The morphology and systematics of Avaceratops, a primitive horned dinosaurfrom the Judith River Formation (Late Campanian) of Montana, with the description of a second skull

The partial skull and skeleton of Avaceratops lammersi, an unusual ceratops id from the Judith River Formation of central Montana, are described. Avace ratops has several characters that are unique among ceratopsids though not among protoceratopsids. Plesiomorphies include: a primitive squamosal; a so lid parietal; lack of a caudosagittal indentation on the parietal; weakly d eveloped olecranon process of the ulna; deltopectoral crest of humerus not as distally expanded as in other ceratopsids; pointed pedal unguals; and re latively small size. A second, recently discovered partial skull that incor porates both centrosaurine and chasmosaurine characters is referred to Avac eratops. Lack of parietal fenestrae and the presence of postorbital horncor es are probably basal characters within the Ceratopsidae. The inferred adul t length of 4.2 m for Avaceratops is the smallest among known ceratopsids. Avaceratops is the least derived known ceratopsid, and hence is close to th e ancestry of Triceratops and the Chasmosaurinae.