Hefty dinosaurs had a trick for sitting on eggs safely

Laying a clutch in the shape of a hollow ring let dinos warm their eggs without squashing them


Small species of oviraptorosaurs (dinosaurs that include the ancestors of birds) sat on their eggs. That’s what the dinosaur in the background illustration is doing. But larger species, like the one in the front illustration, sat in the center of a ring of eggs so they wouldn’t crush them.

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

Birds keep their eggs warm by sitting squarely atop them. This is true from chickadees to ostriches. But ancestors of today’s birds include some hefty dinosaurs. Scientists thought dinosaurs might not be able to sit on their eggs without crushing them. In fact, certain heavy dinos had a clever brooding strategy. They sat in an open space at the center of a ring of eggs, rather than right smack on top of them, new data show.

The researchers studied about three dozen groups of fossilized dino eggs. A group of eggs laid together is known as a clutch. Those in this study belonged to different species of oviraptorosaurs (Oh-vih-rap-TOR-uh-sors). These were toothless, feathered dinosaurs that descended from meat-eaters.

Some fossilized egg clutches were laid in a donut shape, with a hole in the middle. Clutches laid by larger oviraptorosaur species had the largest openings at the center. Kohei Tanaka is a paleontologist at Nagoya University Museum in Japan. His team described its findings May 16 in Biology Letters.

It’s not possible to know the exact species of oviraptorosaur based solely on the eggs. So the researchers just divided the eggs into three size groups.

The smallest eggs were less than 170 millimeters (6.7 inches) long. These likely came from species weighing from a few tens of kilograms to 100 or 200 kilograms (about 400 pounds). That’s similar in size to today’s ostriches and emus. Medium-sized eggs were between 170 and 240 millimeters (6.7 and 9.4 inches) long. The largest eggs were those more than 240 millimeters (9.4 inches) long. The species that laid them may have weighed up to 2 tons.

Clutches from small oviraptorosaurs had a small gap in the middle, compared to the overall clutch size (left). Medium-sized dinosaurs left a slightly larger space in the middle of their eggs (center). For the largest species, the central opening was widest (right).
K. Tanaka et al/Biology Letters 2018

The team measured the diameter of each clutch. If there was a hole at the center of the arrangement, they measured that too. For the largest species, the hole took up most of the area of the clutch, the team now reports. That would have let the biggest parents plop themselves safely on the ground in the center of the clutch.

Scientists don’t know of any modern birds that share the same brooding style. But it let big dinos incubate their eggs — without squashing them flat.

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