What was the first reptile?

The oldest known reptile was the hylonomus. It was around 25 centimeters (10 inches) long. Reptiles developed from amphibians and the first true reptiles had a solid skull with holes only for nose, eyes and a spinal cord. These early reptiles gave rise to another line called synapsids, which had another pair of holes in their skulls behind the eyes. Synapsids developed into mammals.