Amniotes are a group of animals that managed to colonize a dry and terrestrial environment. For this, a series of modifications such as keratinized skin, lung respiration and oviparous reproduction were adapted in a first step. Those who maintained a cranial structure without orifices (anapsid amniotes) are currently represented by reptiles.

However, their arrival on the mainland required, to increase survival, the development, in a second step of the senses of hearing and sight.

For this, they presented modifications of the cranial bone structure forming two groups: synaptic amniotes, who are currently represented by mammals, and diapsid amniotes, who are currently represented by birds.

These modifications that affect the cranial structure allow to maintain a common reptiloid brain in the three groups of amniotes (reptiles, mammals and birds) but mammals and birds were able to develop new brain structures (thermal control, binocular vision and hearing development).