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308 22 The Function of Large Eyes in Protoceratops: A Nocturnal Ceratopsian? NICK LONGRICH among herbivorous dinosaurs, Protoceratops andrewsi is characterized by relatively large eyes. A study of the size of sclerotic rings in extant birds shows that relative eye size correlates with ecological habits: for a given body mass, herbivores, omnivores, and scavengers tend to have modest-sized eyes; diurnal, visually hunting predators have much larger eyes; and nocturnal birds using vision to forage have the largest eyes. Compared to birds, dinosaurs have small eyes relative to their body mass, and so direct comparisons with birds are not informative . However, the dinosaur data show an internally consistent pattern that is similar to birds: predatory theropods tend to have larger eyes than the plant-eating ornithischians and sauropodomorphs. In Protoceratops, however, the sclerotic ring diameter is larger than predicted by the ornithischian regression, and the ring aperture is larger than predicted for either the theropod or ornithischian regression. Whereas large eyes in Protoceratops could have been an adaptation for predatory habits—a hypothesis that is consistent with the presence of binocular vision in that taxon—that hypothesis is inconsistent with other observations, including the morphology of the jaws and dentition, tooth wear, and the relative abundance of Protoceratops in the Djadokhta Formation. Accordingly, a more likely possibility is that large eye size in Protoceratops was an adaptation to a nocturnal lifestyle. The presence of a large aperture is consistent with this interpretation, as is the presence of binocular vision. Nocturnality may have allowed Protoceratops to exploit the hot desert environment of the Djadokhta while minimizing the need for evaporative cooling. In this scenario, Protoceratops foraged at night when the air temperature was relatively low, and sheltered under vegetation or in burrows during the day to prevent overheating. Introduction Few vertebrates, either fossil or extant, are as well-represented in museum collections as is the ceratopsian Protoceratops. In 1922, the first fossils of Protoceratops andrewsi were recovered from the Upper Cretaceous outcrops of the Djadokhta Formation , at Bayn Dzak (Flaming Cliffs). The next year, the Central Asiatic Expedition collected a dozen skeletons of Protoceratops, along with more than sixty skulls, jaws, and partial skeletons. At the end of that fieldwork, in 1925, more than 100 specimens had been collected (Brown and Schlaikjer 1940). Since then, many more fossils of Protoceratops have been recovered from Bayn Dzak (Kurzanov 1972), as well as exposures of the Djadokhta at Tugriken Shireh (Jerzykiewicz et al. 1993; Fastovsky et al. 1997), Ukhaa Tolgod (Gao and Norell 2000), and Bayan Mandahu, in Inner Mongolia (Dong and Currie 1993; Jerzykiewicz et al. 1993; Lambert et al. 2001). This remarkable assemblage is the basis of important insights into the sys- The Function of Large Eyes in Protoceratops 309 FIGURE 22.1. Skull of Protoceratops andrewsi (AMNH 6466) and comparisons with Centrosaurus apertus. (A) A large ?female showing the sclerotic ring; (B) Centrosaurus apertus (AMNH 5351) after Brown (1917); (C) Protoceratops (AMNH 6466) after Brown and Schlaikjer (1940). Note that (B) and (C) are drawn to the same scale and, thus, show the relative size of the sclerotic ring in both ceratopsians. Despite its smaller size, the eye of Protoceratops (inferred from the size of the sclerotic ring) is roughly the same size as that of Centrosaurus. Scale bar is 1 m. tematics and evolution of the Ceratopsia, dinosaur ontogeny, and sexual dimorphism (Brown and Schlaikjer 1940; Kurzanov 1972; Dodson 1976). In terms of morphology, the genus is now known about as well as it is possible to know an extinct animal. But to what extent do we actually understand Protoceratops? The paleobiology of Protoceratops has been the subject of much speculation. Gregory and Mook (1925) suggested that Protoceratops was aquatic, because of its large feet and deep tail. However, the arid paleoenvironments of the Djadokhta Formation pose obvious problems for this hypothesis. Ostrom (1966) briefly discussed the functional morphology of the jaws and teeth of Protoceratops in his study of ceratopsian evolution . Kurzanov (1972), Farlow and Dodson (1975), and Dodson (1976) considered the function of the nasal ornamentation and frill, and what the existence of sexual dimorphism in the animal (Kurzanov 1972; Dodson 1976) might say about its biology. More recently, a clutch of hatchlings found at Tugriken Shireh suggests that the animal was gregarious, at least at some point in its life (You and Dodson 2004). Yet many questions remain unanswered. How did the jaws and teeth function, and what did Protoceratops eat? What was the function...

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