The point I was making was that it is easy to see the differences between the different related species when we have the animals alive with us today. We can visibly see the difference between a brown bear and a polar bear. We can also do post mortems on these animals with a 100% certainty that the bones are either a brown bear or a polar bear.
In the case of fossils however, we don't have the physical form available. We don't have access to the DNA. We are unlikely to have the full skeleton even. There is enough variation within the brown bear stock to confuse leg bones, ribs and necks etc with those of a polar bear. This makes finding a common ancestor virtually impossible.