An Early Nocturnal Ancestor
A majority of living mammals today are nocturnal—and conventional wisdom tells us that this transition to nocturnality occurred as mammals evolved from their early mammal ancestors, synapsids, about 200 million years ago. It’s largely assumed that those synapsids were diurnal—active mostly during the daytime—but The Field Museum’s Kenneth Angielczyk, Associate Curator of Paleomammalogy and co-author Lars Schmitz, Assistant Professor of Biology, Keck Science Department, Claremont McKenna, Pitzer and Scripps Colleges, wanted to put it to the test.
Eye sockets and other small bones that support the eyes give indirect information about the light sensitivity of an animal’s eye. Angielczyk and Schmitz looked mostly at the scleral ossicles of ancient synapsids, a ring of tiny bones located at the front of the eye, supporting muscle movement in life. Measurement of these bones—and then a further statistical analysis—can be used to infer the light sensitivity of the animal, and whether they were active at night or in the day.
However, they also faced the task of sourcing these fragile specimens. These bones were quite delicate and thus are rarely preserved. “They aren't directly connected to the rest of the bones of the skull,” Angielcyzk explains. “So when the eyeball decays, it's very easy for them to just fall out. Because they are small and delicate, even when they are preserved, they can easily be damaged or destroyed during preparation.”
Luckily, fossil preparation techniques are better than ever, allowing such delicate features to be examined. They looked at fossils from museums around the world, and ended up gathering data on 24 synapsid species.
The conclusion they came to provided a much more nuanced picture of these ancient mammal ancestors. The eye bones that Angielczyk and Schmitz examined gave evidence that synapsids were not just diurnal, but in fact showed the full range of daily activities—diurnal, nocturnal, even some that were most active during twilight. The oldest examples of nocturnality occurred in species that were over 300 million years old, 100 million years older than the earliest mammal! This has significant implications to how we understand the behaviors of our mammal ancestors, and shows the impact of ongoing paleomammalogy research going on at The Field Museum.
Learn more about the study Nocturnality in Synapsids Predates the Origin of Mammals by over 100 Million Years, published online on Proceedings of the Royal Society B and covered by Science Daily.
