Claim: Alopecorhinus, a non-mammalian synapsid described in paleontology (the study of prehistoric life forms on Earth) in 1912, is 300 million years old.
Evidence: Alopecorhinus is an extinct genus of therocephalian therapsids. Like other non-mammalian synapsids, therocephalians were once described as "mammal-like reptiles". Synapsids are a group of animals that includes mammals and every animal more closely related to mammals than to the other members of the amniote clade, such as reptiles and birds. This informal term consists of all synapsids that are not therapsids, a monophyletic, more advanced, mammal-like group. The non-mammalian synapsids were described as mammal-like reptiles in classical systematics, but this misleading terminology is no longer in use as synapsids as a whole are no longer considered reptiles. They are now more correctly referred to as stem mammals or proto-mammals. 
Therapsids evolved from "pelycosaurs", specifically within the Sphenacodontia, more than 275 million years ago. Kingdom: Animalia Phylum: Chordata Clade: Therapsida Suborder: †Therocephalia Family: †Scylacosauridae Genus: †Alopecorhinus
Broom, 1912