Here's why:
* Synapsids: These are a group of amniotes characterized by a single temporal fenestra (hole) in their skull. This group includes mammals, as well as their extinct ancestors.
* Therapsids: These are a diverse group of synapsids that emerged in the late Permian period. They are considered the closest relatives of mammals and possessed many features that are transitional between reptiles and mammals, including:
* More upright posture: Therapsids evolved a more erect stance, similar to mammals, unlike the sprawling posture of most reptiles.
* Larger braincase: They developed a larger braincase relative to their body size, suggesting increased cognitive abilities.
* Specialized teeth: Therapsids had differentiated teeth (incisors, canines, molars) for different functions, just like mammals.
* Secondary palate: They had a partially developed secondary palate, which separates the nasal passages from the mouth, allowing breathing while chewing.
* Hair-like structures: Some therapsids have been found to have hair-like structures, indicating the evolution of fur.
* Warm-bloodedness: Evidence suggests that some therapsids were endothermic (warm-blooded), a key trait of mammals.
While there are various therapsid lineages, the group known as cynodonts is considered the most likely direct ancestor of mammals.
It's important to remember that evolution is a complex process with many branching lineages. While therapsids are the closest known relatives, there might be other extinct synapsids that played a role in the evolution of mammals.