No recent animal group has evolved out of reptiles because reptiles did not give rise to reptiles in the first place. To understand this, we need to go back to the origin of vertebrates that adapted to permanent terrestrial life without relying on water for reproduction. While amphibians are the first to come to mind when we think of animals dependent on water for reproduction, it is not reptiles that are independent of water for reproduction. Allow me to explain:

The earliest fully terrestrial vertebrates were not reptiles. They laid eggs with shells, had scales, and walked on sprawling limbs. They were indeed cold-blooded and had a reptile-like appearance, but they were not reptiles or at least not all of them.

The broad term that encompasses all terrestrial vertebrates excluding fish is “Tetrapoda,” which means “four-limbed.” After amphibians adapted to life on land and could lay eggs with shells that did not require water for development, they became part of the group called “Amniota.” This group includes all animals that lay shelled eggs.

At the beginning of amniote evolution, three distinct groups emerged based on differences in their skull structure: Anapsida (no openings behind the eye socket), Synapsida (one opening), and Diapsida (two openings).

All species of Anapsida are now extinct, but every land-dwelling animal alive today belongs to either the Synapsida or Diapsida group. Mammals and their ancestors belong to the synapsid clade, while reptiles are part of the diapsid clade.

Mammals did not evolve from reptiles because the split that gave rise to these two groups occurred before the existence of reptiles. Although the ancestors of mammals shared similarities with reptiles in terms of their appearance, it’s important to consider the presence of openings behind the eye socket. Mammalian ancestors always had a single opening.

Dimetrodon, a well-known extinct creature, is often mistaken for a dinosaur by the general public. However, it is not a dinosaur or even a reptile. It is a synapsid! Surprisingly, it is more closely related to mammals, including humans, than to a T. rex. While not a direct ancestor, it falls within the group of stem-mammals. Despite its basal nature, it exhibits similarities in skull anatomy and already had different types of teeth.