Around 120 million years ago, a colony of flying reptiles once lived on the shores of an ancient lake in China’s Turpan-Hami Basin. Today the environment is within the Gobi Desert—a far cry from lush, temperate lakeshore it once was. But the Gobi’s dry, arid climate makes for excellent fossil preservation.
In 2005, researchers came across evidence of the area’s former inhabitants: pterosaurs, a group of flying reptiles that includes the pterodactyl family. Recent excavations of the site have yielded a plethora of fossil specimens of these ancient reptile residents, including the most juvenile of them—their eggs. A team of paleontologists from China and Brazil found that the unearthed bones and fossilized eggs actually represent a previously unknown genus and species of pterosaur. They published their results today in Current Biology.
Before getting into the nitty-gritty details of the discovery, it’s important to note that pterosaurs aren’t dinosaurs. These reptiles share a different evolutionary history to their dino cousins. Thus, unlike dinosaurs, they aren’t directly related to birds either. Pterosaurs dominated the skies during the Cretaceous and Jurassic periods, but pterosaurs and birds are two evolutionary paths that separately reached the skill set of flight.
In the fossil record, pterosaurs are rare commodities: only one or two fossil specimen define each species, and only four eggs have ever been unearthed—all are smushed or flattened. That makes the Turpan-Hami fossils extremely valuable for analyzing nesting habits. As the researchers write, “sites like the one reported here provide further evidence regarding the behavior and biology of this amazing group of flying reptiles that has no parallel in modern time.”
After the researchers realized they had a huge pterosaur find on their hands, they began systematically excavating the site, and in 2008, they came across their first egg: “I was more excited than surprised,” says Xiaolin Wang, a paleontologist at the Chinese Academy of Sciences in Beijing. With such a fossil-rich site, finding an egg wasn’t out of the realm of possibility.
The team recovered five eggs in total from the site. Microscopic and spectroscopic analysis revealed that the eggs had a thin shell for a top layer, made mostly of calcium carbonate, and underneath that lay a soft, thin membrane. “It is similar to ‘soft’ eggs of some modern snakes; the size and structure are nearly the same,” says Wang.
Beyond cracking the mystery of the eggs, the researchers also wanted to figure out how the Turpan-Hami Basin pterosaurs fit into the larger pterosaur family tree. They had removed fossilized bones from 40 individuals at Turban-Hami, though the entire site could ultimately yield hundreds.
After a closer examination of the bones, they found that these animals had marked differences from other species: a hooked bone at the end of the jaw, wider eye cavities, a well-developed forehead crest, a wrist bone with a protruding spike, and other unique features. Their wingspans ranged from 4 feet to 11 feet, and an evolutionary tree analysis suggested that the individuals belonged to a new genus and species of pterosaur, which the scientists named Hamipterus tianshanensis.
As they unearthed the specimens, the researchers also noticed that some individuals had the same skull (in shape and size) but different head crests: some were large, wrinkled, with a flare at the end of their snout, while others were smaller, smoother, and less protruding. The researchers think they’ve happened upon a sexually dimorphic trait—one that separates the boys from the girls.
Though some modern reptile species do have larger females, the trend in reptiles is big males, small females. So, Wang and his colleagues made the educated guess that, in the case of the pterosaurs, larger crests belong to males and the smaller crests to females.
For creatures like pterosaurs and dinosaurs, figuring out which has male bits and which has female bits can be a window into the lives of these ancient beasts. But as you might expect, sexual dimorphism—though suspected in pterosaurs—can be hard to nail down in fossilized animals. More analysis is needed to say for sure.
In addition, finding the bones and eggs present a picture of gregarious social life and reproduction behaviors that resembles that of modern reptiles. “These pterosaurs nested in the shore of the ancient lake and buried their eggs in the moist sand,” says Wang. The nesting behavior is similar to modern snake species, particularly rat snakes.
Finding in one location evidence of sexual dimorphism and of behaviors that are much more reptile-like than bird-like is quite rare. “This is something of a Holy Grail—a site potentially recording all of these interesting aspects in the same locality,” notes Mark Witton, a paleontologist at the University of Portsmouth who was not affiliated with the study.
"A long-extinct group of flying reptiles may seem unimportant in the big scheme of things, but they're a component of something we must pay more attention to: our changing biosphere,” Witton added. “Looking at the way species and ecosystems have evolved through Deep Time gives us the only long-term insight into the way the natural world works—how it adapts to adversity, when it blooms and diversifies, and so forth.”
It also shows how populations can be snuffed out—near this lake, the nesting pterosaurs also met their demise. The fossil-containing rock layers at Turpan-Hami are divided by the mud and sand deposits traditionally left by huge storms. These storm layers, called tempestites, form when debris from different sediments mix together in the storm’s deluge. Here’s what the scholars think may have happened: “The storm may have killed live pterosaurs, and transported the dead bodies and eggs for a short distance,” says Wang “And then buried them quickly.”
It must have been horrible way to go for these ancient creatures, but a perfect storm for researchers, who now have a better picture of what life was like when lizards dominated the land and sky.