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Archosauria (Cope 1869 emended by Gauthier 1984 "ruling reptiles") sensu stricto, or sensu Gauthier, is the crown clade of Archosauromorpha and is defined as "the common ancestor of birds and crocodiles and all descendants thereof. Such a crown clade definition extracts from Archosauria forms classically considered to be archosaurs and serves no viable purpose. Thus the author prefers a traditional definition of Archosauria, and follows the node-based definition advanced by Benton (1999), in which Archosauria is the common ancestor of Euparkeria and Avesuchia and all its descendants. Benton (1999) applied the new taxon Avesuchia to crown clade archosaurs to supplant the confusing migration of Archosauria to the crown as advanced by Gauthier (1986). Avesuchia is defined by Benton as a node-based taxon including the common ancestor of Avemetatarsalia and Crurotarsi and all its descendants. Crurotarsi, a taxon first erected by Sereno (1991) to replace the philosophically absurd usage of "Pseudosuchia" advanced by Gauthier (1986) is unquestionably holophyletic, but the internal topology of this clade is entirely unresolved. Avemetatarsalia is defined as a stem-based taxon, all avesuchians closer to Dinosauria than to Crocodylia (Scleromochlidae + Ornithodira). Holophyly of Avemetatarsalia is robustly supported (Benton 1999). Holophyly of Archosauria is universally accepted though it has long been noted (e.g., Romer 1956) that Archosauria is not readily definable in comparison with other taxa. Synapomorphies of Archosauria include (after Benton 1990):
- Antorbital fenestra present
- Postfrontal reduced
- Postparietals absent or fused
- Laterosphenoid ossification
- Serrated teeth, laterally compressed
- Trunk intercentra lost
- Ectepicondylar foramen of humerus absent
- Fourth trochanter present
- Benton, M. J. 1990. Origin and interrelationships of dinosaurs. In: Weishampel, D., Dodson, P. & Osmolska, H. (eds.), The Dinosauria, 11-31.
- Benton, M. J. 1999. Scleromochlus taylori and the origin of dinosaurs and pterosaurs. Philosophical Transactions of the Royal Society of London B 354: 1423-1446.
- Benton, M. J. & Clark, J. 1988. Archosaur phylogeny and the relationships of Crocodylia. In: Benton, M. J. (ed.), The Phylogeny and Classification of the Tetrapods, 289-332.
- Gauthier, J. 1986. Saurischian monophyly and the origin of birds. Memoirs of the California Academy of Sciences 8: 1-55.
- Romer, A. S. 1956. Osteology of the Reptiles. University of Chicago Press, Chicago.
- Sereno, P. 1991. Basal archosaurs: phylogenetic relationships and functional implications. Journal of Vertebrate Paleontology (Suppl.) 11: 1-51.