Glossary

**Head-butting** is a behavioral hypothesis proposing that pachycephalosaurid dinosaurs used their massively thickened frontoparietal domes to engage in intraspecific combat by striking their heads together, analogous to the agonistic behavior observed in extant bighorn sheep (Ovis canadensis) and musk oxen (Ovibos moschatus). The dome is formed by the fusion and hypertrophy of the frontal and parietal bones into a hypermineralized osseous structure reaching up to approximately 25 cm in thickness in the largest species, Pachycephalosaurus wyomingensis. The hypothesis is supported by finite element analyses (FEA) demonstrating that the dome could safely withstand impact forces, by a high incidence of cranial pathologies consistent with trauma-induced osteomyelitis concentrated at the dome apex in adult specimens, and by functional morphological comparisons with extant head-striking bovids. However, competing evidence from cranial histology, concerns about the rounded dome's poor suitability for precise head-to-head contact, and alternative interpretations favoring display or flank-butting functions have sustained an active debate in paleontology for over seven decades. The hypothesis remains well-supported but not definitively confirmed, and current consensus increasingly favors a multifunctional interpretation of the dome encompassing both combat and display roles.

**Herding behavior** refers to the social phenomenon in which multiple conspecific (or occasionally heterospecific) individuals persistently aggregate during locomotion, foraging, breeding, and anti-predator defense. In paleontology, it is primarily inferred for non-avian dinosaurs based on three categories of fossil evidence: monospecific bone beds preserving hundreds to thousands of simultaneously killed individuals, parallel trackways documenting coordinated same-direction movement, and colonial nesting sites showing spatially organized communal breeding grounds. The best-documented herding dinosaurs include hadrosaurids such as *Maiasaura peeblesorum*, ceratopsians such as *Centrosaurus apertus*, and sauropods whose parallel trackways have been recorded from the Jurassic through the Cretaceous worldwide. Herding conferred ecological advantages analogous to those observed in extant large herbivorous mammals, including enhanced predator detection through the many-eyes effect, reduced per-capita predation risk via the dilution effect, collective defense of vulnerable juveniles, and improved foraging efficiency. A landmark 2021 study on *Mussaurus patagonicus* from Patagonia (Pol et al., *Scientific Reports*) established that age-segregated herd structures existed by approximately 193 million years ago in the Early Jurassic, pushing back the earliest skeletal evidence of complex social behavior in dinosaurs by roughly 40 million years. Whether carnivorous theropods engaged in true cooperative herding or pack hunting remains one of the most actively debated questions in dinosaur paleobiology.