Chinese paleontologists have unearthed a new kind of dinosaur egg in the eastern Shandong province, suggesting that the ancient creatures were even more diverse than previously assumed.

For decades, academics have debated whether dinosaurs always laid hard-shelled eggs like modern-day birds, or whether some species laid soft-shelled eggs similar to those produced by some turtles, snakes, and lizards. The fossils uncovered in Shandong, however, appear to be from a distinct third category.

The fossilized eggs were originally uncovered in 2019 by a research team from Beijing’s China University of Geosciences. The team, who analyzed the fossils in collaboration with researchers at the Chinese Academy of Sciences and Anhui University, published their findings in the journal Cretaceous Research on April 25.

The part of Shandong where the dig took place — Laiyang City — is known as “the hometown of dinosaurs” in China due to the area’s unusually rich fossil deposits. Paleontologists had previously discovered eggs from 13 different dinosaur species in Laiyang, but the China University of Geosciences team were “astonished” by this latest find, Zheng Hanghai, the researcher who led the dig, told Sixth Tone.

The eggs are presumed to date from the Cretaceous period, around 145 million to 66 million years ago, and belong to the Spheroolithidae family of dinosaur eggs, which are commonly found in East Asia. But their structure is totally distinct from any previously uncovered in the region, according to Zheng.

Despite three of the eggs having been crushed and deformed, they had somehow remained intact. Zheng said they looked like ping-pong balls that had been smashed with a blunt instrument.

This kind of deformation is “extremely unusual,” Zheng said. Only one previous example of a deformed dinosaur egg has been recorded worldwide, and that only had a “tiny, partial deformation,” he said.

This suggests that the eggs were softer and less brittle than normal, hard-shelled dinosaur eggs. But they weren’t soft-shelled eggs either: Further analysis revealed that the eggs had a thin shell around 1.4-2.6 millimeters thick.

In addition, the eggs’ microstructure — with its “bush-like eggshell units in the middle and outer parts of the eggshell” — is different from any spheroolithid eggs previously found in East Asia, Zheng said.

Zheng and his team are still trying to work out what kind of dinosaur may have produced this unique egg, which they have named Spheroolithus phacelus. They expect that they likely belonged to a hadrosaur — a family of fast, agile herbivores with crested skulls that lived in the late Cretaceous period. Many hadrosaur fossils have been found in the same area.

At this stage, Zheng said the team wasn’t ready to speculate about what the eggs reveal about the evolutionary trajectory of the dinosaurs. But recent discoveries of soft-shelled dinosaur eggs have suggested that dinosaurs were more diverse than long assumed. This latest finding will only add to this sense.

“There has been a long-running discussion on whether dinosaurs’ shells were hard or soft,” said Zheng. “The deformed eggs add a third element to discussions on the evolution of dinosaur eggs, which is extremely significant.”

China has become a hotbed of paleontological research in recent years. In 2020, two new species of titanosaurs were discovered in the northwestern Xinjiang Uyghur Autonomous Region. In March, another new species of ankylosaur was uncovered in the eastern Jiangxi province.

Additional reporting: Li Dongxu

(Header image: The five dinosaur eggs researched by Zheng and his colleagues. Courtesy of Zheng)