than the nasal horn—which remained as the only skull trait with potential sexual dimorphism—all previously suggested characters to differentiate hyphotetical males from females were more linked to ontogenic changes and intraspecific variation independent of sex, most notably the neck frill. The geometrics showed no consistent morphological differences between specimens that were regarded as males and females by previous authors, but also a slight support for differences in the rostrum across the sample. Maiorino and team nevertheless, cited that the typical regarded Protoceratops male, AMNH 6438, pretty much resembles the rostrum morphology of AMNH 6466, a typical regarded female. However, they suggested that authentic differences between sexes could be still present in the postcranial skeleton. Although previously suggested for P. hellenikorhinus, the team argued that the sample used for this species was not sufficient, and given that sexual dimorphism was not recovered in P. andrewsi, it is unlikely that it occurred in P. hellenikorhinus. In 2016 Hone and colleagues analyzed 37 skulls of P. andrewsi, finding that the neck frill of Protoceratops (in both length and width) underwent positive allometry during ontongeny, that is, a faster growth/development of this region than the rest of the animal. The jugal bones also showed a trend towards an increase in relative size. These results suggest that they functioned as socio-sexual dominance signals, or, they were mostly used in display. The use of the frill as a displaying structure may be related to other anatomical features of Protoceratops such as the premaxillary teeth (at least for P. andrewsi) which could have been used in display or intraspecific combat, or the high neural spines of tail. On the other hand, Hone and team argued that if neck frills were instead used for protective purposes, a large frill may have acted as an aposematic (warning) signal to predators. However, such strategies are most effective when the taxon is rare in the overall environment, opposed to Protoceratops which appears to be an extremely abundant and medium-sized dinosaur. Tereschenko in 2018 examined the cervical vertebrae series of six P. andrewsi specimens. Most of them had differences in the same exact vertebra, such as the shape and proportions of the vertebral centra and orientation of neural arches. According these differences, four groups were identified, concluding that individual variation was extended to the vertebral column of Protoceratops. In 2020 nevertheless, Andrew C. Knapp and team conducted morphometric analyses of a large