The earliest report of bone material found within the lava tubes at Craters of the Moon dates back to the 1880s. A variety of bovine-like bones were collected from the lava tubes, but are too fragmentary or poorly preserved for definitive identification. The remains of bighorn sheep (Ovis canadensis), including a horn, have also been found in the monument. Packrat middens within the lava tubes contain bone material including the remains of microtine rodents. An grizzly bear (Ursus arctos) skull and femur were found just south of the old monument boundary. In a rare example of fossils preserved in igneous rocks, dozens of tree mold impressions are preserved in the basaltic lava flows and in lava tubes. These impressions were formed as lava flowed around a fallen tree. The molds typically show shrinkage cracks. Moisture in the wood may have prevented incineration of the trees. Similar tree molds are known from other NPS areas including El Malpais National Monument, Hawaii Volcanoes National Monument, Lava Beds National Monument, and Pu'uhonua o Honaunau National Historic Park.