The nocturnal behavior or activity of most of the living mammals might be traced to an older origin among ancient relatives of mammals, says a new study in the Proceedings of the Royal Society journal published on Sept. 3.
The study says that while most living mammals are considered nocturnal or active at nighttime and countless other mammal species are active in twilight conditions, paleontologists have long believed that transition to a nocturnal behavior or activity came at around the same time when mammals evolved about 200 million years back.
Such belief has its basis on features such as big mammal brains—good for information processing—and “details of light-sensitive chemicals in the eyes of mammals.”
So what are these ancient relatives of mammals called? Synapsids.
Paper’s lead author Kenneth Angielczyk says in a statement that these are considered the “most common in the fossil record between about 315 million years ago and 200 million years ago.”
Angielczyk, also a curator in The Field Museum, adds that the usual belief is that these synapsids were active at daytime or diurnal, but they never established hard evidence to prove this was precisely the case.
Titled Nocturnality in Synapsids Predates the Origin of Mammals by 100 Million Years, the findings came from an evaluation of scleral ossicles, which are tiny bones located in the eyes of numerous backboned animals, such as lizards and birds.
Living mammals, however, don’t have scleral ossicles, according to the study.
These tiny bones provide clues to the shape and size of different parts of an eyeball, says Lars Schmitz, a biology professor at Claremont McKenna, Pitzer, and Scripps Colleges in Los Angeles. He adds that these clues allow them to predict the eyes’ light sensitivity, which in turn provides reflection of the time of day the animal is said to be active.
Study says these scleral ossicles are extremely delicate, which is why these aren’t usually preserved in fossils of synapsids. Yet following thorough research and searching museum collections in South Africa and US, the researchers successfully collected data on 24 species representing most major synapsids groups and its scleral ossicles.
Among its findings, the researchers were greatly interested to the oldest synapsids in their dataset, which include the carnivore Dimetrodon. These were discovered to have eye dimensions that are consistent with nocturnal behavior or activity.
“The idea of a nocturnal Dimetrodon was very surprising," says Angielczyk, adding that it only goes to show how little people really know about the everyday lives of several of our ancient relatives.
The results further showed that nocturnality evolved—at least in some synapsids—around 300 million years back or about 100 years ahead than the age of those first mammals, indicating the likelihood that all synapsids’ common ancestor was indeed active in the evening.
Meanwhile, Schmitz says it’s a first for them to finally be able to come up with informed predictions regarding the synapsids’ activity patterns.
The findings of the study is said to be useful to researchers examining the behavior and visual systems of living mammals, among other issues.
