Scleromochlus taylori and the origin of dinosaurs and pterosaurs

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Abstract
The phylogenetic position of Scleromochlus taylori has been disputed recently, in terms of whether it is a basal sister group of Pterosauria or of Dinosauromorpha. The seven specimens, all from the Lossiemouth Sandstone Formation (late Carnian, Late Triassic) of Lossiemouth, near Elgin, north–east Scotland, suggest that Scleromochlus shares no unique features with either Pterosauria or Dinosauromorpha, together the Ornithodira, but is a close outgroup. Scleromochlus retains a primitive ankle structure, and it has a slender humerus, femur and fibula. Scleromochlus shows the classic bird–like characters of a tibia that is longer than the femur, and a closely appressed group of four elongate metatarsals. A new group name, Avemetatarsalia (‘bird feet’), is established here for the clade consisting of Scleromochlus and Ornithodira, and their descendants. A reanalysis of crown–group archosaur relationships confirms the split into Crurotarsi (crocodile relatives) and Ornithodira (bird relatives), as well as the clear division of Ornithodira into Pterosauria and Dinosauromorpha. Relationships within Crurotarsi are, however, much less clear: Ornithosuchidae probably reside within that clade, and there might be a clade ‘Rauisuchia’ consisting of Prestosuchidae and Postosuchus , but support for these relationships is weak. Scleromochlus was probably a bipedal cursor that could adopt a digitigrade stance. However, it is possible that Scleromochlus was also a saltator, capable of leaping long distances.