Final answer:
The incipient frill is a distinctive feature of Marginocephalia, a dinosaur clade including Ceratopsians like Triceratops, which were known for their skull ornamentation.
Step-by-step explanation:
The incipient frill is the principal evolutionary novelty of the Marginocephalia. This group includes dinosaurs like the well-known Triceratops, characterized by a shelf of bone at the back of the skull, among other features. The Marginocephalia is divided into two subgroups: Pachycephalosauria, known for their thick-skulled members, and the Ceratopsia, known for their horns and frills.
The term 'incipient' suggests the initial development or an early stage of a trait. In Marginocephalia, the frill would have started as a simple feature in early members and evolved into a more elaborate structure in later genera. These frills served various functions such as display, temperature regulation, and species recognition.