; Hedges, S. Blair (2009). "The molecular evolutionary tree of lizards, snakes, and amphisbaenians". Comptes Rendus Biologies. 332 (2–3): 129–139. doi:10.1016/j.crvi.2008.07.010. PMID 19281946. S2CID 23137302.
  23. ^ Fry, B.; Vidal, Nicolas; Norman, Janette A.; Vonk, Freek J.; Scheib, Holger; Ramjan, S. F. Ryan; Kuruppu, Sanjaya; Fung, Kim; Blair Hedges, S.; Richardson, Michael K.; Hodgson, Wayne. C.; Ignjatovic, Vera; Summerhayes, Robyn; Kochva, Elazar; et al. (February 2006). "Early evolution of the venom system in lizards and snakes". Nature. 439 (7076): 584–588. Bibcode:2006Natur.439..584F. doi:10.1038/nature04328. PMID 16292255. S2CID 4386245.
  24. ^ Barry C (2009). "Komodo Dragons Kill With Venom, Researchers Find". National Geographic News. Archived from the original on May 21, 2009. Retrieved 2012-03-22.
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