“Missing links” are extinct species in the fossil record that have anatomical features consistent with a possible ancestral anatomy  from which two diverging lineages originated.

The fossil of Archaeopteryx shares features of class Reptilia and class Aves. It is well feathered but has a skeleton that is much more like modern reptiles than modern birds.

Ichthyostega is one of several fossil species that appear to be intermediates between fish and amphibian lineages.