The skull of a relative of the stem-group bird Mononykus

In joint expeditions, researchers from the American Museum of Natural History and the Mongolian Academy of Sciences have recovered over 20 alvarezsaurid (Theropoda: Aves) specimens in the Late Cretaceous beds of Mongolia's Gobi Desert1. Here we describe a new taxon that is closely related to Mononykus2,3. This new taxon is represented by two exquisitely preserved skulls — the first known for Alvarezsauridae — details of which support the theory that the group is related to birds4,5. This theory was first put forward on the basis of primarily postcranial evidence2,3, including the presence of avian characteristics such as the absence of a contact between the jugal and postorbital, and between the quadratojugal and squamosal, articulations. Other earlier evidence that suggested that the alvarezsaurids were birds included the presence of a movable joint between the quadratojugal and quadrate, separate squamosal and braincase articulations of the quadrate, confluence between the caudal tympanic recess and columellar recess, a triradiate palatine, an unusually large foramen magnum, and the loss of a coronoid bone. The configuration of the temporal region of the skull and its articulation with the rostrum indicate the capability for prokinetic movement in which flexing occurs at the junction of the upper jaw and neurocranium, and support the idea that prokinesis preceded other types of avian intracranial kinesis.