First evidence that some dinosaurs were nocturnal

Many dinosaurs and pterosaurs were active both by day and night, and some were entirely nocturnal, a new study suggests.

Lars Schmitz and Ryosuke Motani at the University of California, Davis, wanted to test the assumption that dinosaurs were mainly active by day.

They hit upon a clever way to reconstruct the activity patterns of extinct archosaurs – a group which includes crocodiles, dinosaurs and birds – by comparing the size and shape of their eyes to those of present-day animals.

The soft tissues of the eye, including the iris, rarely fossilise, but most archosaurs have a ring of bones within the eye called the scleral ring, which is preserved in some fossils. In modern archosaurs, the size and shape of this ring relative to the eye socket varies depending on whether the animal is diurnal, nocturnal or is active mainly at dawn and dusk.

A matter of size

Living nocturnal species tend to have a large pupil for the size of their retina, and this helps them see in dim light. Fossils with a large internal scleral ring diameter relative to the diameter of the eye socket, or orbit, probably had a large pupil too – suggesting they were nocturnal.

By contrast species active by day have a small pupil for a given eye size. This means they can avoid overstimulating the retina and contract the iris for better depth of focus.

"If you have a particularly circular orbit and a pretty circular scleral ring you can get a reasonably good idea of the size of the eyeball and the size of the iris within the eye," says David Hone of University College Dublin in Ireland, who was not involved in the study. "There's certainly nothing wrong with applying that to dinosaurs."

Schmitz and Motani measured the eye-socket size and the external and internal scleral ring diameter in 33 Mesozoic archosaur species, and compared them with similar measurements from 1401 living species of mammals, birds, lizards and snakes with known activity patterns.

"What we saw is that the perception that dinosaurs were just day active can be rejected," says Schmitz. "Many were nocturnal and many were day and night active."

Terrestrial predators like Microraptor and Velociraptor emerged as at least partially nocturnal, and the eye shapes of large herbivorous dinosaurs like Psittacosaurus and Diplodocus suggested they were active both by day and by night.

Wider comparisons

"These results are very interesting and certainly plausible, but I would want to see other analyses before I was really convinced," says Hone. He suggests that adding more living species to the analysis, especially those with unusual ecologies – nocturnal species that are part of a wider, mainly diurnal group with a common ancestor, for example – could make a stronger case.

But he agrees that the presence of the scleral ring offers an opportunity to address the question. "We should be grateful [archosaurs] have a scleral ring," he says. "We are trying to reconstruct [the behaviour of] animals which in some cases are 230-odd-million years old."

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