Eggs from the earliest dinosaurs were more like leathery turtle eggs than rigid bird eggs.
Studies of fossilized embryos from two kinds of dinosaurs, one from early in dinosaur history and the other living about 150 million years later, reveal the eggs were enclosed by soft shells, paleontologists report online June 17 in Nature. The discovery marks the first time scientists have identified soft-shelled dinosaur eggs.
Further analyses of these and other dinosaur eggs suggest that hard eggshells evolved independently for each of the three main dinosaur lineages: the long-necked sauropods, plant-eating ornithischians and fierce theropods.
Until now, paleontologists thought that all dinosaurs had hard, mineralized eggshells. But scientists couldn’t explain why eggs from the earliest dinosaurs haven’t appeared in the fossil record or why microstructures within eggshells are so different for each of the main dinosaur lineages.
“This new hypothesis provides an answer to these problems,” says Stephen Brusatte, a paleontologist at the University of Edinburgh who was not involved in the work.
The researchers analyzed a clutch of dinosaur eggs found in Mongolia and dating to between 72 million and 84 million years ago; the collection is attributed to Protoceratops, a sheep-sized ornithischian. The team also analyzed another egg, found in Argentina and dating to between 209 million and 227 million years ago, attributed to Mussaurus, a sauropod ancestor.
The soft eggshells weren’t easy to spot. “When they are preserved, they’d only be preserved as films,” says study author Mark Norell, a paleontologist at the American Museum of Natural History in New York City. When examining the fossilized embryos of both kinds of dinosaurs, the researchers noticed diffuse egg-shaped halos around the skeletons. A closer examination of the halos revealed thin brown layers; the uneven arrangement of these layers suggested the material was organic, or carbon-based, rather than mineralized.
Before a few years ago, “people thought that everything that’s soft and squishy decays away immediately post mortem,” says study author Jasmina Wiemann, a paleontologist at Yale University. But there is growing evidence that this organic material can fossilize. Certain environments can provide the right conditions to preserve soft tissues, she says.
The team used Raman spectroscopy to probe the chemical composition of the proposed soft shells. This nondestructive technique shines a laser on a sample, and the properties of the scattered light indicate what kind of molecules are present. Wiemann previously used the approach to identify pigments in dinosaur eggs (SN: 10/31/18).
The researchers compared the chemical fingerprints of the fossilized eggs with the fingerprints of fossilized hard-shelled dinosaur eggs, as well as eggs from present-day animals. The Protoceratops and Mussaurus eggs were most similar to modern soft-shelled eggs.
Combining this and other eggshell data with the evolutionary relationships of extinct and living egg-laying animals, the researchers calculated the most likely scenario for dinosaur egg evolution. They determined that early dinosaurs had soft-shelled eggs and that hard shells evolved multiple times in dinosaurs — at least once in each major lineage.
These findings suggest it will be important to revisit past conclusions about dinosaur reproductive behavior, which primarily relied on analyses of theropod fossils. For example, oviraptorosaurs sat on eggs in open nests, like modern birds (SN: 5/15/18). But such 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.
“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.”
Now that paleontologists know what to look for, the search is on for 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 in Tallahassee.