The U.S. Fish and Wildlife Service has recommended that the eastern massasauga rattlesnake be listed as a threatened species under the Endangered Species Act.
The massasauga is small and thick-bodied rattlesnake with a heart-shaped head that lives in wetlands and uplands in several states, including Wisconsin, Pennsylvania, Ohio, New York, Minnesota, Michigan, Iowa, Indiana, Illinois, and even Ontario Canada. Here is a picture of the snake, courtesy of Dick Dickinson.
Of all of the locations where the massasauga can be found, less than half of its historical populations have been confirmed to still exist. The population decline of the snake is mostly due to habitat loss. The Fish and Wildlife Service's proposal appears in the September 30, 2015 Federal Register, opening a 60-day public comment period.
Here is a video description of the snake, courtesy of the Michigan Department of Natural Resources.