Simplicidentata

Simplicidentata is a group of mammals that includes the rodents (order Rodentia) and their closest extinct relatives. The term has historically been used as an alternative to Rodentia, contrasting the rodents (which have one pair of upper incisors) with their close relatives the lagomorphs (which have two). However, Simplicidentata is now defined as including all members of Glires (the clade formed by lagomorphs and rodents) that share a more recent common ancestor with living rodents than with living lagomorphs. Thus, Simplicidentata is a total group that is more inclusive than Rodentia, a crown group that includes all living rodents, their last common ancestor, and all its descendants. Under this definition, the loss of the second pair of upper incisors is a synapomorphic (shared derived) feature of Simplicidentata. The loss of the second upper premolar (P2) has also been considered as synapomorphic for Simplicidentata, but the primitive simplicidentate Sinomylus does have a P2.