Editing Protoceratops (section)

===Daily activity===
[[File:Protoceratops AMNH 6466 skull.jpg|thumb|Skull of ''P. andrewsi'' AMNH 6466, preserving sclerotic ring]]
In 2010, Nick Longrich examined the relatively large [[Orbit (anatomy)|orbital]] ratio and [[sclerotic ring]] of ''Protoceratops'', which he suggested as evidence for a [[nocturnal]] lifestyle. Based on the size of its sclerotic ring, ''Protoceratops'' had an unusually large [[eye]]ball among protoceratopsids. In birds, a medium-sized sclerotic ring indicates that the animal is a predator, a large sclerotic ring indicates that it is nocturnal, and the largest ring size indicates it is an active nocturnal predator. Eye size is an important adaptation in predators and nocturnal animals because a larger eye ratio poses a higher sensitivity and resolution. Because of the energy necessary to maintain a larger eyeball and the weakness of the skull that corresponds with a larger orbit, Longrich argues that this structure may have been an adaptation for a nocturnal lifestyle. The jaw morphology of ''Protoceratops''—more suitable for processing plant material—and its extreme [[Abundance (ecology)|abundance]] indicate it was not a predator, so if it was a [[Diurnality|diurnal]] animal, then it would have been expected to have a much smaller sclerotic ring size.<ref name=Longriich20110/>

However, in 2011, Lars Schmitz and Ryosuke Motani measured the dimensions of the sclerotic ring and eye socket in fossil specimens of dinosaurs and pterosaurs, as well as some living species. They noted that whereas photopic (diurnal) animals have smaller sclerotic rings, scotopic (nocturnal) animals tend to have more enlarged rings. Mesopic ([[cathemeral]]) animals—which are irregularly active throughout the day and night—are between these two ranges. Schmitz and Motani separated ecological and [[phylogenetic]] factors and by examining 164 living species and noticed that eye measurements are quite accurate when inferring diurnality, cathemerality, or nocturnality in extinct [[tetrapods]]. The results indicated that ''Protoceratops'' was a cathemeral herbivore and ''Velociraptor'' primarily nocturnal, suggesting that the Fighting Dinosaurs deathmatch may have occurred at twilight or under low-light conditions. Lastly, Schmitz and Motani concluded that [[ecological niche]] was a potential main driver in the development of daily activity.<ref>{{cite journal|last1=Schmitz|first1=L.|last2=Motani|first2=R.|date=2011|title=Nocturnality in Dinosaurs Inferred from Scleral Ring and Orbit Morphology|journal=Science|volume=332|issue=6030|pages=705–708|bibcode=2011Sci...332..705S|doi=10.1126/science.1200043|pmid=21493820|s2cid=33253407}}</ref> However, a subsequent study in 2021 found that ''Protoceratops'' had a greater capability of nocturnal vision than did ''Velociraptor''.<ref>{{cite journal |last1=Choiniere |first1=Jonah N. |last2=Neenan |first2=James M. |last3=Schmitz |first3=Lars |last4=Ford |first4=David P. |last5=Chapelle |first5=Kimberley E. J. |last6=Balanoff |first6=Amy M. |last7=Sipla |first7=Justin S. |last8=Georgi |first8=Justin A. |last9=Walsh |first9=Stig A. |last10=Norell |first10=Mark A. |last11=Xu |first11=Xing |last12=Clark |first12=James M. |last13=Benson |first13=Roger B. J. |title=Evolution of vision and hearing modalities in theropod dinosaurs |journal=Science |date=7 May 2021 |volume=372 |issue=6542 |pages=610–613 |doi=10.1126/science.abe7941 |pmid=33958472 |bibcode=2021Sci...372..610C |s2cid=233872840 |url=https://www.science.org/doi/10.1126/science.abe7941 |language=en |issn=0036-8075}}</ref>