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The cause of death for two baby pterosaurs has been revealed by University of Leicester paleontologists in a post-mortem 150 million years in the making.

Detailed in a study in the journal Current Biology, show how these flying reptiles were tragically struck down by powerful storms that also created the ideal conditions to preserve them and hundreds more fossils like them.

The Mesozoic, or age of reptiles, is often imagined as a time of giants. Towering dinosaurs, monstrous marine reptiles, and vast-winged pterosaurs dominate museum halls and the public consciousness. But this familiar picture is skewed. Just as today's ecosystems are mostly populated by small animals, so too were ancient ones.

The difference? Fossilization tends to favor the largest and the most robust organisms. Small, fragile creatures rarely make it into the paleontological record.

On rare occasions, however, nature conspires to preserve the delicate and the diminutive inhabitants of these lost worlds. One of the most famous examples is the 150-million-year-old Solnhofen Limestones of southern Germany. These lagoonal deposits are renowned for their exquisitely preserved fossils, including many specimens of pterosaurs, the flying reptiles of the Mesozoic.

Yet here lies a mystery: while Solnhofen has yielded hundreds of pterosaur fossils, nearly all are very small, very young individuals, perfectly preserved.

By contrast, larger, adult pterosaurs are rarely found, and when they are, they're represented only by fragments (often isolated skulls or limbs). This pattern runs counter to expectations: larger, more robust animals should stand a better chance of fossilization than delicate juveniles.

Lead author of the study, Rab Smyth, from the University of Leicester's Center for Paleobiology and Biosphere Evolution, said, "Pterosaurs had incredibly lightweight skeletons. Hollow, thin-walled bones are ideal for flight but terrible for fossilization. The odds of preserving one are already slim and finding a fossil that tells you how the animal died is even rarer."

The discovery of two baby pterosaurs with broken wings has helped to solve this mystery. These tiny fossils, though easily overlooked, are powerful evidence of ancient tropical storms and how they shaped the fossil record.

Ironically nicknamed Lucky and Lucky II by the researchers, the two individuals belong to Pterodactylus, the first pterosaur ever scientifically named. With wingspans of less than 20 cm (8 inches), these hatchlings are among the smallest of all known pterosaurs. Their skeletons are complete, articulated and virtually unchanged from when they died.

Except for one detail. Both show the same unusual injury: a clean, slanted fracture to the humerus. Lucky's left wing and Lucky II's right wing were both broken in a way that suggests a powerful twisting force, likely the result of powerful gusts of wind rather than a collision with a hard surface.

Catastrophically injured, the pterosaurs plunged into the lagoon, drowning in the storm-driven waves and quickly sinking to the seabed where they were rapidly buried by very fine limy muds stirred up by the death storms. This rapid burial allowed for the remarkable preservation seen in their fossils.

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Like Lucky I and II, which were only a few days or weeks old when they died, there are many other small, very young pterosaurs in the Solnhofen Limestones, preserved in the same way as the Luckies, but without obvious evidence of skeletal trauma. Unable to resist the strength of storms, these young pterosaurs were also flung into the lagoon.

This discovery explains why smaller fossils are so well preserved—they were a direct result of storms—a common cause of death for pterosaurs that lived in the region.

Larger, stronger individuals, it seems, were able to weather the storms and rarely followed the Luckies stormy road to death. They did eventually die though, but likely floated for days or weeks on the now calm surfaces of the Solnhofen lagoon, occasionally dropping parts of their carcasses into the abyss as they slowly decomposed.

"For centuries, scientists believed that the Solnhofen lagoon ecosystems were dominated by small pterosaurs," said Smyth. "But we now know this view is deeply biased. Many of these pterosaurs weren't native to the lagoon at all. Most are inexperienced juveniles that were likely living on nearby islands that were unfortunately caught up in powerful storms."

Co-author Dr. David Unwin from the University of Leicester added, "When Rab spotted Lucky we were very excited but realized that it was a one-off. Was it representative in any way? A year later, when Rab noticed Lucky II, we knew that it was no longer a freak find but evidence of how these animals were dying.

"Later still, when we had a chance to light up Lucky II with our UV torches, it literally leapt out of the rock at us—and our hearts stopped. Neither of us will ever forget that moment."