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The Ice Age Lizards of Oz

Excavations at Colosseum Chamber. Credit: Gilbert Price

Excavations at Colosseum Chamber. Credit: Gilbert Price

By Gilbert Price

A chance finding in a Queensland cave has revealed that giant and dangerous lizards still lived when the first humans reached Australia.

There’s an old joke in reference to the wildlife in Australia that “everything is trying to kill you”. While that might be a fun way to scare tourists, there is no joking about the murderous killer lizards of the last Ice Age. In fact, we have just uncovered the first fossils to show that those huge lizards were still stalking the bush when the first people migrated from South-East Asia to the Australian continent.

Imagine being one of those first human inhabitants of Australia. It’s around 50,000 years ago, and you’ve just finished a most extraordinary sea journey from tropical South-East Asia. You’ve already said goodbye to your family and friends and are about to begin life in a foreign southern land where the climate, landscape, vegetation and animals are completely different.

It’s a scary enough image as it is, but throw in the giant predators of the last Ice Age and that image becomes truly terrifying.

The biggest mainland carnivorous mammals included a couple of species of “Tasmanian” devil, as well as the thylacine. They were fearsome creatures in their own right, but by far the undisputed top mammalian predator was the marsupial lion Thylacoleo. This was not a lion at all, but rather a 150 kg pouched predator more closely related to wombats and koalas.

The crocodiles included not just the salties and freshies of today but a huge inland beast, the 5-metre long Pallimnarchus. This ancient crocodile was found in the northern half of the continent, from the coastal fringes all the way into the watercourses of the central Lake Eyre basin.

The earliest Australians could have avoided the waterways if they didn’t want to encounter this freshwater giant, but there was no getting away from crocodiles altogether. Quinkana, another Ice Age croc, was slightly smaller in size than Pallimnarchus but what made it unique was that it was land-based, and could spend a long time away from sources of permanent water.

In terms of snakes, the biggest by far was Wonambi. With a girth of 80 cm and length around 6 metres, this 250 kg predator far outweighed any of the modern massive snakes like the boa constrictor and anaconda.

But among the scariest and most ferocious Australian Ice Age predators were the giant monitor lizards. When the first modern humans island-hopped through South-East Asia en route to Australia, they would have gained a feel for what life was like living among big lizards like the Komodo dragon, a species that is today restricted to only a handful of islands in the Indonesian archipelago. But what greeted them on the other side of their marine journey was a big lizard guild unlike anything that they had experienced before.

It may have been a surprise that, despite the treacherous sea crossing, Komodo dragons had already beaten them to the punch and were already established on the continent. In fact, the oldest fossil records of Komodo dragons in Australia date all the way back to the Pliocene, some 3.5 million years ago. It seems likely that Komodos actually evolved in Australia before moving northwards.

The largest monitor lizard was by far the giant Megalania (Varanus priscus). At 5–6 metres in length and weighing 300–500 kg, Megalania was the biggest lizard that ever existed, anywhere and at any time, in the history of the planet. Megalania had a huge distribution too, from the north-east and central regions all the way down to south-eastern Australia – and just about everywhere in between.

To top all that, another massive monitor lizard, intermediate in size between Megalania and the Komodo dragon, has also been reported from central Australian fossil deposits, but this has not yet been formally described in the scientific literature.

Some of the big questions then are what was the nature of the interaction between the earliest people and these predatory beasts? Did humans regularly end up on the dinner plate of a Megalania? Or was it the other way around? When did the giants go extinct, and how?

There are a couple of ways to try to answer these important questions, but one of the most obvious things to do first is to determine the age of the megafauna fossils and any associated archaeology. If you can at least place humans and the extinct predators in the same place at the same time, the hypotheses surrounding their extinction can only then be properly tested.

A Chance Discovery

Over the past 10–15 years, my research team has been exploring the Ice Age fossil record around the Rockhampton area along the central coast of Queensland. We had already discovered some really amazing fossil sites that showed huge habitat changes and extinctions of various species, but most of these records dated to more than 300,000 years ago, long before the first humans ever first stepped onto the continent.

One of the fossil sites that we’ve been really interested in is Colosseum Chamber, which is part of an extensive cave system at the Capricorn Caves tourist park, just north of the city. This particular deposit is around 2 metres deep, and is incredibly rich in fossil vertebrate remains.

If you have ever had the chance to see the cave, you’d ask why we would even be there looking for megafauna. However, big animals were not our target. Most of the fossils in the deposit are tiny, and not “mega” in any kind of way. They belong to little creatures like rats and mice, dunnarts and antechinuses, along with the odd possum and bandicoot.

What is striking, though, is the sheer abundance of fossil material. We once estimated that for every 10 litre bucket of sediment excavated from the floor of the cave, we’d have around 20,000 skeletal elements. That’s a seriously rich fossil deposit. What we were hoping to do was look at how the small fauna responded to climate changes during the last Ice Age, not what happened to the megafauna.

The majority of animals that ended up in the cave were derived from the feeding activities of prehistoric owls. The owls typically roost in the caves during the daytime and leave to hunt at night. It’s pretty hard for an owl to pick up a kangaroo or wallaby, so its diet is mostly restricted to smaller critters like rats and mice.

The owls will return to the cave with their prey and begin to rip them apart with their talons and beak. They swallow their food in small portions and do their best to digest it. It’s hard for feathers, teeth and bone to pass through the digestive system, so they are coughed back up, with the resulting owl pellet ending up on the floor of the cave. It is those pellets, full of the remains of small critters, that make it into the fossil record.

About 2 years ago we were sorting through the bone fragments and noticed something unusual – a very weird but unique fossil of an animal that we were not expecting to ever find in Colosseum Chamber. The fossil was an osteoderm, a type of “skin bone” that grows under the scales of lizards. Its size and shape meant that it could only be from a massive extinct lizard: one of the giant monitors.

This specimen looks pretty boring. It’s only around 1 cm long and resembles a cylinder that has had its ends tapered and then slightly twisted on the long axis. At this size, though, it’s actually up to five times bigger than the equivalent osteoderm in the second-biggest lizard alive today, the South-East Asian water monitor. We’re not able to say definitively which of the extinct giant monitors it belongs to, but it was certainly a whopping animal.

Its input into the cave system is unlikely to have been from an owl. Rather, it is possible that an animal such as a quoll scavenged the carcass of a big dead adult monitor lizard, and dragged some skin-covered tissue into the cave to eat it in peace. Any uneaten skin would have subsequently decayed, leaving the hard, boney osteoderm to survive and become incorporated into the fossil record.

This seemingly ordinary bone told us an amazing story: not just that it was from a giant extinct monitor lizard, but the dating component of our investigation showed that it was just under 50,000 years old. That meant that this cold-blooded killer lizard overlapped in time with the earliest humans living in Australia.

Ice Age Cold Case

Over the past decade there was been enormous debate over who or what wiped out the megafauna. It wasn’t just the predators that were big during the last Ice Age, but there was an assorted range of giant herbivores too, including the 3000 kg wombat-like Diprotodon, a diverse mob of massive kangaroos, as well as huge emu-like birds and even monstrous land turtles. All up, around 90 megafauna species suffered extinction during this period.

Some researchers have argued strongly that humans wiped out the megafauna, either through overhunting or perhaps via indirect landscape modification such as artificially increasing the incidence of fires. However, a close reading of the existing datasets shows that only around 15 species of megafauna actually date to the time of humans in Australia; most of the other 75 or so species either significantly pre-date humans or have no reliable ages at all.

Such was the case with the giant monitors. There have been claims for geologically “young” lizards in archaeological contexts, but these claims have been dismissed by the same researchers who have argued that humans wiped out the megafauna. Paradoxically, though, if you remove those scant records from the existing datasets, there is no reliable evidence at all that humans and giant lizards actually overlapped in time – until now at least.

Prior to our new study, the youngest accepted records for lizards like Megalania were around 80,000 years old. For Komodo dragons they are closer to 300,000 years. In our new study on the Colosseum Chamber specimen, we were able to show, definitely for the first time, that humans and the giant monitors overlapped in time. We essentially extended the younger-end of the timeframe of the giant lizards of Australia by around 30,000 years. Our study has been published in Quaternary Science Reviews (www.tinyurl.com/z6cjh32).

So what does that mean for the lizards? Did humans wipe them out? Did the lizards prey on people? It’s hard to say at this stage, but so far we do not have any palaeontological or archaeological evidence that they actively encountered each other. We’d be surprised if they didn’t though.

We Need More Data

One thing that’s yet to be definitely answered is exactly when the giant lizards became extinct. Our study statistically modelled the possible timing of the extinction event using all of the reliable dates available for the extinct lizards. The models that we applied are well-established and are the exact same ones that have been used to estimate when other famous organisms, like the Dodo, became extinct.

The modelling results showed something that, in a way, was not overly surprising to us: every model suggested that the giant monitors did not go extinct but are still living in Australia today!

We are not arguing that either the Komodo dragon or Megalania are still kicking around on the continent, but our analyses highlight just how patchy the existing datasets are for the extinct species – not just the monitor lizards but all Australian megafauna. With the addition of the Colosseum Chamber specimen, we now have only seven reliably dated records for giant lizards. Worryingly, this is one of the most extensively dated records among all of the Australian megafauna. It’s just not possible to reliably model megafaunal extinctions with such scant datasets, but we are getting closer.

We critically need more reliably dated records to test the leading extinction hypotheses properly. With more research, the temporal datasets for all of the extinct megafauna, not just the giant lizards, can be better developed. Only then will we really be able to establish a strong understanding of what happened to Australia’s Ice Age megafauna.

Gilbert Price is a vertebrate palaeontologist in the School of Earth Sciences at The University of Queensland. You can read his science blog at www.diprotodon.com.