Fossils discovered in Scotland in 1907 shed light on the origins of the first flying vertebrates.
|Tiny Scleromochlus taylori, shown here in an artist’s rendition, was a bipedal, ground-dwelling reptile, and a close relative of pterosaurs, the iconic winged vertebrates of the Age of Dinosaurs. GABRIEL UGUETO|
A mysterious ground-dwelling reptile discovered in a Scottish sandstone over a century ago turns out to be a member of a well-known flying family. Researchers report online October 5 in Nature that the tiny Scleromochlus taylori was a close relative of pterosaurs, the winged reptiles that coexisted with dinosaurs.
The discovery supports the theory that pterosaurs, the first vertebrates to master powered flight, evolved from small, two-legged, fast-moving ancestors.
The research also sheds light on a long-standing mystery: what exactly was S. taylori? "It all comes down to the preservation of this animal," says paleontologist Davide Foffa of National Museums Scotland in Edinburgh.
S. taylori is only known from seven individuals preserved in rocks discovered in 1907, which are difficult to interpret. For starters, there are no actual bones, only impressions on the surrounding rock; the bones have long since been winnowed away. Based on these fossils, numerous studies have been conducted to describe and redescribe the creature. These analyses led to the conclusion that S. taylori was most closely related to dinosaurs, pterosaurs, or even crocodilian ancestors.
The little reptile, which lived about 230 million years ago, had very odd body proportions, according to Foffa. It was less than 20 centimeters long and "fit on the palm of your hand," but its head was much larger than its body. It had a short neck and long hind limbs as well. However, that rough outline is insufficient to identify the creature's closest relatives; finer details of the skull, jaw, body proportions and other features are required.
So Foffa and his colleagues used microcomputed tomography, a noninvasive scanning technology, to collect previously inaccessible data from the fossils, such as the length of its tail, the size of its foot bones, and the shape of its jawline.
Some of the creature's characteristics, such as its massive head, are reminiscent of pterosaurs. Others, such as the orientation of its lower jaw, aren't at all like pterosaurs, the team discovered. According to the researchers, S. taylori lacked any discernible adaptations for flying, jumping, or living in trees. Instead, it was most likely a runner.
One of the most significant new discoveries concerns the structure of the creature's femur. It resembled both pterosaurs and a group of small, ground-dwelling reptiles known as lagerpetids. According to Foffa, the bottom of the femur bone, where it would connect to the lower leg, bears a structure that is characteristic of lagerpetids.
The new information suggests that the creature was almost certainly a lagerpetid. Though lagerpetids did not fly, they are now thought to be very closely related to pterosaurs, as part of a group known as pterosauromorphs. The pterosauromorphs' common ancestor was most likely a small, fast-running reptile.
S. taylori, which has characteristics of both, could be a very early lagerpetid that evolved shortly after the two pterosauromorph lineages split. It was "kind of a surprise" that so many features were present in both, says Martn Ezcurra, a paleontologist at the Argentine Museum of Natural Sciences in Buenos Aires who was not involved in the new study. However, based on the reanalysis of the fossils, he believes the conclusion that S. taylori was an early lagerpetid makes a lot of sense.
Pterosaurs first appeared in the fossil record approximately 220 million years ago, and their anatomy is distinct, with massive heads for their body sizes and super-elongated fourth digits that were part of their wings (SN: 10/12/10). S. taylori has a large head, but its hands are still small, according to Ezcurra. "We're missing several intermediate forms in between that have active flight features," he says. However, this new analysis of old fossils brings scientists one step closer to the time when pterosaurs' distinctive and highly flight-adapted bodies began to evolve (SN: 7/22/21).
According to Hans Sues, a paleontologist at the Smithsonian Institution in Washington, D.C., who was not involved in the new study, it's difficult to say what such a proto-pterosaur might look like. "Scleromochlus is a tiny animal, and it's possible that a related small-bodied form climbed around in trees and gave rise to a proto-pterosaur — perhaps via an intermediate gliding stage."
Scleromochlus and the Early Evolution of Pterosauromorpha, D. Foffa et al. Nature. doi: 10.1038/s41586-022-05284-x. Published online October 5, 2022.
Enigmatic dinosaur precursors bridge the gap to the origin of Pterosauria, according to M.D. Ezcurra et al. Nature, vol. 588, p. 588, December 9, 2020. doi: 10.1038/s41586-020-3011-4.