Megalania

Megalania refers to an extinct giant goanna or monitor lizard,[1] recognised as either Megalania prisca or Varanus priscus. They were part of the megafaunal assemblage that inhabited southern Australia during the Pleistocene. The youngest fossil remains dated to around 50,000 years ago.[2] The first indigenous settlers of Australia might have encountered them and been a factor in their extinction.[3][2][4]

Megalania Temporal range: Pleistocene, 1.5–0.04 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Megalania skeletal reconstruction on Melbourne Museum steps
Scientific classification
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

Taxonomy

Illustration of the original dorsal and cervical vertebrae, 1859

The name Megalania prisca was coined in 1859 by Sir Richard 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."

Owen introduced the genus Megalania to accommodate the species Megalania prisca. Its status as a valid genus remains controversial, with many authors preferring to consider it a junior synonym of Varanus,[5][6] which encompasses all living monitor lizards. As the gender of the genera Megalania and Varanus are respectively feminine and masculine, the specific name prisca (fem.)/priscus (masc.) follows suit.[7]

The genus Megalania is included as a synonym of Varanus by many researchers due to the relationships of the many Varanus species; M. prisca is closely related to other Australian monitors classified as Varanus, so excluding M. prisca 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.[7]

Phylogeny

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.[8] The most recent comprehensive study[9] proposes a sister-taxon relationship with the large Indonesian dragon, Varanus komodoensis, based on neurocranial similarities, with the lace monitor as the closest living Australian relative. Conversely, the perentie is considered more closely related to Gould's and the argus monitors.

Size

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.[6] 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).[10] In 2002, Stephen Wroe considerably downsized megalania, suggesting a maximum length of 4.5 m (15 ft) and a weight of 331 kg (730 lb) with averages of 3.5 m (11 ft) and 97–158 kg (214–348 lb).[11] decrying the earlier maximum length estimate of 7 m (23 ft) as exaggerations based on flawed methods. However, in 2009, Wroe, along with other researchers revised upwards their estimates to at least 5.5 m (18 ft) and 575 kg (1,268 lb).[12]

In a book published in 2004, Ralph Molnar[6] 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 total body length. If it had a long, thin tail like the lace monitor (Varanus varius), then it would have reached a length of 7.9 m (26 ft), while if its tail-to-body proportions were more similar to that of the Komodo dragon (V. komodoensis), then a length around 7 m (23 ft) is more likely. Taking the maximal 7 m (23 ft) length, he estimated a weight of 1,940 kg (4,280 lb), with a leaner 320 kg (710 lb) being average.[6]

[13]

Palaeobiology

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. It had heavily built limbs and body and a large skull complete with a small crest between the eyes, and a jaw full of serrated, blade-like teeth.[7]

Some scientists regard with skepticism the contention that megalania was the only, or even principal, predator of the Australian Pleistocene megafauna.[14] 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 with its 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.

If one were to reconstruct the ecosystems that existed before the arrival of the humans on Australia, introducing Komodo dragons to represent megalania has been suggested to be desirable.[15]

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).[16]

The scales of megalania would possibly be similar to those of their extant relatives possessing a honeycomb microstructure and be durable and resilient to water evaporation.[17]

Venom

Hypothetical megalania skull, at 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.[18][19][20] Closely related varanids use a potent venom found in glands inside the jaw. The venom in these lizards have been shown to be a hemotoxin. 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.[21]

Extinction

Recent paleontological analysis using accelerator mass spectrometry ¹⁴C dating of known fossils shows megalania to have been alive around the Pleistocene epoch 50,000 years ago. An affiliate hypothesis to this dating is that anthropogenic extirpation was the cause of the downfall of megalania and other Australian megafauna in the similar vein as to how a large factor of the extinction of the Northern Hemisphere's megafauna at the end of the Pleistocene epoch was caused by early humans.[2] In addition, a study, which examined the morphology of nine closely related extant varanid lizards and then allometrically scaled and compared them to V. prisca, 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 colonized Australia during that time. This, in coordination with other megafauna that lived at that time such as Quinkana and Thylacoleo carnifex, and possible climate change, could have led to the species' extinction.[22][14][3]

References

1. 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. 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.
3. 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.
4. "Wildfacts - Megalania, giant ripper lizard". BBC. 2008. Archived from the original on 2011-12-27. Retrieved 2012-03-22.
5. 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.)
6. 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.
7. 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.
8. Lee MSY (1996). "Possible affinities between Varanus giganteus and Megalania prisca". Memoirs of the Queensland Museum. 39: 232.
9. 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.
10. 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.
11. 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.
12. Fry BG, Wroe S, Teeuwisse W, et al. (2009). "A central role for venom in predation by Varanus komodoensis (Komodo dragon) and the extinct giant Varanus (Megalania) priscus". Proc. Natl. Acad. Sci. U.S.A. 106 (22): 8969–74. Bibcode:2009PNAS..106.8969F. doi:10.1073/pnas.0810883106. PMC 2690028. PMID 19451641.
13. HEAD, JASON J.; BARRETT, PAUL M.; RAYFIELD, EMILY J. (2009-02-01). "Neurocranial osteology and systematic relationships ofVaranus(Megalania)priscaOwen, 1859 (Squamata: Varanidae)". Zoological Journal of the Linnean Society. 155 (2): 445–457. doi:10.1111/j.1096-3642.2008.00448.x. ISSN 0024-4082.
14. 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.CS1 maint: multiple names: authors list (link)
15. Flannery, Tim (2002). The future eaters: an ecological history of the Australasian lands and people. New York: Grove Press. ISBN 978-0-8021-3943-6.
16. 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.
17. 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.
18. 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.
19. 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.
20. Barry C (2009). "Komodo Dragons Kill With Venom, Researchers Find". National Geographic News. Retrieved 2012-03-22.
21. 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.
22. H., Sobbe, Ian; J., Price, Gilbert; A., Knezour, Robert (2013-06-30). "A ziphodont crocodile from the late Pleistocene King Creek catchment, Darling Downs, Queensland". Memoirs of the Queensland Museum – Nature. 56 (2). ISSN 0079-8835.