Nocturnality in Dinosaurs Inferred from Scleral Ring and Orbit Morphology

Abstract

Variation in daily activity patterns facilitates temporal partitioning of habitat and resources among species. Knowledge of temporal niche partitioning in paleobiological systems has been limited by the difficulty of obtaining reliable information about activity patterns from fossils. On the basis of an analysis of scleral ring and orbit morphology in 33 archosaurs, including dinosaurs and pterosaurs, we show that the eyes of Mesozoic archosaurs were adapted to all major types of diel activity (that is, nocturnal, diurnal, and cathemeral) and provide concrete evidence of temporal niche partitioning in the Mesozoic. Similar to extant amniotes, flyers were predominantly diurnal; terrestrial predators, at least partially, nocturnal; and large herbivores, cathemeral. These similarities suggest that ecology drives the evolution of diel activity patterns.

Response to Comment on “Nocturnality in Dinosaurs Inferred from Scleral Ring and Orbit Morphology”

“…Hall et al observe that many Mesozoic archosaurs plot outside the extant saurian morphospace, especially along discriminant axis 2, and state that it may be impossible to infer diel activity patterns. Contrary to what is stated in (3), scores on discriminant axis 2 are correlated with eye size, the geometric mean of all variables (1,8) [P < 0.001 for both extant and fossil data, calculated with the SMATR package (10) in R 2.13.0 (11)]. Many Mesozoic archosaurs have larger absolute eye sizes than extant saurians (1) and should plot outside the extant morphospace.…”

Comment on “Nocturnality in Dinosaurs Inferred from Scleral Ring and Orbit Morphology”

“…First, the statistical approach employed by Schmitz and Motani (1) does not provide a basis for such definitive interpretations. Their work (1-3) uses discriminant function analysis (DFA), which tests whether groups that are defined a priori differ significantly from one another based on a set of predictor variables (4).…”

“…Our third concern is that nearly half of the fossil archosaurs analyzed do not overlap with extant taxa on discriminant axis 2 in Schmitz and Motani's figure S1 (1). Interpretations beyond the bounds of the originally sampled data are statistically dubious (9).…”

Osteology of a New Late Cretaceous Troodontid Specimen from Ukhaa Tolgod, Ömnögovi Aimag, Mongolia

“…2, 3). The sclerotic ring is also observed in other paravians of various sizes, such as Mei, Microraptor, Tsaagan, and Archaeopteryx (Xu et al, 2003;Xu and Norell, 2004;Mayr et al, 2005;Norell et al, 2006;Schmitz and Motani, 2011). The best-exposed sclerotic plate seems nearly twice as long as wide and almost flat, as in Tsaagan (Norell et al, 2006).…”