Australia's giant, extinct monitor Varanus priscus chases a Bullockornis.
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Art by Peter Trusler, via Monash University on Flickr.
Australia's giant, extinct monitor Varanus priscus chases a Bullockornis.

Australia’s Giant, Venomous Lizard Gets Downsized

From time to  time, I’ve been accused of being a fossil killjoy. I pulverize childhood dreams like Diatryma crushed seeds (and not little horses). I’m not sure how true that is. I’ve yet to quantify how much of my writing destroys dreams versus geeking out over new discoveries. But today I have to own up to being a downer. In case you hadn’t heard, Australia’s extinct, giant monitor lizard wasn’t as monstrous as traditionally thought.

In 1858, when paleontology was still a young science, the anatomist Richard Owen read a paper before London’s Royal Society on some astonishingly large lizard bones that had been collected from Ice Age deposits in Australia. Specifically, Owen described a trio of vertebrae that measured three inches long and two inches high. These were far bigger than any lizard then known – the Komodo dragon wouldn’t be recognized by science for another 54 years – and, through a bit of rough anatomical math, Owen expected that this huge “land lizard” would have reached 20 feet from snout to tail.

Owen dubbed this gargantuan lizard Megalania prisca, and he had a little fun imagining that such a lizard might still clamber through the bush. “Whether among the vast and unexplored wildernesses of the Australian continent any living representative of the more truly gigantic Megalania still lingers, may be a question worth the attention of travellers,” Owen told his audience, although the hard-nosed scientist did concede that the lizard was most likely extinct. Either way, Owen concluded, the huge saurian “must have been carnivorous, and, by its bulk and strength, very formidable.”

While the fossil trail took some confusing turns as Owen and succeeding generations of researchers puzzled over fragments of Australia’s prehistoric reptiles, by 1975 paleontologists had settled the image of Megalania as a truly gigantic monitor lizard that ripped into Volkswagon Bug-sized wombats between 4 million and 30,000 years ago. This was late enough that the first people to arrive on Australia may have encountered the lizard, and Megalania was definitely not a squamate to be trifled with. The skeletal reconstructions put up at the Museum of Victoria and other institutions looked like Komodo dragons pumped up to almost 20 feet long. Despite all that had changed since 1859, Owen was right about one salient point – Megalania was one formidable lizard.

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A reconstruction of Varanus priscus at the Melbourne Museum. Photo by Cas Liber.

But Megalania ain’t what it used to be. For one thing, the lizard’s bones are so similar to those of other monitor species – belonging to the genus Varanus – that paleontologists have taken to calling it Varanus priscus. And while it seems likely that the big lizard was venomous, recent size estimates have shrunk this “dragon in the dust.”

Let’s have a look at the traditional baseline first. In 2004, working with the relationship between vertebrae size and body length, paleontologist Ralph Molnar proposed that mature Varanus priscus could have been between 23 and 26 feet long, depending on the anatomy of the tail. But other researchers think such sizes are major overestimates. In a 2002 study that critiqued “the myth of reptilian domination” in prehistoric Australia, anatomist Stephen Wroe reanalyzed old body size data and calculated that the lizard probably averaged about 11 feet in total length and, citing earlier estimates from Molnar, wouldn’t have grown much longer than 15 feet.

Size estimates in a 2012 paper by paleontologist Jack Conrad and colleagues came out in between the extremes. While describing a new, large Varanus species that once lived in Greece, the researchers also took a look back at Australia’s ever-contentious lizard. Without the tail, the Varanus priscus specimen in their study had an estimated body length of almost seven feet, meaning that this individuals total length was almost certainly longer than the 11 foot average Wroe suggested. Especially large specimens, Conrad and coauthors noted, could have had bodies almost 10 feet long with the tails trailing behind, although these animals still would have been smaller than the monstrous lizards paleontologists used to reconstruct.

The entire back-and-forth over the lizard’s size is only a small part of the story, though. Since Owen’s days, paleontologists have viewed Australia as a place where the Mesozoic clung to life – a land full of marsupials where reptiles tenaciously clung to predatory dominance. That view is changing into a more complex Pleistocene vision, with the island continent’s huge monitor playing the role of ambush predator among a variety of carnivores – mammalian and reptilian – that stalked the Australian Ice Age. At 11 feet or 20, it’d be difficult not to be impressed by a lizard of such scale, but the rapacious role of the long-lost monitor has yet to be pulled from the scraps the reptile left behind.

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[Top image by Peter Trusler, via Flickr.]


Molnar, R. 2004. Dragons in the Dust. Bloomington: Indiana University Press.