Previously, the fossil record indicates that nocturnal behavior began with mammals. The reasoning is that the coordination of neural input necessary for nocturnal activity required a large brain. Kenneth Angielczyk, a curator at The Field Museum, and colleagues from Germany, South Africa, Canada, and the United Sates are the first to find evidence that some mammal-like reptiles (synapsids) were capable of nocturnal activity.
The first mammals developed about 200 million years ago. The oldest known fossil records of synapsids indicate the land-dwelling ancestors of mammals existed as early as 315 million years ago. The research centered on the development of eye structures that would enable animals to see at night or in conditions of low light.
The researchers examined the fossil record for differences in bones found in the eye called scleral ossicles. The bones indicate the size and shape of an animal’s eye and can determine the time of day an animal was capable of being active. Scleral ossicles do not exist in living mammals but can be found in birds and lizards. Due to the small size of the bones, scleral ossicles are rare in the fossil record.
The oldest synapsid examined in the study, the sail-backed carnivore Dimetrodon, had eye bones that are consistent with nocturnal activity. This discovery pushes the advent of nocturnal behavior back in time by 100 million years. Synapsids displayed a variety in eye structure that indicates some but not all of the known synapsids were nocturnal. The most common ancestor of all synapsids and mammals could have been nocturnal but the study does not present evidence that can substantiate this conclusion.