A fossil of an organism between two clearly defined stages of evolutionary development ?

An example of a fossil that represents an organism between two clearly defined stages of evolutionary development is the Archaeopteryx. Archaeopteryx is a transitional fossil that shows characteristics of both dinosaurs and birds, indicating a transition from reptilian ancestors to modern birds. It has features such as feathers and wings like birds, but also teeth and a long bony tail like dinosaurs. This fossil provides evidence for the evolutionary link between dinosaurs and birds.