and colleagues in 1988 described the enamel microstructure of Protoceratops, observing a non-prismatic outer layer. They concluded that enamel shape does not relate to the diet or function of the teeth as most animals do not necessarily use teeth to process food. The maxillary teeth of ceratopsians were usually packed into a dental battery that formed vertical shearing blades which probably chopped the leaves. This feeding method was likely more efficient in protoceratopsids as the enamel surface of Protoceratops was coarsely-textured and the tips of the micro-serrations developed on the basis of the teeth, probably helping to crumble vegetation. Based on their respective peg-like shape and reduced microornamentation, Dauphin and colleagues suggested that the premaxillary teeth of Protoceratops had no specific function. In 1991, the paleontologist Gregory S. Paul stated that contrary to the popular view of ornithischians as obligate herbivores, some groups may have been opportunistic meat-eaters, including the members of Ceratopsidae and Protoceratopsidae. He pointed out that their prominent parrot-like beaks and shearing teeth along with powerful muscles on the jaws suggest an omnivore diet instead, much like pigs, hogs, boars and entelodonts. Such scenario indicates a possible competition with the more predatory theropods over carcasses, however, as the animal tissue ingestion was occasional and not the bulk of their diet, the energy flow in ecosystems was relatively simple. You Hailu and Peter Dodson in 2004 suggested that the premaxillary teeth of Protoceratops may have been useful for selective cropping and feeding. In 2009 Kyo Tanque and team suggested that basal ceratopsians, such as protoceratopsids, were most likely low browsers due to their relatively small body size. This low-browsing method would have allowed to feed on foliage and fruits within range, and large basal ceratopsians may have consumed tougher seeds or plant material not available to smaller basal ceratopsians. David J. Button and Lindsay E. Zanno in 2019 performed a large phylogenetic analysis based on skull biomechanical characters—provided by 160 Mesozoic dinosaur species—to analyze the multiple emergences of herbivory among non-avian dinosaurs. Their results found that herbivorous dinosaurs mainly followed two distinct modes of feeding, either processing food in the gut—characterized by relatively gracile skulls and low bite forces—or the mouth, which was characterized by features associated with extensive processing such as high bite forces and robust jaw musculature. Ceratopsians (including protoceratopsids), along with Euoplocephalus, Hungarosaurus, parkosaurid, ornithopod and heterodontosaurine dinosaurs, were found to be in the former