The researchers used a statistical technique to analyze the tiny bones, comparing them with the bones of living animals to prove that the method was robust.
The findings suggest that some synapsids were active during the daytime and others during the nighttime, and some were active at twilight. Interestingly, some of the oldest synapsids they looked at, including Dimetrodon, had eyes whose size indicates these animals were likely active at night.
The findings help researchers understand the biology of these mammalian ancestors. "How were they living? How were they dividing resources?" Schmitz said. "With these little puzzle pieces, we can start building this image of what life may have been like 250 million years ago."
Jörg Fröbisch, a biologist at Humboldt University of Berlin, in Germany, who specializes in synapsid evolution, said he was "very excited" about the findings. "All the evidence seems to point to the fact that there was a wide variety of behavior" among these mammalian ancestors, said Fröbisch, who knows the authors of the new study but was not involved in it.
Fröbisch's colleague at Humboldt, evolutionary biologist Christian Kammerer, agreed."Mammals are thought to be ancestrally nocturnal, but this study demonstrates that this was not some crucial novelty in mammalian evolution: There had been nocturnal members of the mammalian stem lineage for a hundred million years," Kammerer told Live Science.
However, Kammerer cautioned that the results should not be interpreted as definitive, as the activity patterns in living animals are "often messy," and animals may be active during both day and night.
"The Field Museum shouldn't throw out its mural of a Dimetrodon hunting by daylight quite yet," he said.
More from LiveScience:
Copyright 2014 LiveScience, a TechMediaNetwork company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.
Article originally appeared on LiveScience.