The Permian-Triassic extinction event (aka. The Great Dying) wiped out 90 to 95% of all the known animals on Earth.
Diictodon. Of course, many other animals have survived, like insects, mollusks, spiders, scorpions, fish, amphibians, cynodonts, therocephalians, diapsids, etc. Although these were generally small and survived due to their adaptability and oportunistic behaviour. After this, the world would recover, but synapsids will no longer rule that much, leaving diapsid reptiles to rule the Earth.