Researchers have long-believed the transition to nocturnality occurred around the same time mammals evolved around 200 million years ago.
Mammal's large brains (which are good at processing sensory information) and the light-sensitive chemicals present in their eyes were believed to be clues of a nocturnal lifestyle, the Field Museum reported. New findings suggest nocturnal activity might be even older than previously believed.
"Synapsids are most common in the fossil record between about 315 million years ago and 200 million years ago. The conventional wisdom has always been that they were active during the day (or diurnal), but we never had hard evidence to say that this was definitely the case," said Kenneth Angielczyk, a curator at The Field Museum, the lead author of a paper appearing Sept. 3 in the early edition of Proceedings of the Royal Society B.
The new findings come from an analysis of tiny bones, dubbed scleral ossicles, found in the eyes of a number of vertebrates including birds and lizards.Today's mammals don't have this feature, but they were present in ancient synapsid relatives.
"The scleral ossicles tell us about the size and shape of different parts of the eyeball," said Lars Schmitz, a professor of biology at Claremont McKenna, Pitzer, and Scripps Colleges. "In turn, this information allows us to make predictions about the light sensitivity of the eye, which usually reflects the time of day an animal is active. "
Scleral ossicles are extremely delicate and are rarely preserved, but the researchers scoured U.S. and South African museum collections to collect data on the bones from 24 species that represent the most major group of synapsids. The team then compared the information to a large dataset of similar measurements in today's lizards and birds.
The researchers found the eyes of ancient synapsid species fell into a wide range of light sensitivities. One of the oldest observed dinosaurs, the carnivore Dimetrodon, had eye dimensions that signified nocturnal activity; this suggests nocturnal activity developed 100 million years earlier than the time of the first mammals.
"The idea of a nocturnal Dimetrodon was very surprising," Angielczyk said, "but it shows how little we really known about the daily lives of some of our oldest relatives."
"This is the first time we can make informed predictions about the activity patterns of synapsids," Schmitz concluded. "As we discover more fossils, we can continue to test these predictions and start to address questions such as how many times nocturnality evolved in synapsids and whether the synapsids most closely related to mammals were also nocturnal."
