Here we report the occurrence of a juvenile ankylosaur from the Upper Cretaceous locality Ukhaa Tolgod in southern Mongolia. The locality is well known for its exquisitely preserved theropods, mammals, and squamates, but until now has not yielded diagnostic ankylosaur material, although ankylosaur specimens are common at the site. The new specimen consists of a nearly complete skull with associated mandible and osteoderms that exhibits the following ankylosaurid synapomorphies: two pairs of osteodermal “horns” projecting from the quadratojugals and squamosals; a wide, triangular skull; a premaxillary beak edge that is not continuous with the maxillary tooth row; and absence of premaxillary teeth. We refer the specimen to Pinacosaurus grangeri based on the presence of a large premaxillary sinus, a quadrate not co-ossified with the paroccipital process, and several pairs of accessory openings in the narial region. The new specimen differs from the holotype and other specimens in having five pairs of openings in the narial region, indicating that extensive morphological variability exists in the narial anatomy of P. grangeri.

The specimen is identified as a juvenile based on its small size and the incomplete fusion of secondary dermal ossifications to the skull roof, exposing sutural boundaries. Juvenile ankylosaur skulls are rare, but crucial for understanding the basic anatomy of the highly fused and apomorphic adult skull. Morphological data from the new skull and other specimens are added to existing phylogenetic analyses of the Ankylosauria in order to identify diagnostic characters that aid in resolving ankylosaur relationships.

The specimen also provides data on the ontogenetic sequence of secondary dermal ossification. Presence of dermal ossifications covering only the narial region, quadratojugals, and squamosals supports the hypothesis that osteoderms in these areas appear early in ontogeny. Furthermore, two well-developed osteoderms were found in close apposition but unfused to the ventrolateral edges of the mandible. All known mandibles from adult ankylosaurs exhibit fusion of these elements to the underlying bones. Thus, mandibular osteoderms also appear relatively early but do not become fused until much later.