Horns and Frills:
Contrary to popular belief, the horns and frill structures of the ceratopsians were not primarily for defensive purposes. In fact these structures were quite versatile. For example, the frill contained the muscles controlling jaw movement, and different shapes and sizes probably reflected differences in jaw muscles (Lucas, 1994).
The frill also probably served as a visual display along with the horns and may have expressed sexual dimorphism (Lucas, 1994).
Another possible function of the frill was as a thermoregulator. They were highly vascularized, spreading the blood over a wide surface area, and thus allowing rapid heating and cooling. Consequently, the frill may not have been used as a shield in combat, for it was probably quite vulnerable.
The horns may have served some defensive function, but their use primarily in display, and interspecies competition seems more likely (Lucas, 1994).
The frills of ceratopsians provided sites for the attachment of jaw muscles.
Differences in skull size and shape may have distinguished male and female Protoceratops.
The frill of Triceratops was highly vascularized, covered with channels and grooves for the blood vessels.