Eyes have it: Some dinos were night owls

UC DAVIS (US) — Velociraptors did their hunting at night, according to a new study of the eyes of fossil animals, while plant-eaters scrounged for food around the clock.

The study, published online in the journal Science, goes against conventional wisdom that says early mammals were the first creatures to take to the night. “It was a surprise, but it makes sense,” says Ryosuke Motani, professor of geology at University of California, Davis.

The research is providing insight into how ecology influences the evolution of animal shape and form over tens of millions of years.

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. Motani and postdoctoral researcher Lars Schmitz measured the inner and outer dimensions of the 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, those that are active both day and night, tend to be in between.

The size of these features is affected by a species’ environment (ecology) and by its 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.

The researchers wrote a computer program to separate the “ecological signal” from the “phylogenetic signal.” The results of that analysis are in a separate paper published in the journal Evolution.

By looking at 164 living species, the researchers were able to confirm that eye measurements accurately predict 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 megaherbivores like elephants show the same activity pattern.

Velociraptors and other small carnivores were night hunters. Studying big carnivores such as Tyrannosaurus rex wasn’t possible 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.

The nocturnal Ctenochasma elegans, had a scleral ring with a wide opening to help it see in dim light. (Credit: Lars Schmidt)

The work was funded by the National Science Foundation and a postdoctoral fellowship from the Deutsche Forschungsgemeinschaft (Germany).

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