Published online 14 April 2011 | Nature | doi:10.1038/news.2011.236

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Dinosaur predators hunted in the dark

Study of eye bones reveals details of ancient reptile lifestyles.

dinoThe nocturnal flyer Ctenochasma elegans, a pterosaur, had a scleral ring with a wide opening which enabled it to see in dim light.Lars Schmitz

Palaeontologists have long believed that only mammals were active at night during the Mesozoic Era, but now some dinosaurs have been found to have been nocturnal too.

Lars Schmitz and Ryosuke Motani, palaeobiologists at the University of California, Davis, analysed bones found in the eyes of fossilized dinosaurs and related reptiles that lived between 250 million and 65 million years ago.

The team knew from looking at living animals that if the internal diameter of the 'scleral ring' of bone within the eye was large in relation to its external diameter, then the eye was well adapted to dealing with low light levels. Using this measurement, the team describe which of 33 ancient species were probably nocturnal, and which were probably active during the day. Their findings are published today in Science1.

The eye bones of most Mesozoic predators that the team analysed suggested that they were active at night, like most modern mammal predators.

The nocturnal hunters ranged from Microraptor gui, a winged dinosaur no more than 90 centimetres long, to Megapnosaurus kayentakatae, a pack-hunting species that could grow up to 3 metres, and Velociraptor mongoliensis, the viciously clawed predator made famous in the novel Jurassic Park.

Several pterosaurs, flying reptiles that were closely related to the dinosaurs, also had eye shapes that indicated they were active at night. What these pterosaurs were doing in the dark is unclear, but Schmitz and Motani suggest that they could have been behaving like petrels and albatrosses (Procellariiformes) or waterfowl (Anseriformes), which often forage at night.

"I think this will solidly end the view that all dinosaurs were diurnal and mammals were nocturnal," says Schmitz.

Not all the species that the team analysed were nocturnal. Some pterosaurs and most of the primitive birds that they examined, including Archaeopteryx lithographica and Confuciusornis sanctus, the earliest beaked bird ever found, had eye shapes similar to those of modern birds that are active in daylight.

Stuck in the middle

Most of the 13 large-bodied herbivorous dinosaurs that the team examined, including the long-necked Diplodocus longus and the frilled Protoceratops andrewsi, had scleral rings with shapes between those of nocturnal and diurnal species. Combined with the large overall size of the species' eyes, that suggests they were able to function both at night and during the day.

The conclusion is backed up by studies on living mammals. Species bigger than about 400 kilograms in mass, such as elephants, need to forage for more than 12 hours a day to supply their energy needs, requiring them to be active in both dark and light, because in many parts of the world winter days are shorter than twelve hours.

With this in mind, palaeontologists have argued in recent years that large-bodied herbivorous dinosaurs would have had similar visual needs. Until now, anatomical evidence has been lacking.

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"This work is giving us a glimpse into the details of dinosaur lives that we didn't really think we'd ever get," says Lawrence Witmer, a palaeontologist at Ohio University in Athens.

The findings are also shedding light on how species interacted with one another. In one famous fossil, a Velociraptor is locked in mortal combat with a Protoceratops: the two animals were buried by a sudden sand flow as they wrestled with one another, and their struggles were recorded in stone.

The fossil has helped palaeontologists to confirm that dinosaurs did prey on each other, but when combined with Schmitz and Motani's work, it provides even more information, suggesting that Velociraptor, a nocturnal predator, came upon the Protoceratops while it was resting at night.

"This finding opens up whole new avenues of research into things like environmental use and partitioning of resources," says Witmer. 

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