Pick the difference
Many times an entire fossil species and its evolutionary relationships are claimed on the basis of one or a few bony fragments. But there are pitfalls, even when you have the whole skeleton, in using similarity to prove some evolutionary point.
Look at the three skulls in the diagram. B and C are clearly most similar. Yet A is of a Pekinese dog, B of another common dog, while C is that of a bear, which in evolutionary terms is regarded as only distantly related to dogs.
The two skulls which appear most dissimilar are in fact the same interbreeding species.
(Drawings from Origins: What Is At Stake?, by Wilbert H. Rusch, Sr, Creation Research Society Books, Terre Haute (Indiana), 1991, p. 26. Used with permission.)