New research is changing the understanding of when mammals evolved nocturnal lifestyles. Fossil analysis points to a group of ancient mammal relatives called synapsids thriving at night or in the twilight. This group of animals lived more than 300 million years ago.

That puts the evolution of nighttime behavior at 100 million years earlier than 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, lead author of the paper published in the Proceedings of the Royal Society B: Biological Sciences.

Researchers analyzed tiny bones in the eyes called scleral ossicles. Analyzing these bones proved to be a challenge since they are very delicate. Angielczyk and his team had to scour museum collections to get the data they needed.

The researchers found that the Dimetrodon had eye dimensions that pointed to activity at night.

“The idea of a nocturnal Dimetrodon was very surprising,” said Angielczyk, “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,” added Schmitz. “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.”

The major findings were detailed in a press release. Here’s the big three.

The eyes of ancient synapsids covered the full spectrum of light sensitivities seen in living animals, with some species having eyes best suited to activity under bright conditions during the day, others having eyes best suited to low-light conditions at night, and still others having eyes suited to activity under twilight conditions.

The eyes of the oldest synapsids species considered in the study, which are about 300 million years old, are predicted to have been best suited to activity under low light conditions at night. These animals are about 100 million years older than the oldest fossils of mammals.

The common ancestor of all synapsids (including living mammals) may have been nocturnal (active at night).

The study authors also pointed out what their paper doesn’t say. It doesn’t say “all synapsids were nocturnal” or “all synapsids were diurnal.” It also does not definitively say “whether the most recent common ancestor of living mammals was nocturnal or diurnal.

Image credit: Kenneth Angielczyk

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