The Paleozoic era occurred between 542 and 250 million years (Ma) ago and it was marked by important events in the biosphere, including the vertebrate transition from water to land and the Permian–Triassic mass extinction.
Synapsids were land vertebrates that originated during the Permian period around 270 Ma. The group Synapsidia includes mammals and any other animal more closely related to mammals than to other extant amniotes (snakes, lizards, crocodiles, and birds). The non-mammalian synapsids are described as mammal-like reptiles in classical systematics, and can also be called stem mammals or proto-mammals.
Synapsids were the largest terrestrial vertebrates in the Permian period (299 to 251 Ma), and their diversity was dramatically reduced during the Permian–Triassic extinction. They had a widespread geographical distribution and occupied several different ecological niches, so nowadays we can find fossils of these extinct animals all around the world.
The most common Permian synapsids were the dicynodonts. They were the most successful and diverse group of synapsids, and over 100 species of dicynodonts have been found so far in the fossil record. Around 250 Ma nearly all synapsids disappeared, but a few exceptions survived this dramatic extinction: the dicynodonts, the therocephalians and the cynodonts. The dicynodonts are the ancestors of mammals.
Dicynodonts were herbivorous animals that could weigh from 2kg to 250kg. They had two tusks (hence their name, which means 'two-tooth dog'), short necks and their body was barrel-shaped with short, strong limbs. Dicynodonts had short tails and they were thought to be worm-blooded.