Meanwhile, eggs in open nests (laid by brooding birds) tended to have lower porosity, "so loss of moisture is not as much an issue because gas diffusion is lower,"Zelenitsky said.
Once the researchers figured out that buried eggs tend to have high porosity, and open-nest eggs tend to have low porosity, the researchers turned their attention to the fossilized dinosaur eggs. The eggs were ancient, ranging from 150 million to 70 million years old, but they still retained crucial details, such as porosity.
Most dinosaurs, such as the long-necked sauropods, less-developed theropods and possibly the plant-eating ornithischians, had high-porosity eggs, and likely buried their eggs in nests, the researchers found. But more developed theropods, such as the maniraptorans, had eggs with low porosity, and probably laid their eggs in open nests, they said. It's unclear whether these small theropods also brooded on top of their nests, but there are fossils of small dinosaurs doing just that, which suggests that some did, Zelenitsky said.
However, these well-developed small theropods didn't lay their eggs exactly like today's modern birds. Other fossil evidence shows that early open-nesting dinosaurs still partially buried their eggs, she said.
"It was probably not until modern-looking birds that open nests with fully exposed eggs came to be," Zelenitsky wrote in a statement.
The finding suggests that nest types, and probably incubation styles, changed over time as dinosaurs evolved.
"We don't have eggs for every species of dinosaur, but the more primitive dinosaurs have these buried nests, and the more advanced maniraptoran theropods, which are the closest relatives of birds, laid open-nest eggs that are exposed," Zelenitsky said.
The findings were published online today (Nov. 25) in the journal PLOS ONE.
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