Megalania

Megalania
Temporal range: Pleistocene, 1.5–0.04 Ma
↓
Megalania skeletal reconstruction on Melbourne Museum steps
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Squamata
Suborder: Scleroglossa
Infraorder: Anguimorpha
Parvorder: Platynota
Superfamily: Varanoidea
Family: Varanidae
Genus: ?Megalania (see text)
Species: ?M. prisca (see text)
Proposed binomials

Designation is controversial: EitherorMegalania prisca Owen, 1859[1] orEitherVaranus priscus Owen, 1859

Megalania (Megalania prisca or Varanus priscus) is an extinct giant goanna or monitor lizard.[1] They were part of a megafaunal assemblage that inhabited southern Australia during the Pleistocene. They seem to have disappeared sometime after 50,000 years ago.[2] The first aboriginal settlers of Australia might have encountered them.[2][3]

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,[4][5] 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.[6]

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.[6]

Phylogeny

Several studies have attempted to establish the phylogenetic position of megalania within the Varanidae. An affinity with the perentie, Australia's largest living lizard, has been suggested based on skull-roof morphology.[7] The most recent comprehensive study[8] proposes a sister-taxon relationship with the Komodo dragon based on neurocranial similarities, with the lace monitor as the closest living Australian relative. Conversely, the perentie is considered more closely related to the Gould's and 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.[5] 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).[9] 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).[10] decrying the earlier maximum length estimate of 7 m (23 ft) as an 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).[11]

In a book published in 2004, Ralph Molnar[5] 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 metres (26 ft), while if its tail-to-body proportions were more similar to that of the Komodo dragon (Varanus komodoensis), then a length of 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.[5]

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 like 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 in between the eyes, and a jaw full of serrated blade-like teeth.[6]

Some scientists regard with skepticism the contention that megalanias were the only, or even principal, predators of the Australian Pleistocene megafauna.[12] 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 Thylacoleo carnifex with its wide distribution across Australian Pleistocene deposits. Quinkana, a genus of terrestrial crocodile that grew up to 6m and was present until around 40,000 years ago, has also been marked as another apex predator of Australian megafauna.

It has been suggested that, if one were to reconstruct the ecosystems that existed before the arrival of the humans on Australia, it would be desirable to introduce Komodo dragons to represent megalania.[13]

A study published in 2009 utilizing Wroe's earlier size estimates and an analysis of 18 closely related lizard species estimated a sprinting speed of 2.6–3 m/s (5.8–6.7 mph). This speed is comparable to that of the extant freshwater crocodile (Crocodylus johnstoni).[14]

Venom

Hypothetical Megalania skull, at Museum of Science, Boston

Along with other varanid lizards, such as the Komodo dragon and the lace monitor, megalania belongs to the proposed clade Toxicofera, which contains all known reptile clades possessing toxin-secreting oral glands, as well as their close, non-venomous relatives, including Iguania, Anguimorpha, and Serpentes.[15][16][17] Being a member of Anguimorpha, megalania may have been venomous and if so, would be the largest venomous vertebrate known.[18]

Claims of survival today and in stories

While there are occasional reports from Australia and New Guinea of giant lizards similar to megalania,[19] these reports only began after the species was first described and became publicly known.[5] There is no credible scientific evidence of the existence of a surviving population,[5] with the youngest reliably dated fossil records of giant monitor lizards in Australia being around 50,000 years old.[2]

References

  1. 1 2 3 Owen R. (1859). "Philosophical Transactions of the Royal Society of London" 149: 43–48. JSTOR 108688.
  2. 1 2 3 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. doi:10.1016/j.quascirev.2015.08.013.
  3. ↑ "Wildfacts - Megalania, giant ripper lizard". BBC. 2008. Retrieved 2012-03-22.
  4. ↑ 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. Varanoid lizards of the world. Bloomington: Indiana University Press. p. 45. ISBN 0-253-34366-6.)
  5. 1 2 3 4 5 6 Molnar, Ralph E. (2004). Dragons in the dust: the paleobiology of the giant monitor lizard Megalania. Bloomington: Indiana University Press. ISBN 0-253-34374-7.
  6. 1 2 3 Molnar RE (2004). "History of monitors and their kin". In King, Ruth Allen; Pianka, Eric R.; King, Dennis. Varanoid lizards of the world. Bloomington: Indiana University Press. p. 588. ISBN 0-253-34366-6.
  7. ↑ Lee MSY (1996). "Possible affinities between Varanus giganteus and Megalania prisca". Memoirs of the Queensland Museum 39: 232.
  8. ↑ 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: 445–457. doi:10.1111/j.1096-3642.2008.00448.x.
  9. ↑ 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.
  10. ↑ 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. Retrieved 2012-03-22.
  11. ↑ 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. doi:10.1073/pnas.0810883106. PMC 2690028. PMID 19451641.
  12. ↑ 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.
  13. ↑ Flannery, Tim (2002). The future eaters: an ecological history of the Australasian lands and people. New York: Grove Press. ISBN 0-8021-3943-4.
  14. ↑ Clemente CJ, Thompson GG, Withers PC (2009). "Journal of Zoology". Journal of Zoology 278 (4): 270–280. doi:10.1111/j.1469-7998.2009.00559.x.
  15. ↑ 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.
  16. ↑ 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" (PDF). Nature 439 (7076): 584–588. Bibcode:2006Natur.439..584F. doi:10.1038/nature04328. PMID 16292255.
  17. ↑ Barry C (2009). "Komodo Dragons Kill With Venom, Researchers Find". National Geographic News. Retrieved 2012-03-22.
  18. ↑ 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" (PDF). PNAS 106 (22): 8969–74. Bibcode:2009PNAS..106.8969F. doi:10.1073/pnas.0810883106. PMC 2690028. PMID 19451641.
  19. ↑ "Australian Giant Reptilian Monsters - Queensland Reports". Retrieved 2012-03-22.
This article is issued from Wikipedia - version of the Wednesday, March 30, 2016. The text is available under the Creative Commons Attribution/Share Alike but additional terms may apply for the media files.