Most living mammals are active at night (or nocturnal), and many other mammal species are active during twilight conditions. It has long been thought that the transition to nocturnality occurred at about the same time as mammals evolved, around 200 million years ago. This thinking was based on on features such as the large brains of mammals (good for processing information from senses like hearing, touch, and smell) and the details of light-sensitive chemicals in the eyes of mammals.

It turns out that nocturnal activity might have a much older origin among ancient mammal relatives, called 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," says Kenneth Angielczyk, a curator at The Field Museum. He's the lead author of a paper appearing September 3 in the early edition of Proceedings of the Royal Society B entitled "Nocturnality in Synapsids Predates the Origin of Mammals by 100 Million Years."

The new insights come from an analysis of tiny bones, called scleral ossicles, that are found in the eyes of many backboned animals, including birds and lizards. Living mammals lack scleral ossicles, 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," said Lars Schmitz, a professor of biology at Claremont McKenna, Pitzer, and Scripps Colleges, located 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.

Because scleral ossicles are very delicate, they usually aren't preserved in synapsid fossils. However, by scouring museum collections in the United States and South Africa, and with help from other paleontologists, Angielczyk and Schmitz were able to collect data on scleral ossicles from 24 species that represent most major groups of synapsids. The synapsid data were then compared to a large dataset of similar measurements for living lizards and birds that have known daily activity patterns, using a statistical technique developed by Schmitz.

The technique revealed that the eyes of ancient synapsid species likely spanned a wide range of light sensitivities, with some consistent with activity under bright conditions during the day and others having eyes best suited to low-light conditions at night. Of particular interest was the fact that the oldest synapsids in the dataset, including the famous sail-backed carnivore Dimetrodon, were found to have eye dimensions consistent with activity at night. 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. Indeed, Angielczyk and Schmitz's results raise the possibility that the common ancestor of all synapsids was active at night.

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"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 results should be useful to researchers studying the visual systems and behavior of living mammals, and they also will necessitate the rethinking of some long-held ideas, such as mammals becoming nocturnal to avoid competition with dinosaurs.

Major Findings

1) The eyes of ancient synapsids covered the full spectrum of light sensitivities seen in living aninmals, 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.

2) 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.

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

4) Most plant-eating synapsids that were included in the study are predicted to have been active during the day.

5) Nocturnality may have evolved multiple times in synapsids.

Facts About Synapsids

Synapsida is a large group of backboned animals that live on land.

Synapsids include all living mammals, as well as many extinct mammal relatives.

All fossil synapsids are more closely related to living mammals than they are to any amphibians, reptiles, or birds.

The oldest fossil synapsids are about 315 million years old.

The first mammals evolved about 200 million years ago.

Famous fossil synapsids include the sail-backed predator Dimetrodon (from the Permian Period of Earth history) and Lystrosaurus (from the Triassic Period of Earth History).

Facts About Daily Activity Patterns

Animals that are mainly active during the day are diurnal.

Animals that are mainly active at night are nocturnal.

About 45-55% of living land mammals are nocturnal, and many others are active under twilight conditions.

The light sensitivity of an animal's eyes often reflects their daily activity patterns.

The size and shape of various parts of the eye can be used to predict its light sensitivity.

Scleral ossicles are tiny bones that are found in the eyes of many backboned animals, including birds and lizards. Measurements of the scleral ossicles can be used to make predictions about the light sensitivity of an animal's eyes.

Living mammals do not have scleral ossicles, but the are present in many groups of fossil synapsids.

Scleral ossicles are very delicate, so they often are not preserved in fossils. Therefore, fossil synapsid specimens that preserve scleral ossicles are very rare.

Age

The fossils in this study come from the Permian and Jurassic periods of Earth history.

The Permian Period lasted from 299 million years ago to 252 million years ago

The Jurassic Period lasted from 201 million years ago to 145 million years ago.

The Permian fossils used in this study range in age from about 295 million years ago to about 252 million years ago.

The Jurassic fossils used in this study are about 200 million years old.

The oldest synapsid fossils are about 315 million years old (from the Carboniferous Period of Earth history).

The oldest mammal fossils are about 200 million years old (from near the boundary of the Triassic and Jurassic periods of Earth history).

Geography

Most of the specimens used in this study came from the united Sates (Texas and New Mexico), and South Africa.

Additional specimens used in this study are from Brazil, Russia, and Zambia.

Paper Does not Say