Chasmosaurus had a unique head with a big frill that extended backward from behind the skull. Among different Chasmosaurus species, the size and shape of the frill varied. The frills of some individuals had fenestrae (holes) and patterns of epoccipitals (bony projections), while others were smaller and more solid. The function of the frill was likely multifaceted, including thermoregulation, courtship displays, or territorial defense, as well as possible defensive features.