I saw this yesterday. I'll ask the same question I wondered when I first saw it: How the hell do they know this?
They admit that it is based on assumptions (
http://www.cbsnews.com/8301-205_162-57429334/dinosaur-gas-may-have-warmed-prehistoric-earth/):
"They used a mathematical model to determine how much gas these plant-eating giants would have eaten. They extended data on methane production by modern mammals, based on size, up into the reaches of the sauropods.
In their calculations the researchers used middle-of-the-road numbers: 10 sauropods, each weighing 20,000 pounds (9,071 kilograms), could have roamed 1 square kilometer of lush Mesozoic habitats. "We've taken a middle-ground value," Wilkinson said. "We tried to be reasonably conservative."
They found that these 10 sauropods would have contributed 7.6 tons (6.9 tonnes) of methane every year. Expanding this number to cover the amount of land estimated to be hospitable habitat for these animals (about half the land on Earth at the time), the researchers end up with more than 550 million tons (500 million tonnes) of methane produced every year.
"I was expecting a number like that produced by cows, so the size of the number really surprised me," Wilkinson said. "It's way, way, way ahead of the estimated methane production by modern livestock." (Cows produce 55 to 110 million tons (50 to 100 million tonnes) of methane each year, he estimated.)"