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 eyeball 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 indicate it was not a predator, so if it was a diurnal animal, then it would have been expected to have a much smaller sclerotic ring size. However, in 2011 Lars Schmitz and Ryosuke Motani measured the dimensions of the sclerotic ring and eye socket in fossils 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. However, a subsequent study in 2021 found that Protoceratops had a greater capability of nocturnal vision than did Velociraptor. Paleoenvironment Bayan Mandahu Formation Based on general similarities between the vertebrate fauna and sediments of Bayan Mandahu and the Djadokhta Formation, the Bayan Mandahu Formation is considered to be Late Cretaceous in age, roughly Campanian. The dominant lithology is reddish-brown, poorly cemented, fine grained sandstone with some conglomerate, and caliche. Other facies include alluvial (stream-deposited) and eolian (wind-deposited) sediments. It is likely that sediments at Bayan Mandahu were deposited by short-lived rivers and