Paleontologists have long thought that all dinosaurs laid hard-shelled eggs, much like crocodiles and birds (their descendants) do.
But a new analysis of fossil eggs discovered in the Gobi Desert throws cold water on that theory — and changes our understanding of dinosaur evolution.
According to a study published Wednesday in the journal Nature, two species of dinosaur that lived millions of years apart left behind clutches of eggs that had soft-shells, much like turtle, lizard, and snake eggs do.
“I’ve been excavating in Mongolia for 20 years now, and we find a lot of dinosaur eggs. But these clutches tell us something very different from what we knew before,” Mark Norell, lead author of the study and paleontologist from the American Museum of Natural History, told Business Insider.
Until now, scientists thought that hard-shelled eggs arose just once during the early history of dinosaurs — representing an advantageous evolutionary milestone that allowed the animals to flourish. Hard shells protect developing embryos from drying out, and make it possible for parents to lie on top of their nests without crushing their brood.
But this discovery helped the study authors determine that the oldest dinosaurs laid soft-shelled eggs, and that hard-shelled eggs cropped up much later in the dinosaur fossil record than paleontologists previously thought.
Researchers found eggs that were leathery and soft
Norell and his colleagues found the soft-shelled eggs in 1997, at a site called Ukhaa Tolgod in Mongolia.
They discovered at least 12 eggs and embryos belonging to Protoceratops, a 75-million-year-old sheep-sized herbivore that preceded Triceratops. Six embryos in the Protoceratops clutch preserved nearly complete skeletons, and two of the Protoceratops may have already hatched before being fossilized.
“It was clearly a concentration of very, very small animals curled up in fetal positions, like you’d find inside an egg” Norell said. “It’d barely be four inches long if you stretched one out.”
The other eggs belonged to Mussaurus, a 20-foot, long-necked, herbivore that lived between 227 and 208.5 million years.
Norell brought them back to his museum in New York City for further examination. But it wasn’t until recently that he and other paleontologists had the technology to examine the clutches properly.
He and his colleagues discovered that nine of the Protoceratops embryos were surrounded by egg-shaped, black-and-white halos. Norell’s co-author, Jasemina Wiemann, used a special type of microscope to isolate the minerals left behind in those halos from the surrounding rock and chemically analyzed them.
She found evidence of an dark brown, semi-transparent, multi-layered eggshell membrane — a membrane that lacked all the microscopic mineral elements that would’ve indicated the existence of a hard-shell.
Rather, these Protoceratops eggs were leathery and soft, and the Mussaurus eggs shared the same features.
“It was quite exciting to see that they looked exactly like soft-shelled snake or turtle eggs,” Wiemann told Business Insider.
Constructing a dino egg evolutionary tree
Armed with that data, another study co-author, Matteo Fabbri, helped map out a evolutionary tree that compared the Protoceratops and Mussaurus egg features with those from flying reptiles named pterosaurs, crocodiles, and dinosaurs.
Fabbri and his colleagues found that hard-shelled eggs had evolved at least three times in the dinosaur family tree: once in the Ornithopods, which included duck-billed dinosaurs; once in the giant Sauropods like Titanosaurs; and once in the Therapods (like T. rex) of the late Cretaceous period.
The study authors’ conclusion matches the hard-shelled eggs that have already been found in the fossil record — eggs belonging to species in the aforementioned dinosaur groups.
While paleontologists have found skeletons from countless other, older dinosaur species, they’ve found no eggs to match.
“We have all these other animals, but we don’t have any eggs. It’s bizarre,” Norell said. This new study, though, may provide the answer: “My guess is they were all laying soft-shelled eggs.”
Fossilized soft-shelled eggs don’t weather the test of time as well as their hard-shelled counterparts, which may explain why they haven’t been found intact until now.
The discovery isn’t the only soft-shelled egg find that’s making news.
Giant prehistoric egg from Antarctica
In a second Nature study published Wednesday, a different group of paleontologists announced they had found and analyzed the largest soft-shelled egg ever discovered.
“This is the biggest one by a long shot,” Lucas Legendre, lead author of that second study, told Business Insider, adding: “It’s massive, about the size of an American football and looks exactly like a lizard or snake egg.”
His colleagues discovered the egg in 2011, off the coast of Antarctica on Seymour Island.
The egg has a deflated quality, he said, which suggests the animal inside hatched before the egg was fossilized about 66 million years ago.
Though they did not find any fossil remains of the animal that laid it, the researchers categorized the egg as being a hitherto undiscovered type of animal, which they named Antarcticoolithus bradyi.
According to the study authors, the animal was probably a type of giant, swimming reptile, like a mosasaur or plesiosaur.
Based on the egg’s size (11 inches by 7 inches), Legendre said the animal that laid it would have been more than 20 feet in length. That matches what paleontologists know about plesiosaurs: some of the largest ever plesiosaurs ever found come from Antarctica, Legendre said, reaching lengths of 32 feet or more.
But Legendre’s group’s conclusion — that a plesiosaur or mosasaur laid this egg — challenges the prevailing thought that such creatures did not lay eggs and birthed live young.
“Laying an enormous shelled egg underwater would have created all sorts of problems for hatching infant reptile, which would have needed to surface almost immediately to breathe air,” Ben Kear, a paleontologist from Uppsala University in Sweden, told Business Insider in an email.
Some experts argue the Antarctic egg was laid by a dinosaur
If a mosasaur didn’t lay this egg, what did?
While the Antarcticoolithus egg’s age and massive size suggest it could’ve been laid by giant Sauropods that roamed the Earth during that time, Legendre said that the egg’s shape doesn’t match that of other dinosaur eggs.
Plus, he said, the only dinosaurs that lived in Antarctica 66 million years ago laid hard-shelled eggs.
Kear thinks that Norell’s group’s discovery — that some dinosaurs laid soft-shelled eggs — could be the answer to the Antarctic egg mystery.
“It is now very plausible that Antarctoolithus might have been laid by some form of dinosaur,” Kear said.
“Consequently, after hatching on land, the discarded eggshell could have been washed out to sea, where it remained buoyant for some time because of trapped air before sinking to the seafloor and becoming buried in mud.”