CHEYENNE -- The lichen species thought to have killed nearly 300 elk in extreme south-central Wyoming this year was blamed for sheep and cattle deaths a half-century ago.

Parmelia molliuscula, commonly referred to as ground lichen, grows on the soil surface from Nebraska to North Dakota and west to the Rocky Mountains, according to information provided by the Wyoming Game and Fish Department.

The toxic ingredient the lichen produces is usnic acid, which, in mild cases, causes ataxia, or an inability to move muscles, particularly in an animal's hind legs.

"In severe cases, posterior paralysis is followed by total paralysis of the extremities and the animal is unable to rise," states the literature produced by the Game and Fish Department.

The Wyoming Agricultural Experiment Station and the U.S. Department of Agriculture conducted experiments with the lichen in the last century, prompted by cases of illness in sheep and cattle observed on the range.

Consumption of the lichen in the amount of 1 percent of an animal's body weight per day for five days was found to be lethal, as was a single dose of 3.6 percent of an animal's weight.

The plant remains toxic throughout the year, but most cases of poisoning in animals occur in winter when other forage is most scarce.

Parmelia molliuscula is composed of gray-green, flattened expanses of crisp or leathery tissue that is twisted and tangled into open masses a few inches in diameter.

University of Wyoming Professor of Toxicology Merl Raisbeck said the plant grows about 2 inches tall and can be found in many places across the state.

Raisbeck is not ready to pin the elk deaths on Parmelia molliuscula, because complete test results will not be available until Wednesday, and lichens can be complicated.

He said there are a variety of organic acids that are produced by lichens in general.

A Web site citing F.H. Elmore's "Ethnobotany of the Navajo" from 1944 indicates that the Navajo used the lichen as a remedy for impetigo and to produce a red or yellow-orange dye.