Possible ancestors of pterosaurs were small, fast-moving reptiles.

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The discovery of fossils in Scotland in 1907 sheds light on the ancestry of the earliest creatures capable of flight.

A reptile thought to have lived on the ground for nearly a century has been shown to be a member of a well-known flying family after being discovered in Scottish sandstone. Researchers report online October 5 in Nature that the tiny Scleromochlus taylori was a close cousin of pterosaurs, the flying reptiles that coexisted with the dinosaurs.

Support for the hypothesis that tiny, two-legged, fast-moving predecessors gave rise to pterosaurs, the first flying vertebrates, is provided by this discovery.

The discovery also clarifies a long-standing riddle, revealing what S. taylori really was. According to paleontologist Davide Foffa of Edinburgh's National Museums Scotland, "it all comes down to the preservation of this species."

The only fossilized S. taylori are those of seven individuals found in rocks in 1907. These fossils have proven to be very elusive and difficult to decipher. The bones have long ago dissolved, so all that remains are imprints on the rock. There have been a plethora of research using these fossils to characterize and re-describe the creature. Those studies have led to inferences that S. taylori is most analogous to dinosaurs, pterosaurs, or perhaps crocodilian forebears.

According to Foffa, it is obvious that the little reptile, which existed about 230 million years ago, had quite unusual body proportions. "it would fit in the palm of your hand," at less than 20 cm in length, but its head was disproportionately huge. It had a small head and strong, powerful hind legs. However, more specific information on the creature's skull, jaw, body proportions, and other features is required to determine its closest relatives, thus the outline alone isn't sufficient.

Microcomputed tomography allowed Foffa and his team to get information about the fossils that had previously been unavailable, such as the length of the tail, the size of the foot bones, and the shape of the mouth.

It has several characteristics with pterosaurs, such as its enormous head. But the scientists also uncovered several features that are strikingly unlike to pterosaurs, such as the placement of its lower jaw. The researchers concluded that S. taylori lacked the morphological or behavioral features that would have allowed it to fly, leap, or survive in treetop environments. Perhaps it was a runner instead.

Recent discoveries have shed light on an intriguing aspect of the femur's anatomy. It was very reminiscent of both pterosaurs and lagerpetids, a group of tiny, terrestrial reptiles. Foffa notes that lagerpetids are distinguished by a unique feature on the bottom of the femur bone, near its attachment to the lower leg.

The new evidence strongly indicates that the creature was a lagerpetid. Even though lagerpetids couldn't fly, it's now understood that they are closely related to pterosaurs and so belong to the group of animals known as pterosauromorphs. Pterosauromorphs had a common ancestor with other flying reptiles that was probably tiny and agile.

There is evidence to suggest that S. taylori, which has characteristics with both, is a primitive lagerpetid that arose shortly after the divergence of the two major pterosauromorph lineages. It was "sort of a surprise," says Martn Ezcurra, a paleontologist at the Argentine Museum of Natural History in Buenos Aires who was not involved in the current research, because it turned out to have traits present in both. It makes a lot of sense, he argues, that S. taylori was an early lagerpetid after reexamining the fossil evidence.

About 220 million years ago, pterosaurs begin to show up in the fossil record. Pterosaurs had a unique structure, featuring disproportionately large heads for their bodies and very long fourth fingers that served as a component of their wings (SN: 10/12/10). Ezcurra points out that despite S. taylori's large brain, the organism's hands are rather little. He claims that "many intermediary forms" are absent from our knowledge, all of which have characteristics with active flight. The unusual and extremely flight-adapted bodies of pterosaurs didn't develop overnight, but this new research of ancient fossils does get us closer to that period (SN: 7/22/21).

Paleontologist Hans Sues of the Smithsonian Institution in Washington, D.C., who was not involved in the current research, says it is impossible to describe what such a proto-pterosaur may look like. As the author puts it, "Scleromochlus is a tiny species, and it is possible that a similar small-bodied creature walked about in trees and ultimately gave birth to a proto-pterosaur — potentially via an intermediary gliding stage."

CITATIONS

M.D. Ezcurra et al. Enigmatic dinosaur precursors bridge the gap to the origin of Pterosauria. Nature. Vol. 588, December 9, 2020, p. 588. doi: 10.1038/s41586-020-3011-4.