Final Answer:
The evolutionary novelty shared by ceratopsians and pachycephalosaurs that unites them as marginocephalians is the presence of a unique bony structure on the back of their skulls called the parietosquamosal frill.
Step-by-step explanation:
Marginocephalians, a group of herbivorous dinosaurs, are characterized by a distinctive feature—the parietosquamosal frill. This anatomical structure is a bony shelf or ornamentation extending from the back of the skull. In ceratopsians, such as Triceratops, this frill is often adorned with elaborate patterns or horns.
Pachycephalosaurs, like Pachycephalosaurus, also possess a modified version of this frill, although it is less pronounced compared to ceratopsians. The parietosquamosal frill likely served various purposes, such as display for communication, species recognition, or even defense.
This shared trait indicates a common evolutionary origin for ceratopsians and pachycephalosaurs within the larger group of marginocephalians. Despite the differences in the size and shape of their frills, the presence of this distinctive feature unites them as a monophyletic group.
The evolutionary significance of the parietosquamosal frill showcases adaptations that allowed these dinosaurs to thrive in their respective environments, providing insights into their behavior, social dynamics, and ecological roles during the Late Cretaceous period.