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Incubation behaviours of oviraptorosaur dinosaurs in relation to body size; Kohei Tanaka, et al., Published 16 May 2018.DOI: 10.1098/rsbl.2018.0135
Topic Started: May 16 2018, 08:17 PM (67 Views)
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How 3,000-Pound Dinosaurs Sat on Eggs, But Didn't Crush Them

By Laura Geggel, Senior Writer | May 15, 2018 07:01pm ET

Posted Image
An artist's depiction of giant oviraptorosaurs nesting
Credit: Zhao Chuang

Imagine a giant, bird-like dinosaur that was so heavy, it weighed as much as a modern-day rhinoceros. Given its heft, how did this bulky, feathered beast sit on its eggs without crushing them to smithereens?

If your immediate response is "carefully," that's a good start, but a new analysis takes a deeper dive. These small-to-humongous dinosaurs, known as oviraptorosaurs, laid their oval-shaped eggs in a doughnut-like circle, and these nests had different shapes depending on the size of the dinosaur.

Smaller oviraptorosaurs either had no doughnut hole or a small one where they could sit with their eggs around them, while larger oviraptorosaurs created nests with big holes in the center where the dinosaurs could plop down without squashing the eggs located in a circle around them, a new study finds.

"Oviraptorosaurs appear to have adapted to being able to sit on their clutches, even at giant body size," study co-author Darla Zelenitsky, an assistant professor of paleontology at the University of Calgary in Alberta, Canada, told Live Science.

Posted Image
An illustration of regular-size (top) and giant oviraptorosaur (bottom) nesting. Notice how the smaller dinosaur sits directly on the eggs, while the larger dinosaur sits in the middle of the nest, where it won't squash the developing babies.
Credit: Masato Hattori; Biology Letters 2018

However, no birds alive today practice this trick. (Birds evolved from theropod dinosaurs, which were mainly meat-eating, bipedal beasts such as oviraptorosaurs and Tyrannosaurus rex.)

Nearly all species of modern birds sit on their eggs, Zelenitsky said.

"The largest birds, however, are much smaller than the largest oviraptorosaur," Zelenitsky said.

Egg-cellent nesting

Oviraptorosaurs were bizarre-looking dinosaurs. They had parrot-like heads and toothless beaks, and some sported head crests, much like modern cassowary birds do. These dinosaurs ranged in size from a few dozen pounds (Nomingia, for instance, weighed about 80 lbs., or 37 kilograms) to a few thousand pounds (Gigantoraptor weighed up to 4,400 lbs., or 2,000 kg).

Over the years, researchers have found many well-preserved oviraptorosaur eggs and skeletons, including fossils of oviraptorosaur parents sitting on their nests. (Technically, the nests themselves didn't fossilize, so scientists call the preserved eggs a "clutch." But, for simplicity's sake, we'll refer to them as nests.)

Posted Image
The nest of a regular-size oviraptorosaur dinosaur from China. This nest is about 24 inches (60 centimeters) in diameter.
Credit: Kohei Tanaka; Biology Letters 2018

The nests the researchers examined, about 40 in all, were between 100 million and 70 million years old, Zelenitsky said.

"Oviraptorosaurs seem to have been very picky about how their eggs were arranged in the nest," she said. This proved helpful, because it allowed the researchers to accurately measure the diameters of the entire nests, as well as the doughnut holes in the middle, Zelenitsky said.

The diameters of the entire nests ranged from about 16 inches (40 centimeters) for the pint-size oviraptorosaurs under 88 lbs. (40 kg) to nearly 11 feet (3.3 meters) for the 3,300-lb. (1,500 kg) beasts, Zelenitsky said. After measuring the doughnut holes in the center, the researchers found that smaller oviraptorosaurs sat either directly on their eggs or in a small hole in the center of the nest. Meanwhile, the bigger dinosaurs placed their eggs in a ring farther from the center of the nest, meaning oviraptorosaurs may have had little contact with the eggs when they sat down.

Posted Image
The nest of a giant oviraptorosaur dinosaur from China. The eggs were laid in a giant ring-shaped clutch with a large central opening. The adult would have sat in the middle of the more than 10-foot-wide (3 meters) nest.
Credit: Kohei Tanaka

"This egg-free opening in clutches became larger with increasing species size," Zelenitsky said. "In the largest species, the opening, rather than the eggs, occupied most of the clutch area."

These doughnut holes allowed the adult oviraptorosaurs to sit in the nest and maybe even touch the eggs — perhaps allowing the animals to protect, shelter and provide heat for their developing babies. However, if keeping the eggs warm was the goal, this strategy may have been lacking, the researchers noted.

"This brooding behavior may have been less effective in large species, because there may have been less contact with the eggs due to the modified configuration of their clutches," the researchers wrote in the study.

The study will be published online Wednesday (May 16) in the journal Biology Letters.

https://www.livescience.com/62578-how-giant-dinosaurs-sat-on-eggs.html




Journal Reference:
Kohei Tanaka, et al., Incubation behaviours of oviraptorosaur dinosaurs in relation to body size Published 16 May 2018.DOI: 10.1098/rsbl.2018.0135

Abstract
Most birds sit on their eggs during incubation, a behaviour that likely evolved among non-avian dinosaurs. Several ‘brooding' specimens of smaller species of oviraptorosaurs and troodontids reveal these non-avian theropods sat on their eggs, although little is known of incubation behaviour in larger theropod species. Here we examine egg clutches over a large body size range of oviraptorosaurs in order to understand the potential effect of body size on incubation behaviour. Eggshell porosity indicates that the eggs of all oviraptorosaurs were exposed in the nest, similar to brooding birds. Although all oviraptorosaur clutches consist of radially arranged eggs in a ring configuration, clutch morphology varies in that the central opening is small or absent in the smallest species, becomes significantly larger in larger species, and occupies most of the nest area in giant species. Our results suggest that the smallest oviraptorosaurs probably sat directly on the eggs, whereas with increasing body size more weight was likely carried by the central opening, reducing or eliminating the load on the eggs and still potentially allowing for some contact during incubation in giant species. This adaptation, not seen in birds, appears to remove the body size constraints of incubation behaviour in giant oviraptorosaurs.

http://rsbl.royalsocietypublishing.org/content/14/5/20180135
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