Suborder of reptiles, which appeared in the Carboniferous and became extinct after the early Permian. The suborder includes the earliest known reptiles, retaining some amphibian features, but distinctly reptilian in the form of their skulls, and differing from amphibians in the structure of the pelvic girdle. Because they were ancestral to all later reptile groups, the cotylosaurs are known as ‘stem reptiles’.