Pterosaurs, contemporary reptiles of the dinosaurs, have ruled the skies for over 150 million years. Before disappearing without leaving descendants 66 million years ago. The largest, with a wingspan of up to 12 meters, are the largest flying creatures that the Earth has known.
But this very diversified family – a hundred species counted – also counted birds as small as a sparrow. Hamipterus tianshanensis , a lower Cretaceous species ( 140-100 million years old), was about the size of a large gull. The animal was planted in what is now Xinjiang Province in northwestern China, and was described in 2014 by Zonghe Zhou’s team (Chinese Academy of Sciences, Beijing). Surprise: a set of five eggs accompanied these fossils. The discovery was important because a single pterosaur egg, discovered in 2004 in Argentina, was previously known.
Zonghe Zhou’s team continued their investigations and returns with a much larger harvest. In a new study published by the journal Science , it reveals the discovery of a new deposit containing hundreds of fossils Hamipterus , but also 215 eggs – “but there could be 300, because many appear buried under those who have been exhumed, “ the team reports.
“It’s quite extraordinary ,” says French paleontologist Jean-Michel Mazin, head of a dig site in Crayssac (Lot) where multiple traces of pterosaurs have been uncovered. This deposit will lead to many other publications. That would make you almost jealous. “
The discovery confirms that pterosaurs lay “soft” eggs, covered with a thin, parchment-like film, like current lizards. “Whereas pterosaurs were closer to dinosaurs and crocodiles, to hard-shelled eggs, their spawning is more like that of lizards, more distant on the evolutionary level , “ says Jean-Michel Mazin.
Collective nesting site
This structure implies that the laying should be buried, to protect it from desiccation, as sea turtles do today. “This rules out any form of contact incubation, like that seen in modern birds ,” says Charles Deeming, of Lincoln University in the UK, in Science . However, adults had to look after the nests or protect them, which would explain the presence on the site of many adult pterosaur bones. “
We must therefore imagine a colony, a collective nesting site, which would have been affected by a landslide that facilitated the fossilization of the whole. “A large part of the eggs described are shriveled, crumpled, which suggests an incident occurred at the time of laying,” says Jean-Michel Mazin. Unless the competition to find a place of loose laying has led to the accidental destruction of other nests, as is sometimes observed in marine turtles who inadvertently dig up the offspring of their neighbors.
The Chinese team has another explanation: the study of geological strata suggests that the nesting place, located in a lake environment, could have been dug up by a powerful storm. “The eggs would have been dragged into the lake, floating on its surface, concentrating before being buried with the disjointed skeletons of the adults,” they write.
No teeth observed in the sample
This buried incubation gives rise to other hypotheses: it was expected to last a long time because of a lower temperature than in animal incubators. Does it also mean that the little ones came out of their shells armed for life and theft? This is where the discovery of sixteen embryos takes all its interest. “With rare exceptions, their bones tend to be disarticulated and out of sync with their natural position,” says the Chinese team.
No teeth were observed in the sample, whereas lizard and crocodile embryos have some. Two possibilities: either the pterosaur embryos were petrified whereas their development was not completed, or the eruption of the teeth intervenes later in this species.
And the wings, membranes stretched around the fourth finger of the hand that carries the bulk of the wing? Their joints are also not formed in the sample found, while the femur is well developed. “This means that newborns have to be able to move, but they can not fly ,” Zonghe Zhou and his colleagues write. This leads to the hypothesis that Hamipterus may have been earlier than was generally thought of flying reptiles, and that the pups probably needed some form of parental care. “
“This conclusion depends again on the level of maturity of the studied embryos , however notes Jean-Michel Mazin. It is not known if they had come to an end. “ A shared by Charles Deeming caution: ” It is important to be careful and not to infer too many aspects of the life trajectory of Hamipterus from what remains a limited sample. “
More generally, given the great diversity of this group, we must be careful not to generalize what a species reveals, warns Jean-Michel Mazin. An example ? At Crayssac, the traces of an airstrip were discovered: the two feet first touched the ground, then both hands, and then the animal walked on all four limbs. But again, nothing says that was the general case. “It is only to compare the landing of a swan or a sparrow,” says the Frenchman.
Giants or miniscules, carnivores, piscivores, or vegetarians, fans of flying or flapping wings, these kings of the air remained trapped in the mists of time presented the same maddening diversity as that forged by evolution in birds.