Scleromochlus taylori and the origin of dinosaurs and pterosaurs

The phylogenetic position of Scleromochlus taylori has been disputed recent ly, in terms of whether it is a basal sister group of Pterosauria or of Din osauromorpha. The seven specimens, all from the Lossiemouth Sandstone Forma tion (late Carnian, Late Triassic) of Lossiemouth, near Elgin, north-east S cotland, suggest that Scleromochlus shares no unique features with either P terosauria or Dinosauromorpha, together the Omithodira, but is a close outg roup. Scleromochlus retains a primitive ankle structure, and it has a slend er humerus, femur and fibula. Scleromochlus shows the classic bird-like cha racters of a tibia that is longer than the femur, and a closely appressed g roup 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 r elationships confirms the split into Crurotarsi (crocodile relatives) and O rnithodira (bird relatives), as well as the clear division of Ornithodira i nto Pterosauria and Dinosauromorpha. Relationships within Crurotarsi are, h owever, much less clear: Ornithosuchidae probably reside within that clade, and there might be a clade 'Rauisuchia' consisting of Prestosuchidae and P ostosuchus, but support for these relationships is weak. Scleromochlus was probably a bipedal cursor that could adopt a digitigrade stance. However, i t is possible that Scleromochlus was also a saltator, capable of leaping lo ng distances.