Cranial osteology and ontogenetic variation of the Late Cretaceous ankylosaurid dinosaur Pinacosaurus grangeri

Pinacosaurus grangeri is among the best-known ankylosaurids from the Late Cretaceous of Asia, yet most of the available information is based on juvenile specimens. Here, we describe three cranial specimens of P. grangeri recovered from the Upper Cretaceous Wulansuhai Formation at Bayan Mandahu, Urat Rear Banner, Inner Mongolia, China. These three specimens include the smallest (IVPP V 16855) and largest (IVPP V 34380) cranial individuals currently known for this species, thereby expanding the documented ontogenetic range. Based on the new morphological data, we refine the diagnosis of P. grangeri, identify cranial features that vary through ontogeny, and recognize additional variation that may represent sexual dimorphism. These results provide new insights into ankylosaurid cranial development, taxonomy, and intraspecific variation.