From the Billings Gazette:
Five bison dropped dead in Yellowstone National Park apparently after being overwhelmed by toxic gases in a geothermal area.
The bison carcasses were found March 10 along the Gibbon River near Norris Geyser Basin. It’s likely the animals were killed by hydrogen sulfide or carbon dioxide trapped at ground level by unusually cold and windless weather, according to a report on the incident released Tuesday.