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 a lvarezsaurid (Theropoda: Aves) specimens in the Late Cretaceous beds o f Mongolia's Gobi Desert(1), Here we describe a new taxon that is clos ely related to Mononykus(2,3). This new taxon is represented by two ex quisitely preserved skulls-the first known for Alvarezsauridae-details of which support the theory that the group is related to birds(4,5). This theory was first put forward on the basis of primarily postcrania l evidence(2,3), including the presence of avian characteristics such as the absence of a contact between the jugal and postorbital, and bet ween the quadratojugal and squamosal, articulations, Other earlier evi dence that suggested that the alvarezsaurids were birds included the p resence of a movable joint between the quadratojugal and quadrate, sep arate squamosal and braincase articulations of the quadrate, confluenc e between the caudal tympanic recess and columellar recess, a triradia te palatine, an unusually large foramen magnum, and the loss of a coro noid bone. The configuration of the temporal region of the skull and i ts articulation with the rostrum indicate the capability for prokineti c movement in which flexing occurs at the junction of the upper jaw an d neurocranium, and support the idea that prokinesis preceded other ty pes of avian intracranial kinesis.