The most famous of the giant Monitors is obviously Megalania, a huge lizard that inhabited southern Australia up until the arrival of humans, when it was wiped out in the Anthropogenic-extinction. Known only from fairly incomplete remains, despite being a very recent animal, Megalania has been the subject of much debate, chiefly over its size. Original estimates from when it was first discovered placed it at 7 meters in length, what that would have made it a truly enormous animal, but in 2002, Zoologist Stephen Wroe found it to be a great deal smaller, at only 4.5 meters in length. This was again changed in 2009, when Wroe himself along with other researchers upsized it to 5.5 meters. In truth, we have no idea how large Megalania really was, since we do not know of its proportions. Ralph Molnar said in 2004 that if it's proportions were like those of a Komodo dragon, it would have been 7 meters long, while if they were similar to a Lace monitor, it would be upwards of 8.
Varanus priscus, by Vlad Konstantinov
Another important thing to note about Megalania is that while it was certainly in a league of its own, far greater in size than any other known lizard, it still appears to have been a member of the genus Varanus, with its scientific name being Varanus priscus. This is a testament to the astounding diversity within Varanus, a genus which includes both the Komodo dragon and the Dampier Peninsula monitor. Because of this great diversity, it is also difficult to place V. priscus in the family tree, as while it looks superficially similar to the Komodo dragon, it may not have been particularly closely related to it at all. The fact that the Varanus keeps producing giants, many of which seem to have evolved independently of each other, means that there are probably many other species out there, waiting to be uncovered. Megalania was distributed across southern Australia, and the Komodo dragon once across the entirety of Flores, but the other islands in the archipelago most likely also hosted giant monitors at some point, even if we have not yet discovered them.
A Komodo dragon, the largest extant lizard
A particularly interesting point about the Komodo dragon is that while it is often portrayed as a case of island gigantism, an animal which evolved large size due to insular conditions and a lack of competition, the opposite may in fact have been true. Fossil evidence shows that the ancestors of the Komodo dragon actually evolved on mainland Australia, only recently spreading to Indonesia. While the Australian dragons are extinct, the great size difference between mainland giants such as Megalania and the Komodo dragon show that it may in fact have been an island dwarf.
While Varanus seems to have a propensity for spawning giant lineages, the question still remains as to how they lived, and what role they played in the ecosystems they inhabited. Obviously we can still study the Komodo dragon, but it is an insular species, and lives in an ecosystem mostly devoid of other species. It was not always like this however. 13,000 years ago, Flores was a very different place from today, populated by dwarf mastodons, tiny humans, giant storks, and many other odd island endemics. In this ecosystem, it seems that the Komodo dragon may have been the apex predator. Today the only remaining dragons are from small islands such as Komodo, and are probably smaller than the inland dragons, but we can assume that their sizes were fairly similar, give or take a few meters. In this environment, Komodo dragons may actually have preyed on the dwarf mastodons and tiny humans,
Megalania chasing a prehistoric Emu
While the Komodo dragons were sole rulers of their island kingdoms, mainland Australia was filled with predators of all shapes and sizes, some of which were larger than even Megalania. Thylacoleo, the marsupial lion, was, as the name suggests, a lion sized beast, and would have been one of the top predators at the time. Likewise, members of the now extinct Thylacine lineage would also have been abundant in prehistoric Australia, with some species significantly larger than the so-called "Tasmanian tiger" that was hunted to extinction in the early 20th century. But the true apex predators of Pleistocene Australia were not Mammals, but instead huge terrestrial crocodiles of the Mekosuchid lineage. While a predominantly small group for most of their history, they began rapidly increasing in size during the Pleistocene, eventually producing the genus Quinkana, which at 6 meters in length was possibly the largest fully terrestrial predator on the planet. Though possibly not as long as Megalania, this species would have been far heavier and more powerful, possibly even capable of chasing the giant Monitor away from its kills. While not the apex predator, Megalania would probably still have been one of the largest carnivores in its environment, dwarfing any of its contemporary Mammal predators.
But like all the other great Australian beasts, Megalania is now extinct, and has been so for 40,000 years. There has long been debate as to whether Megalania was driven to extinction by humans, or died out shortly before their arrival, but after a new study has firmly demonstrated that the early Aboriginals did coexist with them, it now seems likely that their demise was indeed at our hands. It has been suggested by some conservationists that Komodo dragons should be released onto mainland Australia to serve as ecological proxies for Megalania, which could in theory help heal the heavily depleted ecosystems of the continent. While I personally find this to be an interesting endeavor, I doubt it will happen anytime soon considering the country's current political climate.
Komodo, the island from which the Komodo dragon gets its name
While it is a great shame that the giant Monitors of yore are now gone, the remains they have left behind allow us to gain a glimpse of the world they inhabited. Still poorly understood, and lacking any complete fossils, there is yet much to be learned about them. In time however, if we continue looking, we may very well begin to uncover more of their secrets, and bolster our understanding of these fascinating animals. I look forward to the day when our understanding has progressed to the point where I can confidently declare this post obsolete.