Dinosaur Egg Study Supports Evolutionary Link Between Dinosaurs And Birds

Paleontologists studying dinosaur egg fossils have reported a new discovery which they say strengthens the evolutionary link between dinosaurs and birds.

Researchers Darla Zelenitsky at the University of Calgary and David Varricchio at Montana State University examined the shell fossils of a small, carnivorous dinosaur called Troodon in order to better understand whether dinosaurs completely buried their eggs in nesting materials, like crocodiles, or if they left them partially uncovered like brooding birds.

In the case of the Troodon the researchers report, the dinosaur only partially buried its eggs in mud.

"Based on our calculations, the eggshells of Troodon were very similar to those of brooding birds, which tells us that this dinosaur did not completely bury its eggs in nesting materials like crocodiles do," said study co-author Zelenitsky, an assistant professor of geoscience.

The Troodon, a small dinosaur of the late Cretaceous period, stood about waist high, with long slender limbs, suggesting the creature could run quickly. Troodons reportedly built dish-shaped nests from sediments, but not plant matter.

"Both the eggs and the surrounding sediments indicate only partial burial; thus an adult would have directly contacted the exposed parts of the eggs during incubation," said lead author Varricchio, an associate professor of paleontology.

There has always been a scarcity of evidence of dinosaur incubation behaviors, but scientists have turned to characteristics of crocodiles and birds for insight.

We know that when crocodiles or birds bury their eggs completely, the eggs tend to have lots of pores, which allows for respiration.

The eggs of brooding species tend to have far fewer pores.

Researchers counted the pores on the shells of the Troodon eggs and assessed how water vapor would have been conducted through the shell and compared that with what's known of the eggs from contemporary crocodiles, mound-nesting birds and brooding birds.

Zelenitsky and Varricchio are optimistic their methods can be applied to other dinosaur species' fossil eggs to show how they may have been incubated.

"For now, this particular study helps substantiate that some bird-like nesting behaviors evolved in meat-eating dinosaurs prior to the origin of birds. It also adds to the growing body of evidence that shows a close evolutionary relationship between birds and dinosaurs," Zelenitsky says.

The study is published in Paleobiology.