synapsiden

Synapsida is a clade of amniote vertebrates that includes mammals and their extinct relatives. They are distinguished by a single temporal fenestra, a hole in the skull behind each eye. This opening allowed for the attachment of jaw muscles, leading to stronger bites. The earliest synapsids appeared in the Late Carboniferous period, around 312 million years ago.

Initially, synapsids were a diverse group, occupying a wide range of ecological niches. Some early forms were small and insectivorous, while others evolved into larger, herbivorous or carnivorous species. A key evolutionary development within synapsida was the gradual increase in size and complexity of the brain, as well as the development of a more upright posture and differentiated teeth, including incisors, canines, and molars.

The group experienced a significant evolutionary radiation during the Permian period. However, the end-Permian extinction event, also known as the Great Dying, severely impacted synapsid diversity. Despite this setback, a lineage of synapsids survived and eventually gave rise to the therapsids, which in turn produced the cynodonts. Cynodonts possessed many mammalian characteristics, including a secondary palate and a reduced number of bones in the jaw. Ultimately, one group of cynodonts evolved into the first true mammals. Today, mammals are the sole surviving lineage of the once widespread synapsid clade.