Mammals have capacity to see and move about in the night. This capability was believed to have been developed during the evolution of mammals some 200 million years ago. The fact that mammals had large brains which helped in processing information pertaining to sight, smell and hearing and this helped them to move hunt and survive in the night. Mammal’s nocturnal habits were further confirmed by studying the details of light sensitive chemicals present in their eyes.
However recent studies have indicated that nocturnal activity might be of older origin and could have been present in the ancestors of mammals, synapsids. This conclusion was arrived after an analysis of tiny bones, called scleral ossicles. These scleral ossicles were found in the eyes of number vertebrates like birds and lizards. However many mammals lack scleral ossicles which were present in many of their ancient synapsid relatives.
Kenneth Angielczyk, lead author of a new paper on the subject and a curator at The Field Museum says, “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.”
Since scleral ossicles are very delicate they are seldom preserved in synapsid fossils. The researchers studied museum collections in the United States and South Africa. Ably assisted by other paleontologists, Angielczyk and Schmitz were successful in collecting information about scleral ossicles from 24 species that characterize most major groups of synapsids. Using a statistical technique developed by Schmitz, the synapsid data was then compared to a large dataset of similar measurements for living lizards and bird
It was revealed that the sight organs of ancient synapsid species were sensitive to a wide range of light sensitivities. Some of the species had eyes which were suited for bright conditions while others were best suited for poor light conditions.
The study will be useful for researchers who are studying the visual systems and behavior of living mammals. It will also lead to a rethinking of long held ideas that mammals became nocturnal to compete with dinosaurs.