Here’s how hefty dinosaurs sat on their eggs without crushing them

Sitting in the center of a ring of eggs kept dinos’ weight off — and warmth near — the eggs

EGG SITTERS  Small species of oviraptorosaurs (background illustration) — dinosaurs that include birds’ ancestors — sat on their eggs, but larger species (foreground) sat in the eggless center of the clutch so as not to crush the eggs.

Masato Hattori, K. Tanaka et al/Biology Letters 2018

Brooding birds from chickadees to ostriches sit squarely on their eggs. But scientists thought some of the heftier dinosaur ancestors of birds might not be able to do that without crushing the clutches. Now, a new study finds that certain dinos with a little extra junk in the trunk also had a clever brooding strategy: They sat within an open space at the center of a ring of eggs, rather than right smack on top of them.

The researchers studied about three dozen fossilized egg clutches belonging to different species of oviraptorosaurs, a group of feathered meat-eating dinosaurs. Clutches laid by larger oviraptorosaur species also had the largest openings at the center, a team led by paleontologist Kohei Tanaka of Nagoya University Museum in Japan reports May 16 in Biology Letters.

Although it’s not possible to determine the exact species of oviraptorosaur from the eggs alone, the researchers divided the eggs into three classifications based on size. The smallest eggs, at less than 170 millimeters long, were assigned to the group Elongatoolithus, which likely included species with body masses ranging from a few tens of kilograms up to 100 or 200 kilograms — similar to today’s ostriches and emus. Medium-sized eggs were assigned to the group Macroolithus and the largest eggs, more than 240 millimeters long, to the group Macroelongatoolithus. The dinos that laid the biggest eggs may have had body masses as high as about 2,000 kilograms.

The team then measured the diameter of the clutch and — if there was one — the diameter of the hole at the center of the clutch. For the largest species, the hole took up most of the area of the clutch, the team found. That, the researchers say, allowed the biggest oviraptorosaur parents to plop themselves in the center of the clutch, reducing the weight load on the eggs while still keeping the eggs warm. No modern birds are known to share that same brooding style.

Carolyn Gramling is the earth & climate writer. She has bachelor’s degrees in geology and European history and a Ph.D. in marine geochemistry from MIT and the Woods Hole Oceanographic Institution.

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