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.
“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.
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.
“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.
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.
“It was quite exciting to see that they looked exactly like soft-shelled snake or turtle eggs,” Wiemann told Business Insider.
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.
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.
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.
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.