Most mammals around today are active at night or during twilight hours. For a long time, we thought that the transition to nocturnal behavior happened early in mammalian history around 200 million years ago. Not so, according to researchers studying ancient mammal relatives: Being nocturnal predates the origin of mammals by more than 100 million years. The findings are published in Proceedings of the Royal Society B this week.
The carnivorous Dimetrodon with its sail-like finback seems like an unlikely mammal relative, but all mammals and pelycosaurs (which include Dimetrodon) are members of a group called Synapsida. Fossil synapsids are more closely related to living mammals than they are to reptiles or birds. Although synapsids are sometimes called dinosaurs or “mammal-like reptiles,” those are technically incorrect. Pre-mammalian synapsids dominated the land until losing ground to dinosaurs. In fact, all branches of synapsids are extinct, except for mammals.
"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," says Kenneth Angielczyk from the Field Museum in a news release. Nocturnality was thought to have occurred when mammals evolved because larger brains were good for processing sensory information and because of the light-sensitive chemicals in the eyes of mammals.
Angielczyk and Lars Schmitz of the Claremont Colleges examined tiny, ring-shaped bones called scleral ossicles, which are found in the eyes and eye sockets of many animals. Mammals nowadays don’t have these, but ancient relatives of mammals did. By studying these bony features in museum collections, the team reconstructed the light sensitivity of the eyes of 24 species of synapsids that lived during the Carboniferous to Jurassic periods, from 315 million to 190 million years ago.
The arrow in the synapsid skull photo to the right indicates where the fossil scleral ring is found. The duo compared their synapsid data to scleral measurements of living lizards and birds with known daily activity patterns. Turns out, the eyes of ancient synapsids spanned the full spectrum of light sensitivities: Some were consistent with activity under bright conditions during the day, others had eyes best suited to low-light conditions at night. The latter included the oldest synapsid in the dataset, Dimetrodon.
Nocturnal activity, the findings suggest, evolved in synapsids by at least 300 million years ago -- that’s more than 100 million years before the appearance of mammals. Furthermore, the researchers say the common ancestor of all synapsids may have been nocturnal, though they can't say for sure if synapsids most closely related to mammals were also nocturnal.
“It shows how little we really know about the daily lives of some of our oldest relatives,” Angielczyk adds.
We might also have to rethink some long-held ideas, such as mammals becoming nocturnal to avoid competition with dinosaurs, who showed up after synapsids.
Images: Marlene Hill Donnelly (top), Kenneth Angielczyk (middle)