Hefty dinosaurs had a trick for sitting on eggs safely | Science News for Students

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
Jul 3, 2018 — 6:45 am EST
an illustration of oviraptorosaurs sitting on eggs

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.

fossilized egg clutches
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.

Power Words

(more about Power Words)

biology     The study of living things. The scientists who study them are known as biologists.

birds     Warm-blooded animals with wings that first showed up during the time of the dinosaurs. Indeed, dinosaurs are their ancestors. Birds are jacketed in feathers and produce young from the eggs they deposit in some sort of nest. Most birds fly, but throughout history there have been the occasional species that don’t.

brood      A group of related animals that emerges in a specific region in the same year. Depending on the animal type, the collective group is sometimes also known as a year class. (verb) The act of guarding and/or incubating eggs.

clutch     (in biology) The eggs in a nest or the hatchlings from that collective group of eggs.

diameter     The length of a straight line that runs through the center of a circle or spherical object, starting at the edge on one side and ending at the edge on the far side.

dinosaur     A term that means terrible lizard. These ancient reptiles lived from about 250 million years ago to roughly 65 million years ago. All descended from egg-laying reptiles known as archosaurs. Their descendants eventually split into two lines. For many decades, they have been distinguished by their hips. The lizard-hipped line are believed to have led to the saurichians, such as two-footed theropods like T. rex and the lumbering four-footed Apatosaurus (once known as brontosaurus). A second line of so-called bird-hipped, or ornithischian dinosaurs, appears to have led to a widely differing group of animals that included the stegosaurs and duckbilled dinosaurs. But a new 2017 analysis now calls into question that characterization of relatedness based solely on hip shape.

egg     The unfertilized reproductive cell made by females.

emu     A large, flightless bird. Only the ostrich is a larger bird. The emu is native to Australia, Indonesia, the Philippines and some South Pacific islands. The animal belongs to a very primitive family within modern birds. It eats fruits, seeds and insects and tends to travel over a broad region rather than settling down in one small area.

oviraptorosaur     A group of feathered dinosaurs with beaks. Their short, toothless heads sported an ornamental crest on top. Some were as small as sheep. Others had bodies that stretched up to 6.7 meters (22 feet). As theropod dinosaurs, they evolved from meat eaters. But once they lost their teeth, their diet likely broadened to include plants.

paleontologist     A scientist who specializes in studying fossils, the remains of ancient organisms.

species     A group of similar organisms capable of producing offspring that can survive and reproduce.

strategy     A thoughtful and clever plan for achieving some difficult or challenging goal.


Journal: K. Tanaka et al. Incubation behaviours of oviraptorosaur dinosaurs in relation to body size. Biology Letters. Vol. 14, May 2018. doi:10.1098/rsbl.2018.0135.