Avemetatarsalia

Avemetatarsalia

Avemetatarsalia (meaning "bird metatarsals") is a clade name established by British palaeontologist Michael Benton in 1999 for all avesuchians (crown group archosaurs) that are closer to dinosaurs than to crocodiles.

In 1999 Benton published an improved description of the small archosaur Scleromochlus together with a cladistic analysis of its phylogenetic position. This analysis showed that Scleromochlus was more closely related to the dinosaurs than to the Crurotarsi (to which the crocodiles belong) but outside of the node clade Ornithodira as originally interpreted by Jacques Gauthier: the group containing the last common ancestor of the dinosaurs and the pterosaurs and all of its descendants. Paul Sereno had in 1991 given a formal (and different) definition of Ornithodira, one in which Scleromochlus was explicitly added. However, as yet there existed no clade concept emphasizing the possible basal position of species within the archosaurian branch leading to dinosaurs (as opposed to that leading to crocodiles) so Benton created a new stem clade 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.

Avesuchia was a new term: as Benton rejected the use of a crown group Archosauria sensu Gauthier, but continued to use the term in its traditional content (thus equal to Gauthier's Archosauriformes), in the same publication he created the name Avesuchia for the crown group. However, Avesuchia was again defined referring to Avemetatarsalia, making the definition structure circular.

In 2005 Sereno gave a new definition, based on species instead of groups, of Avematetarsalia: all species closer to Passer domesticus than to Crocodylus niloticus. Sereno considered there was little need for a concept Ornithodira, not even to make a contrast with basal Avemetatarsalians, as these are presently unknown: recent analyses show Scleromochlus within Ornithodira sensu Gauthier. Avemetatarsalia is seen as more useful because it neatly divides Archosauria sensu Gauthier in two, together with a Crurotarsi, if the latter is used as a stem clade also, as Sereno does since 2005.

References

  • Benton, M.J. (1999), "Scleromochlus taylori and the origin of dinosaurs and pterosaurs", Philosophical Transactions of the Royal Society of London 354 1423-1446
  • Benton, M.J. (2005). Vertebrate Palaeontology (3rd ed.). University of Bristol.

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