Definitions for "Pelycosaurs"
Reptiles with large, sail-like fins that may have helped to regulate body temperature (also called "sail-back reptiles")
Finback reptiles, such as Dimetrodon, which were the dominant reptile group in the Permian Geologic Range: Pennsylvanian to Late Permian
The more primitive of the two groups of (extinct) synapsids or mammal-like reptiles (cf. therapsids). Several sported large, sail-like dorsal fins.