The dome-headed pachycephalosaurians are among the most enigmatic dinosaurs. Bearing a hypertrophied skull roof and elaborate cranial ornamentation, members of the clade are considered to have evolved complex sociosexual systems1–3. Despite their importance in understanding behavioural ecology in Dinosauria, the absence of uncontested early diverging taxa has hindered our ability to reconstruct the origin and early evolution of the clade4–7. Here we describe Zavacephale rinpoche gen. et sp. nov., from the Lower Cretaceous Khuren Dukh Formation of Mongolia, the most skeletally complete and geologically oldest pachycephalosaurian discovered globally. Z. rinpoche exhibits a well-developed frontoparietal dome and preserves the clade’s first record of manual elements and gastroliths. Phylogenetic analysis recovered Z. rinpoche as one of the earliest diverging pachycephalosaurians, pushing back fossil evidence of the frontoparietal dome by at least 14 Myr and clarifying macroevolutionary trends in its assembly. We found that the earliest stage of dome evolution occurred by means of a frontal-first developmental pattern with retention of open supratemporal fenestra, mirroring proposed ontogenetic trajectories in some Late Cretaceous taxa. Finally, intraskeletal osteohistology of the frontoparietal dome and hindlimb demonstrate decoupling of sociosexual and somatic maturity in early pachycephalosaurians, with advanced dome development preceding terminal body size.
