Killer dinosaurs may have hunted mainly under cover of darkness.
By examining the shape of dinosaur eyes, Lars Schmitz and colleagues at the University of California Davis concluded that many dinosaurs were nocturnal, particularly the predators.
Animals with night vision tend to have large pupils relative to their eyeballs for maximum light entry. Eyeball size can be inferred from the skull, and many dinosaurs had a ring of bone surrounding their pupil called the ‘scleral ring’, which fossilizes and accurately reflects the size of the pupil.
By determining the eye-socket-to-scleral-ring-ratio in modern birds and lizards, which are close living relatives of dinosaurs, the group was able to predict whether animals used their eyes at night, in twilight or during the day.
“So we knew what we should expect to see in dinosaurs: if the dinosaur was night-active then we’d expect to see an eye shape that resembled modern nocturnal birds or lizards,” explained Schmitz.
The team then analyzed 33 fossilized species from the Mesozoic era (between 250 and 65 million years ago), including 19 dinosaurs and 8 ‘pterosaurs’, a type of flying reptile.
The pattern of when dinosaurs were active turned out to be remarkably diverse. This contradicts the conventional view of dinosaurs as dominating the day, while small mammals scurried in the undergrowth at night.
“We can totally reject that there was this split between diurnal [day-active] dinosaurs and nocturnal [night-active] mammals, and we can tell by looking at eye shape,” said Schmitz.
Predators, such as Velociraptor depicted in Jurassic Park, tended to be night or twilight-active, presumably when their prey was most vulnerable. Herbivores could see in both dim and bright light, allowing them to forage for most of a 24 hour period whilst avoiding the mid-day heat (large animals are more prone to over-heating).
This is similar to modern mammals, with large herbivores like elephants foraging for up to 15 hours a day, mainly at dawn and dusk, and smaller predators hunting at night. Flying pterosaurs were found to be mainly day-active, just like their modern-day bird counterparts.
The key conclusion is that animal lifestyle determines activity patterns irrespective of whether the animals are reptiles, mammals or birds.
“Ecology, lifestyle and differences in the physical environment influence the evolution of shape, or the structure of these animals... We see essentially the same patterns both today and also in the Mesozoic,” said Schmitz.
The research also raises intriguing questions, such as how predatory dinosaurs, which were mainly night-active, evolved into the predominantly day-active modern birds.
The group are now extending their research to more dinosaur species, including Tarbosaurus, a close relative of the iconic Tyrannosaurus rex, in the hope of discovering more about dinosaur behaviour, ecology and evolution.
“The key”, says Schmitz, “is to look at their eyes.”
Lars Schmitz, University of California Davis, with evidence that predatory dinosaurs were noctural.