category, indicating that Protoceratops and relatives had strong bite forces and relied mostly on its jaws to process food. Ontogeny Brown and Schlaikjer in 1940 upon their large description and revision of Protoceratops remarked that the orbits, frontals, and lacrimals suffered a shrinkage in relative size as the animal aged; the top border of the nostrils became more vertical; the nasal bones progressively became elongated and narrowed; and the neck frill as a whole also increases in size with age. The neck frill specifically, underwent a dramatic change from a small, flat, and almost rounded structure in juveniles to a large, fan-like one in fully mature Protoceratops individuals. In 2001 Lambert and colleagues considered the development of the two nasal "horns" of P. hellenikorhinus to be a trait that was delayed in relation to the appearance of sexual-discriminant traits. This was based on the fact that one small specimen (IMM 96BM2/1) has a skull size slightly larger than a presumed sexually mature P. andrewsi skull (AMNH 6409), and yet it lacks double nasal horns present in fully mature P. hellenikorhinus. Makovicky and team in 2007 conducted a histological analysis on several specimens of Protoceratops from the American Museum of Natural History collections in order to provide insights into the life history of Protoceratops. The examined fossil bones indicated that Protoceratops slowed its ontogeny (growth) around 9–10 years of life, and it ceased around 11–13 years. They also observed that the maximum or latest stage of development of the neck frill and nasal horn occurred in the oldest Protoceratops individuals, indicating that such traits were ontogenically variable (meaning that they varied with age). Makovicky and team also stated that as the maximum/radical changes on the neck frill and nasal horn were present in most adult individuals, trying to differentiate sexual dimorphism (anatomical differences between sexes) in adult Protoceratops may not be a good practice. David Hone and colleagues in 2016 upon their analysis of P. andrewsi neck frills, found that the frill of Protoceratops was disproportionally smaller in juveniles, grew at a rapid rate than the rest of the animal during its ontogeny, and reached a considerable size only in large adult individuals. Other changes during ontogeny include the elongation of the premaxillary teeth that are smaller in juveniles and enlarged in adults, and the enlargement of middle neural spines in the tail or caudal vertebrae, which appear to grow much