Their eyes tell a tale

PALEONTOLOGY

Steven Spielberg and Michael Crichton were bang on target when they portrayed those smart velociraptors as efficient nocturnal hunters. Those raptors indeed hunted by night while the big plant-eater dinosaurs browsed around the clock, says a new study.

Analysing the eyes of fossil dinosaurs, the new research overturns conventional wisdom that dinosaurs were active by day while early mammals scurried around at night. There are at least two historical reasons behind the conventional wisdom.

“First, several decades ago, dinosaurs were portrayed as sluggish, "cold-blooded" animals. It seemed unlikely that these animals were capable of being active at night, when ambient temperatures were cold compared to the day. Second, the idea of diurnal dinosaurs may have arisen because many paleobiologists were trying to explain why majority of mammals is nocturnal - and the picture of dinosaurs occupying the diurnal niche, pushing the mammals into the dark just seemed to fit well,” Lars Schmitz, a researcher at the University of California in Davis told Deccan Herald.

How ecology shapes evolution

In a paper published in Science on April 14, Davis and his colleague Ryosuke Motani, not only explained the hunting habits of dinosaurs but provided insight into how ecology influences the evolution of animal shape and form over tens of millions of years. The scientists worked out the dinosaur's daily habits by studying their eyes. "It was a surprise, but it makes sense," Motani said.

Dinosaurs, lizards and birds all have a bony ring called the "scleral ring" in their eye, a structure that is lacking in mammals and crocodiles. The researchers measured the inner and outer dimensions of this ring, plus the size of the eye socket, in 33 fossils of dinosaurs, ancestral birds and pterosaurs. They took the same measurements in 164 living species.

Day-active, or diurnal, animals have a small opening in the middle of the ring. In nocturnal animals, the opening is much larger. Cathemeral animals -- active both day and night -- tend to be in between.

The size of these features is affected by a species' environment (ecology) as well as by ancestry (phylogeny). For example, two closely related animals might have a similar eye shape even though one is active by day and the other by night: The shape of the eye is constrained by ancestry.

Schmitz and Motani wrote a computer program to separate the “ecological signal” from the “phylogenetic signal” and analysed the signals differently.

“We found striking similarities between the Mesozoic and today’s biosphere. Large herbivores, just like living megaherbivores, were active both day and night, probably because of foraging needs (they just had to eat most of the time), except for the hottest hours of the day when there was risk of overheating,” Schmitz said.

“Small carnivores such as velociraptor were nocturnal hunters. Flying species, including early birds and pterosaurs (like Scaphognathus above, with scleral ring highlighted in blue colour) were mostly day-active (although some of the pterosaurs were nocturnal). These ecological patterns are also found among today's living mammals, lizards, and birds,” he said.

By looking at 164 living species – lizards and birds – the UC Davis team was able to confirm that eye measurements are quite accurate in predicting whether animals are active by day, by night or around the clock.

They then applied the technique to fossils from plant-eating and carnivorous dinosaurs, flying reptiles called pterosaurs, and ancestral birds. The measurements revealed that the big plant-eating dinosaurs were active day and night, probably because they had to eat most of the time, except for the hottest hours of the day when they needed to avoid overheating. Modern mega-herbivores like elephants show the same activity pattern, Motani said.

“It was known that they must have had large foraging budget, and overheating was recognised as a potential reason for nocturnal activity. But until now we could not test this with real morphological data,” Schimtz said.

Velociraptors and other small carnivores were night hunters. They were not able to study big carnivores such as Tyrannosaurus rex, because there are no fossils with sufficiently well-preserved scleral rings.

Flying creatures, including early birds and pterosaurs, were mostly day-active, although some of the pterosaurs, including a filter-feeding animal that probably lived rather like a duck, and a fish-eating pterosaur, were apparently night-active.

The ability to separate out the effects of ancestry gives researchers a new tool to understand how animals lived in their environment and how changes in the environment influenced their evolution over millions of years, Motani said.