Flexi Says: During the Carboniferous Period (about 359 to about 299 million years ago), two groups of amniotes evolved: the Sauropsida and the Theropsida. The sauropsids ("lizard-face") gave rise to reptiles, dinosaurs, and birds, and the theropsids (“beast-face”), also known as synapsids, gave rise to mammals. The earliest known sauropsid reptiles were called Hylonomus, and the earliest known theropsid was called Archaeothyris. These small lizard-like animals quickly gave rise to many descendants.
