Final answer:
The frill in ceratopsians had multiple proposed functions, including mating displays, combat protection, and thermoregulation.
Step-by-step explanation:
The frill in ceratopsians, such as Triceratops, is believed to have served multiple functions. One proposed function is that it acted as a display structure for mating and species recognition. The frill could have been used by males to attract females or by individuals of the same species to identify each other. Another proposed function is that it provided protection and support to the neck and head during combat. The frill was made of bone and would have acted as a shield, absorbing and distributing the impact of blows. Additionally, the frill may have helped regulate body temperature in these large dinosaurs, functioning as a radiative surface for heat exchange with the environment.