Archaeopterygiformes

The Archaeopterygiformes were an order of early birds that lived during the Jurassic and possibly Cretaceous periods. They include one of the most well known and earliest birds known, Archaeopteryx. Archaeopterygiformes are distinguished from other early birds by long bony tails, and in some species, by the presence of a hyperextendible second toe.

Taxonomy
The order Archaeopterygiformes was coined by Lambrecht in 1933 to contain the single family Archaeopterygidae and genus Archaeopteryx. While the order has never been given a phylogenetic definition, a definition for Archaeopterygidae was proposed by Paul Sereno in 2005: the clade comprising all birds closer to Archaeopteryx than to Neornithes.

The family Dromaeosauridae, traditionally considered to be non-avian dinosaurs, have been included in this group by at least one author. Discoveries of a number of primitive forms have muddied the relationships of early birds, making it possible that Velociraptor and similar dinosaurs are actually members of Aves. Gregory S. Paul placed dromaeosaurids in Archaeopterygiformes for these reasons, though most cladistic analyses since have found them to be slightly more primitive, and therefore outside clade Aves.

As its name suggests, Protarchaeopteryx was also originally referred to this order, but most paleontologists now consider it an oviraptorosaur. Other referred genera, like Jurapteryx, Wellnhoferia, and Proornis, are probably synonymous with Archaeopteryx (the former two) or do not belong into this group (the last). Jinfengopteryx was originally described as an archaeopterygid, though it was later shown to be a troodontid.