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 attachment of jaw muscles, allowing for stronger bites. 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.