A preference for a nocturnal life style, favored by many of today's mammal species, started much earlier in Earthly life forms than was previously believed, researchers say.

There is fossil evidence for a nighttime lifestyle in a group of ancient mammal predecessors called synapsids 300 million years ago -- at least 100 millions years earlier than the time the first true mammals made their debut on the evolutionary stage, they say.

Little ring-shaped bones known as scleral ossicles, embedded in eye sockets of the ancients creatures, are evidence that some of them were active at night or in deep twilight, a study published in the Proceedings of the Royal Society B reports.

"The scleral ossicles tell us about the size and shape of different parts of the eyeball," says biologist Lars Schmitz of Claremont McKenna, Scripps and Pitzer Colleges, near Los Angeles. "In turn, this information allows us to make predictions about the light sensitivity of the eye, which usually reflects the time of day an animal is active."

The researchers studied the scleral ossicles in species representing major groups of synapsids, finding evidence of considerable differences in light sensitivity.

While some indicated the animals would have been active in bright daylight conditions, others had eyes obviously better adapted to low-light nighttime conditions, they report.

The discovery of nighttime activity far back in the line that led to mammals contradicts a long-held belief about nocturnal lifestyles only emerging with the coming of true mammals around 200 million years ago, and that they evolved the preference to avoid the possibility of becoming a meal for the dinosaurs they were sharing the Earth with.

The new findings suggest that belief needs to be reexamined.

Even the oldest of the synapsids, a sailbacked carnivore known as a Dimetrodon, had eye dimensions that suggested activity at night, the researchers say.

"The idea of a nocturnal Dimetrodon was very surprising," says lead study author Kenneth Angielczyk of The Field Museum in Chicago, "but it shows how little we really know about the daily lives of some of our oldest relatives."

"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," he says.

The new fossil findings, in addition to adding to the knowledge of the evolution of the earliest mammal relatives, can also be of use to scientists studying visual systems and the behavior of today's mammals, the researchers say.

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