Numerous well-preserved bird, mammal, and insect-larva? tracks were discovered in the lowermost part of the delta facies of the middle Eocene Green River Formation. The area was probably a near-shore shelf area of the Eocene Uinta Lake that frequently became an emergent mudflat. The preservation of the tracks provides a record of an organic community that is not known from conventional skeletal remains in this area. The level to which animal tracks may be classified taxonomically depends upon the group represented, the geologic age of the tracks, the tracking conditions, and the variations resulting from differences among the individuals of a particular species.