Synapsids are a group of animals that include mammals and their ancestors. They are characterized by having a single temporal fenestra, an opening in the skull behind the eyes, which distinguishes them from other reptiles. This feature is important for the evolution of jaw muscles and has played a significant role in the development of mammalian traits. The earliest synapsids appeared during the Carboniferous period over 300 million years ago. They evolved into various forms, including the well-known dimetrodon, which lived during the Permian period. Over time, synapsids gave rise to mammals, making them a crucial part of vertebrate evolution.