Reassessment of the Triassic archosauriform Scleromochlus taylori: neither runner nor biped, but hopper.

S Christopher Bennett

doi:10.7717/peerj.8418

Abstract

The six known specimens of Scleromochlus taylori and casts made from their negative impressions were examined to reassess the osteological evidence that has been used to interpret Scleromochlus’s locomotion and phylogenetic relationships. It was found that the trunk was dorsoventrally compressed. The upper temporal fenestra was on the lateral surface of skull and two-thirds the size of the lower, the jaw joint posteriorly placed with short retroarticular process, and teeth short and subconical, but no evidence of external nares or antorbital fossae was found. The posterior trunk was covered with ~20 rows of closely spaced transversely elongate dorsal osteoderms. The coracoid was robust and elongate. The acetabulum was imperforate and the femoral head hemispherical and only weakly inturned such that the hip joint was unsuited to swinging in a parasagittal plane. The presence of four distal tarsals is confirmed. The marked disparity of tibial and fibular shaft diameters and of proximal tarsal dimensions indicates that the larger proximal tarsal is the astragalus and the significantly smaller tarsal is the calcaneum. The astragalus and calcaneum bear little resemblance to those of Lagosuchus, and the prominent calcaneal tuber confirms that the ankle was crurotarsal. There is no evidence that preserved body and limb postures are unnatural, and most specimens are preserved in what is interpreted as a typical sprawling resting pose. A principal component analysis of skeletal measurements of Scleromochlus and other vertebrates of known locomotor type found Scleromochlus to plot with frogs, and that finding combined with skeletal morphology suggests Scleromochlus was a sprawling quadrupedal hopper. Phylogenetic analyses found that Scleromochlus was not an ornithodiran, but was either within the Doswelliidae or outside the clade consisting of the most recent common ancestor of the Erythrosuchidae and Archosauria and all its descendants.

Highlights

Among the tetrapod fossils from the Carnian Lossiemouth Sandstone Formation near Elgin, Scotland, those of Scleromochlus taylori, a tiny archosauriform with extremely long hindlimbs known from seven largely articulated skeletons on small slabs, have probably had the greatest impact on interpretations of diapsid evolution
Benton & Walker (1985) reviewed the vertebrate fauna of the Lossiemouth Sandstone, which consisted of the herbivores Stagonolepis, Hyperodapedon (1.3 m) and Leptopleuron (40 cm), the carnivores Ornithosuchus, Erpetosuchus (60 cm) and Saltopus (60 cm), the omnivore Brachyrhinodon (25 cm) and smallest of all, Scleromochlus (17 cm per Benton, 1999), which made up 5% of the fossils
Benton & Walker (1985) described the Lossiemouth Sandstone as lying atop a claystone or siltstone and consisting of up to 30 m of eolian sandstone with uniform grain size and large-scale cross-beds. They cited several authors (e.g., Gordon, 1892) writing while the Lossiemouth Sandstone was being quarried and its fossils collected, who stated that the vertebrate fossils were found only in a thin bed of soft yellowish sandstone at the base of the formation below the main thickness of hard siliceous sandstone that was quarried for building stone

Summary

Introduction

Among the tetrapod fossils from the Carnian Lossiemouth Sandstone Formation near Elgin, Scotland, those of Scleromochlus taylori, a tiny archosauriform with extremely long hindlimbs known from seven largely articulated skeletons on small slabs, have probably had the greatest impact on interpretations of diapsid evolution. The first specimens were briefly described and named by Woodward (1907), who interpreted Scleromochlus as a small bipedal running or leaping dinosaur. How to cite this article Bennett SC. Reassessment of the Triassic archosauriform Scleromochlus taylori: neither runner nor biped, but hopper. PeerJ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __

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Conclusion