Therapsids were the ancestors of mammals. The group appeared in the Permian; however, the oldest known mammal only dates back to the Jurassic. The fossil record of synapsids is one of the most extensive of any groups of vertebrates. This fossil record has been used to illustrate the concept of evolution and to test macroevolutionary patterns. The largest gap in this fossil record is between the Permo-Carboniferous synapsids and therapsids.