Together with other large plant-eating mammals that are now extinct, they released around 9.6 million tonnes of the gas each year, experts estimated.
When the ”megafauna” disappeared there was a dramatic fall in atmospheric methane which may have altered the climate.
Analysis of gases trapped in ice cores suggests that the loss of animal emissions accounted for a large amount of the decline.
”The changes in methane concentration at this time seem to be unique,” said the researchers, writing in the journal Nature Geoscience.
Methane is a powerful greenhouse gas. The scientists, led by Dr Felisa Smith from the University of New Mexico in the US, pointed out that a ”cold event” hit the Earth at about the same time that methane levels plunged.
”Our calculation suggest that decreased methane emissions caused by the extinction of New World megafauna could have played a role..” they wrote.