April 14, 2011 - Nightlife during the Mesozoic era was full of activity and fraught with dangers, suggests a new study that found some dinosaurs and prehistoric reptiles were nocturnal, with carnivorous dinosaurs likely sneaking up on victims after dark.
The discovery, published in the latest issue of Science, counters the traditional view that, in order to conserve energy, these animals were active only by day, leaving the night to small mammals.
This plant-eating dinosaur, Protoceratops andrewsi, was active both day and night.
Nighttime was, in fact, the right time for some pterosaurs, including a fish-eating species and a filter feeder that probably lived like a duck. Numerous predatory dinosaurs and reptiles that lived 250 to 65 million years ago also came alive at night.
"This makes sense because given their small body size they were probably able to approach their prey very closely before attacking," co-author Lars Schmitz told Discovery News. "Even if the prey had a good sensory system to notice a predator closing in, the success rate of a nocturnal attack may be higher than a diurnal attack."
The skeleton of the nocturnal flyer, Ctenochasma elegans, a pterosaur, is shown.
Schmitz, a researcher in the Department of Evolution and Ecology at UC Davis, and colleague Ryosuke Motani made the determination by studying a bony eye feature known as the scleral ring (shown here in the day- and night-active dinosaur, Saurolophus osborni). Diurnal, or day-active, animals with the ring have a small opening, whereas nocturnal animals have a much larger opening to maximize light at night.
Schmitz and Motani 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 also took the same measurements in 164 living species, which confirmed their belief that this type of investigation accurately predicts what time of day an animal is active.
The measurements revealed big, plant-eating dinosaurs, including Diplodocus longus, shown here, tended to be active during both the day and night, probably because they had to keep fueling their huge bodies. Elephants today show the same pattern, resting primarily during the hottest hours of the day to avoid overheating.
No known fossils for big carnivores, such as Tyrannosaurus rex, (shown here) retain sufficiently well-preserved scleral rings, so these impressive hunters weren't included in this study. The researchers can make some educated guesses, though.
"T. rex was very large compared to the predators that we analyzed in our study," Schmitz said. "Given its enormous size, we think it was likely day-and-night active, an opportunistic predator that did not have to rely on the dark mantle of the night to sneak up on its prey."
He added, "Day-and-night activity would also fit with the scenario of T. rexbeing a scavenger.
The scientists further determined that the sizes and shapes of an animal's eye features are affected by ecology and ancestry. This helps to explain why two closely related animals might have similar eye shapes even though one animal is diurnal and the other is nocturnal.
The researchers even wrote a computer program, outlined in the journal Evolution, which explains how to tease apart a species' ecological influences from its ancestral ones.
This program, along with other analysis techniques could allow paleontologists to learn more about how these species lived and how their environments influenced their evolution.
This is a close-up of the skull of the nocturnal carnivore Juravenator starki.
A close-up of the eye socket of the plant-eating Protoceratops.
Lawrence Witmer, a world-renowned paleontologist and professor of anatomy at Ohio University, describes this latest research as "truly exciting, in part because of what it tells us about dinosaurs and their activity patterns, but also, it's a great example of the kind of modern work that's being done on extinct animals that draws inferences based on meticulous comparisons in the modern-day realm."
This is the fossil of a nocturnal carnivore, Velociraptor mongoliensis.
Witmer believes we're now "in a golden age of dinosaur biology," with important discoveries on dinosaur physiology being made too.
Such research, he concluded, "sheds a new light on dinosaurs…which is handy because a lot of them were running around in the dark!"