Paleontologists always thought that dinosaurs had hard, mineralized eggshells. But recent evidence of two fossilized embryos from two kinds of dinosaurs reveal that dinosaur eggs were soft-shelled.

A defining characteristic of modern birds is hard-shelled eggs, which are thought to have played a vital role in their survival during the dinosaur extinction 66 million years ago (Cretaceous–Palaeogene extinction). So, it was assumed that all dinosaurs would also have laid hard shelled eggs, as scientists couldn’t explain why eggs from the earliest dinosaurs haven’t appeared in the fossil record or why microstructures within eggshells are different for each of the main dinosaur lineages.

Fossilized embryos were studied by paleontologists from two kinds of dinosaurs, one from early in dinosaur history and the other living about 150 million years later, the results were published in the journal – Nature. After careful analysis of these dinosaur eggs along with others suggested that hard eggshells evolved independently for each of the 3 main dinosaur lineages: fierce theropods, plant-eating ornithischians, and the long-necked sauropods.

Clutch of dinosaur eggs discovered in Mongolia and Argentina

A clutch of dinosaur eggs were discovered by the team of paleontologists in Mongolia that dated between 72 million and 84 million years ago; the collection is attributed to Protoceratops, a small ornithischian with horns. Another dinosaur egg was analyzed by the time which was discovered in Argentina and dated back to 209 million and 227 million years ago, attributed to Mussaurus, a plant-eating sauropod ancestor.

The soft eggshells are hard to spot. “When dinosaur eggs are preserved, they’d only be preserved as films,” says study author Mark Norell, a paleontologist at the American Museum of Natural History. As the scientists examined fossilized embryos of both types of dinosaurs, they noticed diffuse egg-shaped halos around the skeletons. A closer look at the halos showed thin brown layers; the uneven sorting of these layers suggested the material was organic, or carbon-based, rather than mineralized.  

dinosaur egg fossil
Image Credit: M. Ellison (AMNH) | ScienceNews
The arrow points to an embryo that preserves remnants of a soft shell.

Until recently, “people thought that everything that is soft and squishy decays away immediately post mortem,” says study author Jasmina Wiemann, a paleontologist at Yale University. But new evidence suggests that this organic material can fossilize as certain environments can provide the right conditions to preserve soft tissues, she says.  

Raman spectroscopy was used by the researchers to determine the chemical composition of the soft egg shells. This nondestructive technique shines a laser on a sample, and the properties of the scattered light indicate what kind of molecules are present. Chemical fingerprints of the fossilized dinosaur eggs were correlated with the fingerprints of fossilized hard-shelled dinosaur eggs, as well as eggs from present-day animals. The results showed that eggs from both Protoceratops and Mussaurus eggs were almost similar to modern soft-shelled animal eggs.

By studying the data from recent and past egg shells the team determined the evolutionary relationships between extinct and present-day egg-laying animals, suggesting that hard shells evolved multiple times in dinosaurs — at least once in each major lineage. “If you have a soft-shelled egg,” Norell says, “you’re burying your eggs: there’s not going to be a lot of parental care. It makes the dinosaurs of that soft-shelled egg more primitive reptilian than birdlike in many ways.”

These research findings show that it is imperative to revisit past conclusions about reproductive behavior of dinosaurs, which primarily relied on analyses of theropod fossils. For example, oviraptorosaurs sat on eggs in open nests, like modern birds, but this behavior may not reflect general dinosaur practices. Because dinosaur eggs evolved independently, what researchers have deduced about parental care may represent just one lineage, Wiemann says.

Now that paleontologists know what to search for, researchers will try to discover and study more soft-shelled dinosaur eggs. “I would not be surprised if other people come forward with other specimens,” says Gregory Erickson, a paleontologist at Florida State University.

Read More: Ancient shells reveal Earth days were shorter 70 million years ago


STORY

Fossil discoveries suggest the earliest dinosaurs laid soft-shelled eggs
Jack J. Lee | June 24, 2020 | ScienceNews

RESEARCH ARTICLE

The first dinosaur egg was soft
Nature