are usually found in articulation, suggesting possible mass mortality events. Stephan N. F. Spiekman and colleagues reported a partial P. andrewsi skull (RGM 818207) in the collections of the Naturalis Biodiversity Center, Netherlands in 2015. Since Protoceratops fossils are only found in the Gobi Desert of Mongolia and this specimen was likely discovered during the Central Asiatic Expeditions, the team concluded that this skull was probably acquired by the Delft University between 1940 and 1972 as part of a collection transfer. Species and synonyms Protoceratopsid remains were recovered in the 1970s from the Khulsan locality of the Barun Goyot Formation, Mongolia, during the work of several Polish-Mongolian paleontological expeditions. In 1975, Polish paleontologists Teresa Maryańska and Halszka Osmólska described a second species of Protoceratops which they named P. kozlowskii. This new species was based on the Khulsan material, mostly consisting of juvenile skull specimens. The specific name, kozlowskii, is in tribute to the Polish paleontologist Roman Kozłowski. They also named the new genus and species of protoceratopsid Bagaceratops rozhdestvenskyi, known from specimens of the nearby Hermiin Tsav locality. In 1990 the Russian paleontologist Sergei Mikhailovich Kurzanov referred additional material from Hermiin Tsav to P. kozlowskii. However, he noted that there were enough differences between P. andrewsi and P. kozlowskii, and erected the new genus and combination Breviceratops kozlowskii. Though Breviceratops has been regarded as a synonym and juvenile stage of Bagaceratops, Łukasz Czepiński in 2019 concluded that the former has enough anatomical differences to be considered as a separate taxon. In 2001 Oliver Lambert with colleagues named a new and distinct species of Protoceratops, P. hellenikorhinus. The first known remains of P. hellenikorhinus were collected from the Bayan Mandahu locality of the Bayan Mandahu Formation, Inner Mongolia, in 1995 and 1996 during Sino-Belgian paleontological expeditions. The holotype (IMM 95BM1/1) and paratype (IMM 96BM1/4) specimens consist of large skulls lacking body remains. The holotype skull was found facing upwards, a pose that has been reported in Protoceratops specimens from Tugriken Shireh. The specific name, hellenikorhinus, is derived from Greek hellenikos (meaning Greek) and rhis (meaning nose) in reference to its broad and angular snout, which is reminiscent of the straight profiles of Greek sculptures. In 2017 abundant protoceratopsid material was reported from Alxa near Bayan Mandahu, and it may be referable to P. hellenikorhinus. Viktor Tereshchenko and Vladimir R. Alifanov in 2003 named a new protoceratopsid dinosaur from the Bayn Dzak