Researchers have finally solved the enigma of Scleromochlus taylori anatomy

Reassessment of an enigmatic fossil reptile from the Triassic of Scotland (around 230 million years ago) reveals that it was a close cousin of the flying pterosaurs. The study, published in Nature today, sheds light on one of the closest relatives of the first vertebrates to fly.

Flying reptiles

Reptiles of the Mesozoic Era (approximately 252–66 million years ago) are known for their remarkable size: dinosaurs include the largest known land animals, and their relatives, the pterosaurs, include the largest creatures ever to fly. Their fossil record spans over 150 million years, from the Late Triassic until the end of the Cretaceous. However, the evolutionary history of pterosaurs is uncertain due to anatomical gaps between these highly modified reptiles and their closest relatives.

Recently, lagerpetids—a group of small-bodied, terrestrial archosaurs previously thought to be close to dinosaur ancestry—were shown to be the sister taxon of pterosaurs, together forming Pterosauromorpha. This discovery reduced the anatomical distance between pterosaurs and their close relatives but left many questions regarding the tempo and mode of the morphological transformations occurring during the origin of pterosauromorphs unanswered.

To gather the missing pieces, Davide Foffa and colleagues turned towards Scleromochlus taylori- a small (less than 7.8 inch (20 cm) long) reptile with a large head, short neck, slender body, spindly legs, and a long tail. Because it is the oldest known non-pterosaur pterosauromorph from the Northern Hemisphere, Scleromochlus is fundamental to understanding early pterosauromorph evolution.

Its fossils were discovered in 1907, leaving many scientists confused about its features. Its remains are poorly preserved, so scientists placed this enigmatic reptile in several groups.

Solving the enigma of Scleromochlus taylori

In the current study, researchers used microcomputed tomographic scans (µCT) and image processing to show the whole-skeletal reconstruction of Scleromochlus. This non-destructive analysis allows the visualization of anatomical details covered by the sedimentary rock where the fossil is preserved.

To perform the µCT, each specimen was first embedded in broken sandstone blocks consisting of multiple parts. These blocks were first re-assembled and held together with rubber bands before µCT scanning started. The additional step was necessary to: capture the specimens in their entirety in a single scan; to minimize the risk of misidentification of individual elements that have been broken into separate sections and preserved in different parts of the original articulated blocks.

Results revealed new anatomical details, suggesting that Scleromochlus belonged to Pterosauromorpha — the clade that includes pterosaurs and a group of small reptiles called lagerpetids. Scleromochlus is more similar to lagerpetids than pterosaurs, making it one of the earliest branching lagerpetids. Scans also revealed that Scleromochlus lacks adaptations for flying, arboreality (animals living in trees), or leaping like a frog.

Instead, its unusual proportions and hindlimb morphology support the hypothesis that the common ancestor of pterosaurs and lagerpetids was a tiny, probably digitigrade, ground-dwelling (potentially bipedal) runner rather than a sprawling quadruped, frog-like hopper. The lack of flight-related adaptations in Scleromochlus and lagerpetids suggests that the evolution of pterosaurians remains to be found approximately 18 million years ago between the first pterosaurs and the origin of pterosauromorphs.