New subclass of birds from the Cretaceous of South America

Current classification of birds recognizes three subclasses which are morphologically distinct: the Archaeornithes for Archaeopteryx, the Odontornithes for the Hesperornithiformes and the Ichthyornithiformes, and the Neornithes for all modern birds and their extinct immediate relatives. (Some authorities¹ prefer different names for some of these taxa.) I have examined new material recently discovered in the Upper Cretaceous rocks of Argentina which indicates the existence of a group of birds having features so different from those of the currently recognized subclasses that they seem to represent a fourth subclass, here named the Enantiornithes (`opposite birds'). I describe unique features of the Enantiornithes which include a reduced outer metatarsal, in some forms an extreme modification of the remaining elements of the tarsometatarsus, a highly modified pectoral girdle, and sometimes a characteristic perforation in the proximal end of the humerus.