An extinct and specialized group of land fossil birds that lived during the Cretaceous Period. Enantiornithes represents the most diverse avian group in the Mesozoic. This group was not discovered until 1981, at which time some very unusual, isolated bones from the Late Cretaceous of Argentina were recognized as belonging to a new group of birds. Enantior- nithine birds are now known to be widely distri- buted, with remains from Argentina, North America, Mexico, Mongolia, Australia, Spain, and China. Because their anatomy includes a mixture of specialized and primitive features, they are thought to represent an evolutionary side branch in early avian evolution that became extinct at the end of the Cretaceous and left no living descendants.