An exceptionally preserved herd of the ceratopsian dinosaur Psittacosaurus was recovered from the Lujiatun Beds of the Lower Cretaceous Yixian Formation, Liaoning Province, north-east China. This specimen provides the earliest direct evidence of post-nestling gregarious behaviour in this important dinosaur clade. The sediment in which the skeletons occur represents a lahar (volcanic mudflow), indicating that the six individuals present were entombed simultaneously during a catastrophic mass mortality event. Reconstruction of the herd age profile shows it was composed of offspring from at least two clutches, revealing that group fidelity in these dinosaurs extended beyond time spent together in the nest. This demonstrates that sociality appeared much earlier in ceratopsian evolution than is generally acknowledged and that such behaviour preceded the evolution of the elaborate horns and frills seen in derived ceratopsids.