Don't mess with homo sapiens sapiens.
Yesterday, I stumbled across an Animal Planet special about a probably-extinct giant reptile called the megalania. Topping out at twenty feet long and upwards of half a ton in weight, the megalania's bite injected a toxic bacteria into its victims which caused them to expire of blood poisoning in the unlikely event the prey managed to escape. It is believed that the creature could see into the infrared spectrum and hunted by heat signature.
The megalania has the distinction of being the largest reptile to coexist with man, and the aboriginal settlers had the misfortune of being the humans who ran into this fearsome predator. The aboriginal oral histories preserve accounts of the beast.
But the megalania proved to be even more unfortunate, meeting a much deadlier foe. Fast, strong and largely unaffected by the humans' primitive weaponry, the aboriginal settlers realized there was no way to stand toe-to-toe with the reptile. After losing people to the megalania, the aboriginals quickly adapted. Recognizing that lizards are slower in the morning, they scouted the general section of the grasslands where the creature lived and broke out their weapon of mass destruction: Fire.
Staking out that area in the early morning, the settlers set fire to the region. By the time the megalania roused itself sufficiently to the danger, it was barbeque.