Eggshells Reveal How Dinosaurs Nested

A look at dinosaur eggs suggests most dinosaurs buried their eggs in nests covered with dirt and vegetation, a tactic also used by modern crocodiles.

The fragile remains of 150-million-year-old eggshells are helping researchers figure out what kinds of nests dinosaurs created for their eggs, according to a new study.

A comprehensive look at 29 types of dinosaur eggs suggests that most dinosaurs buried their eggs in nests covered with dirt and vegetation, a tactic also used by modern-day crocodiles.

But some small theropods (mostly meat-eating, bipedal dinosaurs) that were closely related to birds used another strategy: They laid their eggs in open nests, much like most birds do today, the researchers found. [Image Gallery: Dinosaur Daycare]

"The evolution of open nests and brooding behavior could have allowed small theropod dinosaurs, and obviously birds, to move to other nesting locations other than on the ground," which may have helped their evolutionary success, said study co-researcher Darla Zelenitsky, an assistant professor of paleontology at the University of Calgary in Canada.

Researchers have spent years piecing together limited evidence about how dinosaurs raised their young, but there are few dinosaur egg fossils to study, said study lead author Kohei Tanaka, a doctoral student in the faculty of science at the University of Calgary.

"Dinosaur nest structures and nesting materials are usually not preserved in the fossil record," Tanaka said in a statement. "In the past, this lack of data has made working with dinosaur eggs and eggshells extremely difficult to determine how dinosaurs built their nests and how the eggs were incubated for hatching young."

Luckily, researchers can compare the fossilized eggs of dinosaurs to those of the dinosaur's closest living relatives: crocodiles and birds. Crocodiles bury their eggs in nests on the ground and cover them with sand, dirt and rotting vegetation, which keeps the eggs warm. In contrast, birds usually lay their eggs in open nests, and brood on them during incubation.

These modern eggs - specifically, the number and size of the pores in their shells - gave researchers clues about the nest type. They collected data on the eggs and nests of more than 120 modern species of birds and crocodiles, and found stark differences between the two. [Album: Discovering a Duck-Billed Dino Baby]

Buried eggs tended to have a high porosity, or larger and more holes in the shell that allow for vapor and gas exchange between the outside world and the embryo. However, burying the eggs helps to retain their humidity and moisture as the embryo develops inside, Zelenitsky told Live Science.

Meanwhile, eggs in open nests (laid by brooding birds) tended to have lower porosity, "so loss of moisture is not as much an issue because gas diffusion is lower,"Zelenitsky said.

Once the researchers figured out that buried eggs tend to have high porosity, and open-nest eggs tend to have low porosity, the researchers turned their attention to the fossilized dinosaur eggs. The eggs were ancient, ranging from 150 million to 70 million years old, but they still retained crucial details, such as porosity.

Most dinosaurs, such as the long-necked sauropods, less-developed theropods and possibly the plant-eating ornithischians, had high-porosity eggs, and likely buried their eggs in nests, the researchers found. But more developed theropods, such as the maniraptorans, had eggs with low porosity, and probably laid their eggs in open nests, they said. It's unclear whether these small theropods also brooded on top of their nests, but there are fossils of small dinosaurs doing just that, which suggests that some did, Zelenitsky said.

However, these well-developed small theropods didn't lay their eggs exactly like today's modern birds. Other fossil evidence shows that early open-nesting dinosaurs still partially buried their eggs, she said.

"It was probably not until modern-looking birds that open nests with fully exposed eggs came to be," Zelenitsky wrote in a statement.

The finding suggests that nest types, and probably incubation styles, changed over time as dinosaurs evolved.

"We don't have eggs for every species of dinosaur, but the more primitive dinosaurs have these buried nests, and the more advanced maniraptoran theropods, which are the closest relatives of birds, laid open-nest eggs that are exposed," Zelenitsky said.

The findings were published online today (Nov. 25) in the journal PLOS ONE.

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Article originally appeared on LiveScience.

A duckbill dinosaur (left) stands next to its eggs buried in the ground, and a birdlike oviraptorid dinosaur (right) incubates its eggs in an open nest.

Paleontologists have just assembled the most comprehensive family tree of meat-eating dinosaurs. Published in the journal Current Biology, the family tree reveals how diverse carnivorous dinosaurs were and how birds eventually evolved from them. Tyrannosaurs, including

Tyrannosaurus rex

, are one key group on the meat-loving dino family tree. Lead author Steve Brusatte of the University of Edinburgh's School of GeoSciences told Discovery News, "The most iconic dinosaurs of all, tyrannosaurs were more than just the 13-meter-long (nearly 43 feet long), 5-ton monster predator

T. rex

." "Tyrannosaurs were an ancient group that originated more than 100 million years before

T. rex

, and for almost all of their evolutionary history they were small carnivores not much bigger than a human in size."

New T-Rex Tracks Add to Pack-Hunting Theory

"Some of the rarest theropods (two-legged carnivorous dinos) of all, compsognathids are represented by about half a dozen species," Brusatte said. "They were small, sleek meat-eaters which ate small prey like lizards." One of the more recent finds, Juravenator from Germany, is known from a nearly complete fossil.

Dinosaurs That Loved the Nightlife

Ornithomimosaurs were theropods called "ostrich mimic" dinosaurs -- for a reason. "Like living ostriches, they could run fast on their long legs and used their sharp, toothless beaks to eat a varied diet of small prey, plants, and perhaps even small shrimps in the water just like living flamingos," Brusatte explained. "A recent find in Canada showed that not only were ornithomimosaurs feathered, but they also had complex feathers on their arms that would have formed something of a wing, although they couldn't fly."

Brusatte describes therizinosaurs as "perhaps the weirdest theropods of all." "These were big, bulky, cumbersome dinosaurs that ate plants. They had fat barrel-shaped chests, stocky legs, and big claws on their arms," Brusatte said. For many years paleontologists argued about which group this dinosaur belonged to, only recently settling on theropods. This means they were fairly closely related to birds, despite their weird anatomy.

Nests of Big-Clawed Dinosaurs Found in Mongolia

Alvarezsaurs were among the smallest dinosaurs of all, measuring just a few feet long and weighing less than 5 kilograms (10 pounds). "Some of them had only a single functional finger on their hand, which they probably used to prod deep into the nests of bugs, which were one of their main food sources," Brusatte said.

New Dinosaur Bolsters Bird-Dino Connection

Oviraptorosaurs, were a group of small omnivores that were lightweight, lacked teeth and had tall, hollow crests on their skulls. The recently discovered Anzu -- the so-called "

chicken from hell

" -- came by its nickname honestly. It towered more than five feet tall, weighed more than 400 pounds, and was covered in a coat of feathers.

"Troodontids were probably the smartest dinosaurs of all, as they had the largest brains relative to their body size of any dinosaur group," Brusatte said. "Most troodontids were small, fast-running dinosaurs that probably ate both meat and plants." Among the most recently discovered of this group are the small, feathered Anchiornis and Xiaotingia, which lived in China about 160 million years ago. "They look eerily similar to birds, so much so that some researchers think they could be primitive birds rather than troodontids with wings and feathers," Brusatte said.

World's Earliest Bird Discovered

Dromaeosaurids were "raptor dinosaurs" that include Velociraptor from "Jurassic Park" fame. These dinosaurs were pack hunters who wielded a sharp, hyper-extendable "killer claw" on their second toe. "One of the most recently discovered dromaeosaurids is Balaur, a poodle-sized terror from Romania which had not one, but two 'killer claws' on each foot."

Velociraptor Inspires Fast Running Robot

"The oldest birds, like Archaeopteryx that lived 150 million years ago in Germany, are very hard to distinguish from their closest dinosaurian relatives," Brusatte said. "Unlike living birds, they had teeth, sharp claws on their wings, and long tails." "Over the past two decades," he continued, "over 50 new species of Mesozoic birds have been discovered in northeastern China, in the same rock units as the famous 'feathered dinosaurs.' So many birds are preserved here because entire ecosystems were buried by volcanic eruptions, turning animals to stone like a dinosaur version of Pompeii."

Tiny Feathered Dinosaur Found

"The 10,000 species of birds that live today -- from hummingbirds to ostriches -- are modern dinosaurs," Brusatte said. "They are dinosaurs in the same way that humans are mammals. The classic body plan of living birds -- feathers, wings, wishbones, air sacs extending into hollow bones -- did not evolve suddenly but was gradually assembled over tens of millions of years of evolution. But, when this body plan finally came together completely, it unlocked great evolutionary potential that allowed birds to evolve at a super-charged rate." "They underwent a burst of evolution early in their history, which eventually led to the 10,000 species alive today -- more than twice the number of mammals."

Photos: Audubon Report Names Birds at Climate Risk