|MadSci Network: Zoology|
Size matters, Talitha ! Once you have decided that the bone belongs to a land mammal, the length should give you a fairly accurate guide to the size range of the animal. Even in a continent with as diverse a fauna as N. America (did it come from there ?) there are relatively few animals to choose from within each size range. Having narrowed down your choice on the basis of size, the next thing to do is assign it to a major group, probably at the order level. Is it a carnivore, an artiodactyl, a lagomorph, etc ? One clue would be to look for signs of the fibula. In some mammal groups the fibula is fused to the tibia, in others it is free but may have distinctive articulations at one or both ends. It is the shape of the articulating surfaces at each end of a limb bone which give the most reliable clues to identification. The only reliable way to discover the nature of these articulations, and then to move on to your final identification, is with reference material. The differences in shape between limb bones are often so subtle that even the best drawings and photos are of limited use, and you need to be able to hold your bone side by side with a known example for a conclusive identification. There is also the added problem that very few such books exist, and they are all out of print, as far as I know ! Professional conservationists and zooarchaeologists always have a reference collection available for critical identifications. Don't give up just because you haven't got access to a top-class reference collection ! (You might have, if you ask at your local natural history museum or university zoology/archaeology department). If you can persuade your parents to have a roast leg of lamb next weekend - there's your artiodactyl reference specimen, and your school may have a dog or cat skeleton set up in the lab - there's your carnivore. Good luck - and remember that being able to explain the reasoning behind each step of your identification is as important as arriving at a final species name, which is not always possible.
Try the links in the MadSci Library for more information on Zoology.