Archaeornithes

Of those known, all fell into either the relatively modernly built birds with fused ribcage and the breastbone extended into a keel, or the "Urvogels" of the Solnhofen Plattenkalk of late Jurassic age.

[1] The evolution of the modern avian traits such as the compact body, clawless wing and the alula are now known to appear over successive stages.

[3] According to Romer, the Archaeornithes are characterised by having clawed wings, a reptilian style ribcage without a large carina and the presence of a long, bony tail.

They fall between Romer's description of Archaeornithes and Neornithes, in that they have clawed wings, but reduced tails with a rod-like pygostyle (as opposed to the ploughshare-shaped one in modern birds) and the presence of a small carina.

[3] While rarely used by palaenthologists today, the term was revived by the ornithologists Livezey and Zusi in 2007, for a group comprising Archaeopterygidae and the Confuciusornithidae.