How we know Velociraptor hunted by night

As dramatic fossils go, it’s hard to beat the Mongolian fighting dinosaurs – a Velociraptor and a Protoceratops locked in mortal combat. The Protoceratops, an early horned dinosaur, has the raptor’s arm in its mouth, and the raptor appears to be kicking its prey in the neck. The two combatants were killed in this pose, around 75 million years ago. And according to a new study, they probably met and died sometime around dawn or dusk.

Most dinosaur reconstructions portray the animals walking about in bright sunlight but of course, we know that living animals are active at all times of the day. The diurnal ones prefer the daylight hours, while nocturnal species haunt the night. Crepuscular animals favour twilight hours, while cathemeral ones are active in short bursts throughout the day.

It’s easy enough to work out which group a living animal falls into, but the task becomes far more difficult if the animal in question is extinct. With the exception of tracks, burrows or other trace fossils, behaviour doesn’t fossilise easily. But Lars Schmitz and Ryosuke Motani have developed a clever way of working out when dinosaurs were active, using something we have in abundance – their skulls.

The eyes of all birds and many reptiles are reinforced by a bony disc called a scleral ring (which you can clearly see in my photo of Deinonychus above). The form of these rings closely follows their function. In nocturnal animals, the ring has a wide hole to let in as much light as possible. In diurnal species, the ring is thicker and has a narrower hole. That gives them sharper vision (think about how your focus gets better when you squint) without overloading the retina. Crepuscular or cathemeral species have rings that are somewhere in between, but they also tend to have larger-than-average eyes for their body size.

Schmitz and Motani developed a new model that takes the size of an animal’s scleral ring and eye socket, and works out when it would have been active. The model also accounts for the evolutionary relationships between different species. The duo tested it using data from 164 living animals, and found that it could accurately predict their daily habits. Next, they used it to analyse the skulls of 23 dinosaurs, as well as 10 other extinct reptiles, including eight of the flying pterosaurs.

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They found that all the plant-eating dinosaurs in their sample, such as Diplodocus and Protoceratops were cathemeral or crepuscular, with only one truly diurnal species. Schmitz and Motani think that this pattern was probably driven by their size and diet. Even today, many large plant-eaters need to forage for significant chunks of the day to get enough food, while avoiding the hottest part of the day to avoid overheating. To keep cool while getting enough to eat, their lives bridge both night and day.

Most of the flyers in their sample were diurnal, including Archaeopteryx and three other extinct birds, as well as three pterosaurs (although four of the pterosaurs were nocturnal, including the famous Rhamphorhynchus). Meanwhile, the predatory dinosaurs in their sample, including Velociraptor and Microraptor, were mostly nocturnal, with a few cathemeral species. Again, modern animals share the same pattern. Most meat-eating mammals, for example, are nocturnal.

These new results fit with those of previous studies. For example, Martin Kundrát and Jiří Janáček speculated that the small predator Conchoraptor might have been a nocturnal hunter, based on brain structures that suggested excellent hearing. Meanwhile, based on the eye sockets of various meat-eating dinosaurs, Daniel Chure suggested that the small ones were nocturnal while the large ones hunted during daylight. Based on Schmitz and Motani’s study, Chure was at least half-right. Unfortunately, no one has yet been able to study the scleral rings of the large tyrannosaurus-sized hunters.

Schmitz and Motani say that their study goes against a commonly held assumption that dinosaurs generally walked around during the day, while the mammals scurried about under cover of darkness. Clearly, that’s not the case – the dinosaurs had the entire 24-hour cycle covered.

Reference: Schmitz & Motani. 2011. Nocturnality in Dinosaurs Inferred from Scleral Ring and Orbit Morphology. Science http://dx.doi.org/10.1126/science.1200043

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