230-million-year-old mystery fossil sheds light on origins of pterosaurs

Since 1907, paleontologists have been flummoxed by strange fossils that formed some 231 million years ago in the ancient sand dunes of what’s now Scotland. The fossils don’t preserve any bones: just the outlines of them etched into grainy sandstone. To study these imprints, scientists once had to pour wax or plastic onto the slabs and peel off molds—techniques that revealed an oddball. The peeled materials offered hints of an eight-inch-long reptile with what looked like long hindlimbs, a short neck, bizarrely short ribs, and an oversized head.

This creature, named Scleromochlus taylori, has bounced around the reptile family tree ever since its discovery, with generations of scientists trying to pin down its identity. Researchers also have struggled to reconstruct how it lived, such as whether the creature jumped through ancient sand dunes like today’s jerboas and other hopping rodents. Now, after more than a century, Scleromochlus has been unmasked thanks to new anatomical discoveries—which could help scientists understand the evolution of pterosaurs, the flying reptiles that lived alongside the dinosaurs.

High-resolution X-ray scans, published today in the journal Nature, reveal never-before-seen anatomical features that place Scleromochlus within a group of reptiles called the lagerpetids, which lived from roughly 240 million years ago to the end of the Triassic period about 201 million years ago. “Scleromochlus, at the time it was discovered, was just this weird and bizarre creature … so it was very difficult to understand,” says lead study author Davide Foffa, a paleontologist at Virginia Tech and the United Kingdom’s University of Birmingham who performed the research at the National Museums Scotland.

Lagerpetids are themselves enigmatic: Until recently, most known fossils included just hindlimbs and bits of the skull. But in 2020, a landmark study led by Martín Ezcurra showed that the lagerpetids shared many anatomical traits with the pterosaurs. This discovery has helped scientists chip away at a gap in the fossil record that has obscured pterosaurs’ evolutionary origins.

Because the fossils of Scleromochlus are almost complete skeletons, “they give us for the first time a much more complete look at the anatomy of lagerpetids,” says Ezcurra, a paleontologist at Argentina’s Bernardino Rivadavia Museum of Natural Sciences who wasn’t involved with the new study.

Unmasking Scleromochlus 

Foffa’s journey to unmasking Scleromochlus began in 2018, when he joined the National Museums Scotland to study a group of Triassic fossils called the Elgin reptiles, so named because they hail from the rocks near the Scottish town of Elgin.

These reptiles provide a snapshot of a critical time in evolutionary history. Some 252 million years ago, at the end of the Permian period, Earth underwent the worst known mass extinction: a cataclysmic hothouse driven by the release of vast amounts of warming gases from volcanoes in what’s now Siberia. More than 95 percent of species went extinct in this event, which is known as the “Great Dying.”

But in the early Triassic period that followed, life rebounded and rapidly diversified, setting the stage for today’s groups of land vertebrates. “Nature just went experimental—you can see it was just trying new things, it just went off the rails,” says paleontologist Natalia Jagielska, a Ph.D. candidate at Scotland’s University of Edinburgh who wasn’t involved with the study. (Read more about Jagielska’s research on a spectacularly complete pterosaur fossil.)

To research these reptiles and their outré anatomies, Foffa and his colleagues aimed to scan them at high resolution with X-rays, stacking many 2D images to reconstruct the fossils’ contours in 3D. After successfully scanning an Elgin reptile known as Leptopleuron, Foffa next set his sights on the legendary Scleromochlus itself.

As generations of scientists had learned before him, it’s hard to study a fossil that’s in absentia. To build his 3D models, Foffa spent more than a year identifying the air pockets in his X-rays of Scleromochlus’s sandstone slabs, all while accounting for cracks in the stone. But eventually, Foffa and his colleagues could see details that were simply too small and too fine for waxes to record.

The creature’s ribs were longer than previous studies had recorded; so were its forelimbs and tail. Foffa could reconstruct an entire hand and foot that hadn’t been seen before. And crucially, Foffa could see the ends of Scleromochlus’s femur—which confirmed that the creature was a lagerpetid.

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Making sense of the pterosaurs 

Now that Scleromochlus has joined the lagerpetids’ ranks, the fossil can help scientists make sense of the pterosaurs, the first vertebrates to achieve powered flight. Weird and wondrous to our modern eyes, these creatures were a wildly diverse group unlike anything alive today that included some of the biggest animals ever to fly. But piecing together their evolutionary story has been tricky. (Read more about pterosaurs—including one as tall as a giraffe with a fighter jet's wingspan.)

In part, the challenge comes down to the fossil record’s many biases. Pterosaurs’ bones were hollow, which made them lightweight for flight but extremely fragile. And in the early Triassic, when pterosaurs first arrived on the scene, there simply weren’t as many fossil-friendly settings across the ancient Earth as there were in later time periods.

As a result, there was a roughly 30-million-year gap in the fossil record of pterosaurs’ origins. The oldest known pterosaurs, which lived some 220 million years ago, were fully formed fliers, providing little hint of what came before. Ezcurra’s 2020 discovery that lagerpetids were a sister group to pterosaurs narrowed that gap to about 18 million years.

However, the remaining gap has proven stubborn to fill: Neither Scleromochlus nor any other lagerpetid has elongated fourth fingers, which are how pterosaurs supported their wings. “It’s not easy, but we can’t stop searching,” Ezcurra says.

While no bone of Scleromochlus has survived, the imprints of its skeleton combine to give scientists the most complete lagerpetid yet known. It also appears to belong to one of the oldest branches of the lagerpetid family tree, which means it sheds further light on the forerunners of pterosaurs.

Scleromochlus didn’t have any clear adaptations for climbing, a trait long hypothesized for pterosaurs’ gliding ancestors. Its pelvis also lacks the kinds of bony reinforcements seen in the skeletons of jumping creatures such as jerboas. Instead, Scleromochlus probably could scamper about on its hind limbs or slip into walking on all fours, which implies that the ancestors of lagerpetids and pterosaurs may have moved in a similar fashion.

“When you look at the common ancestor [of pterosaurs and lagerpetids], it’s not very flighty at all: it looks very grounded, with a big sturdy foot,” Jagielska says. “The story might be more interesting.”

Future work on the fossils could provide more details: Foffa’s goal is to build an atlas of Scleromochlus bones, using the scan data to build a permanent digital record of the fossil. By releasing the data, Foffa’s team hopes to foster ongoing discussion of this strange creature.

“If the debate goes a hundred years further, that’s fine,” he says. “That’s how things work!”