Barbaturex morrisoni, or the Bearded King Morrison, evolved to its unusual size in response to elevated global temperature during a period known as the Paleocene greenhouse during the Eocene epoch.
Lead researcher Jason Head from the University of Nebraska-Lincoln believes his study will show how climate helped the reptile evolve.
The giant lizard weighed up to 60 pounds, and unlike modern large reptiles, which are carnivores, it ate only plants. "The large size of the Lizard King certainly protected it from many predators. But there is no doubt that it was hunted by mammalian carnivores of the day," Head said in a release.
But competition with herbivorous mammals and predation by carnivores didn't prevent the Bearded King Morrison from becoming so large.
Lizards are cold-blooded, and depend on external warmth to heat their bodies. When their environment warms they become more active, eating and growing more.
Global average temperatures are now only about 2.5 degrees Celsius short of where they were 40 million years ago, but climate change likely won't result in larger lizards.
"We're changing the atmosphere so fast that the rate of climate change is probably faster than most biological systems can adapt to. So instead of seeing the growth and spread of giant reptiles, what you might see is extinction," he said.
The Doors' late frontman was known to stand up for environmental causes, and Head said he "was listening to The Doors quite a bit during the research."
"Some of their musical imagery includes reptiles and ancient places, and Jim Morrison was of course 'The Lizard King,' so it all kind of came together," said Head.
Head and his team publish their findings Wednesday in the research journal Proceedings of the Royal Society B.