TheArchaeornithes, classicallyArchæornithes, is an extinct group of the first primitive, reptile-likebirds. It is anevolutionary grade oftransitional fossils, the primitive birds halfway between non-aviandinosaur ancestors and the derived modern birds (avian dinosaur).
Fossils of early birds were poorly known until the late 20th century. Of those known, all fell into either the relatively modernly built birds with fused ribcage and thebreastbone extended into akeel, or the "Urvogels" of theSolnhofen Plattenkalk of lateJurassic age. As the physiological and anatomical difference between the two was so great, the subclass Archaeornithes was erected for the latter.
With the unearthing of several well preserved early bird fossils in the last decades of the 20th century and early 21st century, our knowledge of the evolution of birds has increased dramatically.[1] The evolution of the modern avian traits such as the compact body, clawless wing and thealula are now known to appear over successive stages. Today the Archaeornithes are classified into a series of nestedmonophyletic groups, and the name is rarely used in modern literature.
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| Cladogram of 19th century Class Aves (~Avialae) |
In traditional classification, it is one of twosubclasses of birds, the other subclass being theNeornithes, the birds with a short, modern tail. This classification was erected byHans Friedrich Gadow in 1893 and followed byAlfred Romer (1933) and subsequent authors through most of the 20th century.[2] Othermesozoic birds like the toothed, but otherwise modern, birds likeHesperornis were included under the latter in their own superorder, theOdontognathae.[3]
According toRomer, the Archaeornithes are characterised by having clawed wings, a reptilian style ribcage without a largecarina and the presence of a long, bony tail.[4] The known members of the group by the time of its erection wereArchaeopteryx andArchaeornis.[5] The two are now thought to represent a single species,Archaeopteryx lithographica, theArchaeornis being the Berlin specimen ofArchaeopteryx.[6] TheConfuciusornithidae andEnantiornithes were found a century after Gradow's organization of birds into two subclasses. 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 theploughshare-shaped one in modern birds) and the presence of a small carina.[3]
While rarely used by palaenthologists today, the term was revived by theornithologists Livezey and Zusi in 2007, for a group comprisingArchaeopterygidae and theConfuciusornithidae.[7]
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