phenacodontids

Phenacodontids are an extinct group of synapsids, a class of mammals that includes therapsids and mammals. They lived during the Late Permian and Early Triassic periods, approximately 270 to 245 million years ago. Phenacodontids are notable for their unique features, such as the presence of a single pair of incisors in the upper jaw, which is a characteristic shared with mammals. This feature, known as the "mammalian dental formula," is one of the key synapomorphies that define the group.

Phenacodontids were small to medium-sized animals, with some species reaching lengths of up to 1.5 meters. They had a diverse range of adaptations, including some that were well-suited for aquatic environments. Their fossils have been found in various parts of the world, including North America, Europe, and South Africa. The most famous phenacodontid is Phenacodus, a genus that includes several species with well-preserved fossils.

Despite their mammalian-like features, phenacodontids are not considered direct ancestors of mammals. Instead, they are thought to have been part of a diverse group of synapsids that evolved during the Permian-Triassic extinction event. The extinction of many phenacodontid species during this event may have contributed to the eventual rise of mammals in the Triassic period.