An improved description of the small archosaur Scleromochlus together with a cladistic analysis of its phylogenetic position demonstrated that Scleromochlus was more closely related to the dinosaurs than to the Crurotarsi (to which the crocodiles belong) but outside of the node-basedcladeOrnithodira as originally interpreted by Jacques Gauthier: the group containing the last common ancestor of the dinosaurs and the pterosaurs and all of its descendants.1
Paul Sereno had in 1991 given a formal (and different) definition of Ornithodira, one in which Scleromochlus was explicitly added.2 However, at that point, there was no named clade that could encompass species with a basal position on the archosaurian branch leading to dinosaurs (as opposed to that leading to crocodiles) so Benton named a new branch-basedclade for this purpose: Avemetatarsalia, named after the birds (Aves), the last surviving members of the clade, and the metatarsal ankle joint that was a typical character of the group. Avemetatarsalia was defined as: all Avesuchia closer to Dinosauria than to Crocodylia.
References
^ ab Benton, M.J. (1999). Scleromochlus taylori and the origin of dinosaurs and pterosaurs. Philosophical Transactions of the Royal Society of London354:1423-1446.
^ Sereno, P. C. 1991. Basal archosaurs: phylogenetic relationships and functional implications. Journal of Vertebrate PaleontologyMemoir 2, 11(4, Supplement):1-53.
