The movie ‘Jurassic Park’ certainly got one thing right. New research findings show that not all dinosaurs dined by day.
Velociraptors hunted at night while the large plant eating dinosaurs ate around the clock, according to a new paper on the eyes of fossil animals to be published later this week in Science Express.
These findings overturn the conventional wisdom that dinosaurs were active during the day while the small, early mammals snuck around at night, said Ryosuke Motani, a geologist at the University of California and co-author of the paper.
It is also providing insight into how ecology and the environment influences evolution of animal forms and shapes over tens of millions of years, according to Motani’s collaborator Lars Schmitz, a postdoctoral researcher at UC Davis.
“The authors conclusion that these dinosaurs were active nocturnally and diurnally challenges the general dogma — that night-time activity was generally restricted to mammals,” says H. Richard Lane, program director in the NSF’s Division of Earth Sciences.
It’s In The Eyes
Motani and Schmitz worked out the dinosaurs’ daily habits by studying their eyes.
Dinosaurs, birds and lizards all have a circular bony ring called the “scleral ring” in their eyes. This ring is absent in mammals and crocodiles.
Schmitz and Motani measured the inner and outer dimensions of this ring and the size of the eye socket in 33 fossils of dinosaurs, ancestral birds and pterosaurs, and in 164 living species.
Day-active, or ‘diurnal’ animals have a small opening in the middle of the ring while the opening is much larger in nocturnal animals.
Cathemeral animals (active during the day and night) tend to be in between.
Though the size of these features can also be affected by ancestry.
For example, two closely related animals might have a similar eye shape despite one being active by day, and the other by night: the shape of the eye is constrained by ancestry, and that would cause bias in the results.
Schmitz and Motani designed and developed a computer program to differentiate the “ecological signal” from this “phylogenetic signal.”
The results of that particular analysis will be in a separate paper published simultaneously in the journal Evolution.
They then applied the technique to fossils, including plant-eating and carnivorous dinosaurs, flying reptiles called pterosaurs and ancestral birds.
The big plant-eating dinosaurs were active day and night, probably because they had to continually eat to support their massive bodies. Modern mega-herbivores like elephants show the same pattern, Motani said.
Velociraptors and other small carnivores were night hunters.
Unfortunately, Schmitz and Motani were unable to study the larger carnivores such as Tyrannosaurus rex, because there are no fossils with sufficiently well-preserved eyes.
Flying creatures, including pterosaurs and early birds, were mostly active during the day, although some of the pterosaurs — including a filter-feeding animal that probably lived like a duck, and a fish-eating pterosaur — were apparently nocturnal.
“This strongly suggests that ecology drives activity,” Schmitz said.
By separating out the effects of ancestry, researchers now have a tool to understand how animals lived in their environment and how changes in the ecology influenced their evolution over millions of years.
Andy Fell, UC-Davis (530) 752-4533 firstname.lastname@example.org
Cheryl Dybas, NSF (703) 292-7734 email@example.com