As ceratopsians, protoceratopsids were herbivorous, with constantly replacing tooth batteries made for slicing through plants and a hooked beak for grabbing them.

Compared to more derived ceratopsians, protoceratopsids had a deep and wide oral cavity, though more narrow than in predecessors like Psittacosaurus, which may have aided in breathing or thermoregulation.

Granger and Gregory recognized Protoceratops's close relationship to other ceratopsians, but considered it primitive enough to warrant its own family, and perhaps suborder.

Sereno's definition ensures that Protoceratopsidae is monophyletic, but probably excludes some dinosaurs that have been traditionally thought of as protoceratopsids (for example, Leptoceratops and Montanoceratops).

Derived characters shared by these dinosaurs include a narrow strap-shaped paroccipital process, a very small occipital condyle, and an upturned dorsal margin of the predentary.

[9] The relationships of Graciliceratops to other protoceratopsids remain unclear due to its fragmentary nature, and it is regarded as a metaspecies with highly variable phylogenetic positions.

[10][11] In 2003, Vladimir Alifanov named, but did not define, a new ceratopsian family Bagaceratopidae to include Bagaceratops, Platyceratops, Lamaceratops and Breviceratops.

Based on cranial characters such as presence or absence of premaxillary teeth and an antorbital fenestra, P. andrewsi is the basal-most protoceratopsid and Bagaceratops the derived-most one.

Protoceratops's mouth structures and general abundance indicate it was not a predator, so if it were also diurnal, then it would have been expected to have a much smaller sclerotic ring size.

If Protoceratops was nocturnal, it could avoid the hottest parts of the day and survive in an arid environment without highly developed cooling mechanisms.

[14] Leonardo Maiorino and his team used geometric morphometrics to analyze the dimorphism in Protoceratops andrewsi and concluded that there is no difference in male and female structures.

The nature of these groups is not completely known, though herds of young likely formed for protection from predators, and adults are believed to have come together for communal nesting.

[19] The skeleton of Protoceratops juveniles indicates that protoceratopsids were able to employ facultative bipedalism when young and became obligate quadrupeds in adulthood.

The second dispersal was 15 million years later, this time of Ceratopsidae's ancestors across the Bering Land Bridge into North America between 120Ma and 140Ma.