Ancient Mammal Relatives Thrived at Night: Nocturnal Eyes Evolved Earlier Than Expected

It turns out that the ancestors of ancient mammals may have been nocturnal. The new research reveals that the transition to nocturnality occurred far earlier than previously expected.

Most living mammals are active at night, and many other species are active during twilight conditions. In the past, researchers believed that this transition to being active at night occurred around the same time that mammals evolved, around 200 million years ago. Now, though, it seems that this particular trait may have appeared far earlier in ancient mammal relatives, called synapsids.

"Synapsids are most common in the fossil record between about 315 million years ago and 200 million years ago," said Kenneth Angielczyk, one of the researchers, in a news release. "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."

In contrast, it seems as if these creatures preferred night to day. The scientists analyzed tiny bones, called scleral ossicles, that are found in the eyes of many backboned animals. While living mammals lack these bones, they were present in many synapsid relatives. These bones can actually tell researchers about the size and shape of different parts of the eyeball, which can allow them to predict the light sensitivity of the eye. In all, the scientists collected data on scleral ossicles from 24 species that represented most major groups of synapsids.

So what did they find? It turns out that the eyes of ancient synapsid species spanned a wide range of light sensitivities. Some were consistent with activity under bright conditions during the day. Others, though, had eyes that were better suited to low-light conditions at night. In fact, the oldest synapsids had eyes that were meant for nighttime conditions. This, in particular, shows that nocturnality evolved in at least some synapsids about 100 million years earlier than the age of the first mammals.

The findings reveal a bit more about the evolution of mammals, and could be useful to researchers studying the visual systems and behaviors of living mammals. In addition, the research means that scientists will need to rethink some long-held ideas, such as mammals becoming nocturnal to avoid competition with dinosaurs.

The findings are published in the journal Proceedings of the Royal Society B: Biological Sciences.