Nearly all vertebrate fossils are represented by the bones of the deceased specimen. These can be in the form of partial or complete skeletal remains, but for many species individual bones are all that has been found. This partial evidence of a species may lead to disputes as to its classification and many are classed as Nomen dubium, indicaring that their relationship with other species is not known. Sometimes direct impressions such as skin, feathers or scales are left behind, usually because the sediment is fine and the fossil hasn't undergone any major changes such as compression. Other times trace fossils are left, in the form of coprolite (feces), footprints, burrows and the like.