Archaeopteryx is more similar to small theropod dinosaurs than it is to modern birds. However, the evolutionary history of Archaeopteryx has never been that simple. Mammals evolved from a group of reptiles called the synapsids. These reptiles arose during the Pennsylvanian Period (310 to 275 million years ago). A branch of the synapsids called the therapsids appeared by the middle of the Permian Period (275 to 225 million years ago). So we concluded that both birds and mammals were evolved from reptiles.