Final answer:
The unique evolutionary feature of the ceratopsids is their very large skulls with bony frills and nasal horns.
Step-by-step explanation:
Unique evolutionary features of the ceratopsids include very large skulls. Ceratopsids, a group of herbivorous dinosaurs from the Cretaceous period, are characterized by their distinctively large skulls with bony frills and nasal horns. Notably, they do not possess smooth frills, small nostrils, or nasal fenestrae, which distinguishes them from other dinosaur groups. Their enormous skulls could support large jaw muscles, allowing ceratopsids to eat tougher vegetation.