Talk about vibrant nightlife. We thought the threat of predatory dinosaurs forced early mammals to become nocturnal. But it looks like their ancestors were night owls long before that.
We can tell when an animal is most active by looking at its eyes: big ones are a sign of nocturnal habits, while smaller eyes indicate an animal is active during the day, or diurnal. Eyes don鈥檛 fossilise, but a bony ring that surrounds some animals鈥� eyes does, revealing eye size in extinct animals and hinting at what time of day the animal was active.
Mammals don鈥檛 have this ring, but it was present in the mammal-like reptiles or 鈥溾�� they evolved from. So at the W.M. Keck Science Department in Claremont, California studied the bony rings of 300-million-year-old synapsid fossils.
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With at the Field Museum of Natural History in Chicago, he found many synapsids were most active at night. When mammals appeared 100 million years later, they may have inherited their nocturnal ways from synapsids.
That might be a stretch, says at the University of Groningen in the Netherlands. Transitions from nocturnal to diurnal habits occurred many times, he says, so mammals may have become nocturnal independently.
But if there is a direct link between nocturnality in early synapsids and mammals, we may have misunderstood why mammals adapted to night life. Dinosaurs only appeared 240 million years ago, so it鈥檚 unlikely the synapsids became nocturnal to avoid being eaten. In fact many nocturnal synapsids were top predators, like , a 4.5-metre beast with a prominent fin on its back.
鈥淭oday, predators are largely active in dim light conditions,鈥� says Schmitz, and his earlier work suggested that some predatory dinosaurs were nocturnal. 鈥淚t鈥檚 easier to attack prey.鈥� He thinks synapsids, too, probably used cover of darkness to kill, not to escape being killed.
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