Transitional fossils

Transitional fossils are the mineralized remains of organisms that provide insight in the evolutionary transition of one major category of organisms into another.

When Charles Darwin's Origin Of Species was first published, the fossil record was poorly known, and the claim that there was a lack of transitional fossils was perfectly reasonable. However, the discovery of Archaeopteryx only two years later was seen as a stunning triumph for Darwin's theory of common descent. Similarly, O.C. Marsh provided groundbreaking evidence of transitional fossils in the evolution of horses. Since that time, many more transitional fossils have been found.