Nocturnal Behavior May Predate The Earliest Mammals By 100 Million Years

Chuck Bednar for – Your Universe Online
Originally believed to have occurred around the same time that mammals evolved some 200 million years ago, researchers from the Field Museum of Natural History in Chicago now report that the transition to nocturnal behavior actually occurred more than 100 million years earlier.
Previous theories regarding nocturnal behavior were based on the large brains of mammals, which allow them to better process information from senses such as hearing, touch and smell, and the details of light-sensitive chemicals in the eyes of mammals, the researchers explained.
Now, however, Field Museum curator Kenneth Angielczyk and Claremont McKenna, Pitzer, and Scripps Colleges biology professor Lars Schmitz report in Wednesday’s early edition of the journal Proceedings of the Royal Society B that nocturnal activity might have actually originated within ancient mammal relatives known as synapsids.
“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,” Angielczyk, lead author of the study, said in a statement.
Angielczyk and Schmitz based their work on the analysis of scleral ossicles, tiny bones found in the eyes of birds, lizards and many other types of backboned animals. Modern living mammals lack these bones, the researchers explained, but they were present in many of their ancient synapsid relatives.
“The scleral ossicles tell us about the size and shape of different parts of the eyeball. 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,” Schmitz said.
The bones are extremely delicate, and as such are not typically preserved in synapsid fossils. However, Angielczyk and Schmitz were able to locate data on scleral ossicles from two dozen species, representing most major groups of synapsids, through a review of museum collections and by recruiting other paleontologists to assist on the project.
The information they collected was then compared to similar measurements for living lizards and birds known to have daily activity patterns. They used a special statistical technique, developed by Schmitz, to find that the eyes of ancient synapsid species likely spanned a wide range of light sensitivities, some of which were consistent with activity under bright daytime conditions and others possessing eyes better suited to low-light nighttime conditions.
“The oldest synapsids in the dataset, including the famous sail-backed carnivore Dimetrodon, were found to have eye dimensions consistent with activity at night,” the Field Museum explained. “Based on the ages of the rocks in which these fossils are found, the results indicate that nocturnality had evolved in at least some synapsids by about 300 million years ago or 100 million years earlier than the age of the first mammals.”
The results raise the possibility that the common ancestors of all synapsids were active at night, and the researchers said their findings could help scientists studying the visual systems and behaviors of living mammals. It will also require experts to rethink some long-standing viewpoints, particularly suggesting that mammals became nocturnal in order to avoid competition with dinosaurs.
Furthermore, Angielczyk explained that their research “shows how little we really known about the daily lives of some of our oldest relatives,” and Schmitz added that as he and his colleagues 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.”
Image 2 (below): This is the skeleton of Dimetrodon, an ancient relative of mammals. New research suggests that at least some species of Dimetrodon were active at night (nocturnal). Credit: The Field Museum
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