Final answer:
The earliest Marginocephalian dinosaurs are believed to have originated in Asia, as supported by fossil evidence of species such as Protoceratops and similar dinosaurs discovered by early Scythians in Central Asia.
Step-by-step explanation:
The earliest marginocephalian dinosaurs are believed to have originated in Asia, notably represented by dinosaurs such as the Protoceratops. This group of dinosaurs, characterized by a distinctive shelf or margin at the back of the skull, is part of the larger clade called the Cerapoda, which also comprises the ornithopods and the pachycephalosaurs. Marginocephalians are known from fossil evidence to have roamed the earth during the Late Jurassic to the Late Cretaceous period.
Adrienne Mayor, in her historical work, indicates how early Scythians could have discovered Protoceratops skeletons in Central Asia. This region, along with evidence from dinosaurs like Turanoceratops and Sinoceratops, supports the theory that marginocephalians, among others, originated there. Another significant fact relates to the Triassic Period, during which the first dinosaurs branched off from the reptiles and the supercontinent Pangea began to separate, resulting in the land division that created Laurasia and Gondwanaland, with dinosaurs spreading across these emerging lands.