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Some dinosaurs loved the nightlife

Skeleton of the nocturnal flyer Ctenochasma elegans, a pterosaur. The scleral ring has a wide opening which enabled this flyer to see in dim light.
[Image courtesy of Lars Schmitz]

Some dinosaurs and other reptiles from the Mesozoic era (about 250 to 65 million years ago) could see in dim light and were likely active at night, according to a study of these animals' eye shapes.

These findings challenge the conventional wisdom that, for energy reasons, these animals were only active during the day, leaving the night to small and more energetically flexible mammals.

Lars Schmitz and Ryosuke Motani of the University of California, Davis analyzed the eye structures of many living species, including mammals, reptiles and birds, with known activity patterns. That is, these animals were diurnal, nocturnal or cathemeral (active in bouts throughout a 24-hour cycle). These activity patterns are related to the size of different structures in the eye that help the animals see in various light conditions, the researchers found.

This plant-eating dinosaur, Protoceratops andrewsi, was active day and night, like many other herbivorous dinosaurs.
[Image courtesy of Lars Schmitz]

Schmitz and Motani then analyzed fossils from 33 Mesozoic archosaurs (the group that includes dinosaurs and pterosaurs) and inferred their activity patterns based on these same eye structures.

Their findings suggest that archosaurs from the Mesozoic displayed all three types of activity patterns. Schmitz and Motani also found many similarities between living and extinct groups. For example, flying animals like birds and pterosaurs were largely diurnal, carnivores were largely nocturnal and herbivores were largely cathemeral.

The authors argue that, similar to mammals today, the Mesozoic archosaurs had a variety of different activity patterns and that these patterns were similarly shaped by ecology. The research is being published online by the journal Science, at the Science Express Web site, on 14 April 2011.