In contrast, today's largest insects have average wingspans of less than a foot!
So, why the shrinkage in size? The prehistoric atmosphere contained 50 percent more oxygen than our current environment. This extra O2 gave prehistoric creatures more energy per breath, allowing them to power larger bodies.
According to new research by Matthew Clapham, a paleobiologist at the University of California Santa Cruz, suggests we have the evolution of birds to thank for not being constantly attacked by giant insects when we venture outside.
During the Jurassic period (even as oxygen levels were rising), insects stopped growing larger as birds started to evolve. Clapham's work suggests that birds reversed the evolutionary favoring of large sizes. Large insects quickly became much less maneuverable, making them easy targets for agile birds.
Like the insect, evolutionary pressure also led to a decrease in the size of sloths. During the Pleistocene period (roughly 2.6 million years ago to 10,000 years ago), the Megatherium roamed South America. This sloth relative weighed in at a whopping 7 tons and was outfitted with foot-long toenails.
It's believed that the evolution of a superior predator, in this case early humans, caused the extinction of Megatherium and led other sloth species to decrease dramatically in size, making them faster and more able to hide. Today's sloths weigh approximately 15 pounds, about the equivalent of one of those giant claws!
To learn more about other prehistoric giants (including some that are still roaming the Earth), listen to our latest episode of A Blue View: