Hundreds Of Fossilized Pterosaur Eggs, Some With Perfectly Preserved Embryos, Found In World Record Discovery

An illustration of a pterosaur flock.
Corey Ford / iStock Images

A treasure trove of 120-million-years-old pterosaur eggs emerged from a Lower Cretaceous site in northwestern China, and paleontologists can barely contain their excitement.

This is truly a momentous find, not only due to the impressive number of fossils recovered in the largest collection of fossilized pterosaur eggs ever unearthed but also through the surprising new details it brings to light on the early development of these ancient winged creatures.

Hailed as “a world-class find,” the amazing discovery yielded hundreds of beautifully preserved pterosaur eggs, with at least 215, and possibly, as many as 300, National Geographic reports. Along with the fossilized eggs, researchers also found remains of male and female adult pterosaurs, as well as skeletons of hatchlings and juveniles, all belonging to a species known as Hamipterus tianshanensis.

Perhaps the most striking element of this record-breaking discovery is the finding of pterosaur embryos in 16 of the fossilized eggs. Although none of the eggs rendered a complete skeleton, these are by far the most detailed pterosaur embryo fossils ever uncovered.

“This is a one-of-a-kind record for pterosaurs — for the first time, we have the whole spectrum,” says Juliana Sayão, a bone-structure expert at the Federal University of Rio de Janeiro (UFRJ), Brazil.

Sayão, who co-authored a study on the amazing find, which was published December 1 in the journal Science, believes these bones have much to tell us about the species’ features in all its stages of development.

This incredible discovery is credited to Xiaolin Wang, study lead author, and Shunxing Jiang, both paleontologists at the Chinese Academy of Sciences. Wang and Jiang came across the remarkable bounty of pterosaur fossils while on a dig in the Turpan-Hami Basin, in northwestern China, for a project that started more than a decade ago, LiveScience notes. Now, the project has come to fruition in the most unexpected of ways.

The small fossilized eggs, measuring no more than 2.3 to 3.1 inches (six to eight centimeters) in length, were unearthed from a 10-foot (three-meter) long block of sandstone, where they had been preserved in excellent 3D condition. In other words, many of the recovered pterosaur eggs were not flattened or crushed; a happy occurrence that doesn’t come along too often in paleontology digs.

Inside 16 of the eggs, researchers were delighted to find the remains of pterosaur embryos, namely partial wing and skull bones, as well as one complete lower jaw, Phys.org reports.

“This is by far the most exciting discovery that I know of,” says Alexander Kellner, study co-author and paleontologist at UFRJ’s National Museum.

To understand more about the life cycle of the mysterious Hamipterus tianshanensis, a pterosaur species Wang and Kellner only discovered a mere three years ago, the team analyzed the fossilized embryos using 3D computerized tomography scans.

Much to their surprise, the researchers stumbled upon the astonishing realization that pterosaur hatchlings “were likely not as precocial as previously thought.”

CT scans revealed well-developed thigh bones, suggesting newly-hatched pterosaurs were able to walk but uncovered underdeveloped chest bones “in potentially late-term embryos.” This indicates the embryos had weak pectoral muscles, which prompted scientists to believe pterosaur hatchlings couldn’t fly and were highly dependent on their parents.

Since the hind limbs were more developed than the forelimbs, as revealed by one particularly well-preserved Hamipterus tianshanensis embryo, the team deduced pterosaur hatchlings “might have been flightless and less precocious than previously assumed.”

“Bones related to flight were less developed, or ossified, than bones of the hind limb, which indicates that hatchlings might be able to walk, but not fly,” Kellner shows in a statement.

Furthermore, unlike dinosaur embryos previously recovered, the embryonic remains found at the Turpan-Hami site appeared to be toothless. From this, the team inferred that “hatchlings might have not been able to hunt for themselves, relying on their parents to feed them,” explains an accompanying article published in the same journal.

In addition, the newfound pterosaur embryos yielded other important clues regarding Hamipterus tianshanensis’ early development. The absence of teeth in the skull bones could indicate a delayed dental growth compared with modern reptiles, such as lizards and crocodiles, whereas one particular embryo suggests “pterosaurs had long incubation periods.”

Judging by the anatomy of this embryo, which belonged to a 2-year-old pterosaur that was “still growing at the time of its death,” it seems these winged reptiles took a while to reach adulthood and had a slow developmental rate.

The embryos aside, the eggs themselves provide unique insight into the pterosaurs’ egg-laying behavior, the researchers point out. The team observed the pterosaur eggs had soft, parchment-like shells, which means they most likely needed to be buried in a moist place to ensure their survival.

“All are deformed to a certain extent, which indicate their pliable nature,” notes the team in their study.

The fossils were formed 120 million years ago when the Turpan-Hami site (now part of the Gobi Desert) was still a lush environment the study authors refer to as “pterosaur Eden.”

The staggering number of eggs, combined with the discovery of adult pterosaur remains at the site, indicate these winged reptiles probably used the same nesting grounds on multiple occasions, returning to the exact same spot to lay their eggs; a behavior now seen in modern-day sea turtles.

This suggests “pterosaurs may have exhibited breeding site fidelity, similar to rookery-breeding seabirds,” the authors wrote in their paper.

“Thus, the similarity between these two groups goes beyond wings,” the researchers added.

The fossilized pterosaur eggs were discovered spread out among four distinct layers of sediments, which hints the nesting grounds was flooded several times, possibly by rainstorms, and that the eggs were not deposited all at once but in separate increments over a longer period of time.

“The geological context, including at least four levels [of sediments] with embryos and eggs, indicates that this deposit was formed by a rare combination of events, with storms acting on a nesting ground. This discovery supports colonial nesting behavior and potential nesting site fidelity in the Pterosauria,” the researchers show in the study.

All things considered, this impressive find truly captures “the life history of pterosaurs,” paleontologists believe.

“Since these are extremely fragile fossils, we were very surprised to find so many in the same place,” Kellner says. “Because of this discovery, we can talk about the behavior of these animals for the first time.”

To fully grasp the importance of this discovery, it is worth mentioning the Turpan-Hami cache considerably expands the pterosaur egg fossil record. Until now, only a handful of fossilized eggs had been retrieved from various sites in China and Argentina, including these five Hamipterus tianshanensis eggs Wang unearthed in the same location back in 2014, when he first identified the previously unknown species.

Additionally, the Turpan-Hami fossils boast no less than 16 distinct pterosaur embryos; an outstanding find considering the scarcity of pterosaur eggs and embryos in the fossil record. In fact, the first pterosaur embryo ever discovered, along with eggshell fragments, wing membranes, and skin imprints, was also unearthed by Wang in a Chinese site in 2004.

Hamipterus tianshanensis is a pterosaur species that lived in northwestern China more than a hundred million years ago. These flying reptiles ruled the skies during the time of the dinosaurs. Standing about four feet tall, an adult Hamipterus tianshanensis had a wingspan of 10 feet and most likely fed on fish.

Pterosaurs rose to power in the Cretaceous and Jurassic periods as airborne cousins of dinosaurs and followed a different evolutionary path than the mighty wingless reptiles. This explains why pterosaurs, including pterodactyls, are not dinosaurs, nor are they directly related to birds, Smithsonian.com clarified at the time of the 2014 discovery. Nevertheless, pterosaurs shared their cousins’ dire fate and perished in the same extinction event that killed off most of the dinosaurs.