were regarded as representing male P. hellenikorhinus. The other group of skulls is characterized by low nasals that have undeveloped horns; a relatively longer antorbital length; and more oblique nostrils. These individuals were considered as females. The team however, was not able to produce deeper analysis regarding sexual dimorphism in P. hellenikorhinus due to the lack of complete specimens. Also in 2001, Tereschhenko analized several specimens of P. andrewsi in order to evaluate sexual dimorphism. He found 19 anatomical differences in the vertebral column and pelvic region of regarded male and female Protoceratops individuals, which he considered to represent actual sexual characters. In 2012 Naoto Handa and colleagues described four specimens of P. andrewsi from the Udyn Sayr locality of the Djadokhta Formation. They indicated that sexual dimorphism in this population was marked by a prominent nasal horn in males—trait also noted by other authors—relative wider nostrils in females, and a wider neck frill in males. Despite maintaining the skull morphology of most Protoceratops specimens (such as premaxillary teeth), the neck frill in this population was straighter with a near triangular shape. Handa and team in addition found variation across this Udyn Sayr sample and classified them in three groups. First group includes individuals with a well-developed bony ridge on the lateral surface of the squamosal bone, and the posterior border of the squamosal is backwards oriented. Second group had a fairly rounded posterior border of the squamosal, and a long and well-developed bony ridge on the posterior border of the parietal bone. Lastly, the third group was characterized by a curved posterior border of the squamosal and a notorious rugose texture on the top surface of the parietal. Such skull traits were regarded as marked [[Genetic variability |intraspecific variation]] within Protoceratops, and they differ from other populations across the Djadokhta Formation (like Tugriken Shireh), being unique to the Udyn Sayr region. These neck frill morphologies differ from those of Protoceratops from the Djadokhta Formation in the adjacent dinosaur locality Tugrikin Shire. The morphological differences among the Udyn Sayr specimens may indicate intraspecific variation of Protoceratops. A large and well-developed bony ridge on the parietal has been observed on another P. andrewsi specimen, MPC-D 100/551, also from Udyn Sayr. However, Leonardo Maiorino with team in 2015 performed a large geometric morphometric analysis using 29 skulls of P. andrewsi in order to evaluate actual sexual dimorphism. Obtained results indicated that other