Archaeornithes
Encyclopedia
Evolutionary grade
In alpha taxonomy, a grade refers to a taxon united by a level of morphological or physiological complexity. The term was coined by British biologist Julian Huxley, to contrast with clade, a strictly phylogenetic unit.-Definition:...
of transitional fossils, the primitive birds halfway between dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
ancestors and the derived modern birds.
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 the breastbone
Sternum
The sternum or breastbone is a long flat bony plate shaped like a capital "T" located anteriorly to the heart in the center of the thorax...
extended into a keel, or the "Urvogels" of the Solnhofen Plattenkalk of late Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
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. The evolution of the modern avian traits such as the compact body, clawless wing and the alula
Alula
The alula, or bastard wing, is a small projection on the anterior edge of the wing of modern birds. The alula is the freely moving first digit, a bird's "thumb," and is typically covered with three to five small feathers, with the exact number depending on the species...
are now known to appear over successive stages. Today the Archaeornithes are classified into a series of nested monophyletic
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
groups, and the name is rarely used in modern literature.
Classification
Basal bird phylogeny simplified after Chiappe, 2007 All but the Ornithurae cladeClade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
fall within the Archaeornithes
In traditional classification, it is one of two subclasses
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
of birds, the other subclass being the Neornithes, the birds with a short, modern tail. This classification was erected by Hans Friedrich Gadow
Hans Friedrich Gadow
Hans Friedrich Gadow was a German ornithologist.Gadow was born in Pomerania, the son of an inspector of the Prussian royal forests. He studied at the universities of Berlin, Jena and Heidelberg. At Jena he studied under Ernst Haeckel and at Heidelberg under the anatomist Karl Gegenbaur...
in 1893 and followed by Alfred Romer
Alfred Romer
Alfred Sherwood Romer was an American paleontologist and comparative anatomist and a specialist in vertebrate evolution.-Biography:...
(1933) and subsequent authors through most of the 20th century. Other mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
birds like the toothed, but otherwise modern, birds like Hesperornis
Hesperornis
Hesperornis is a genus of flightless aquatic birds that spanned the first half of the Campanian age of the Late Cretaceous period . One of the lesser-known discoveries of the paleontologist O. C. Marsh in the late 19th century Bone Wars, it was an important early find in the history of avian...
were included under the latter in their own superorder, the Odontognathae
Odontognathae
Odontognathae is an extinct superorder of birds found widely in Kansas. The superorder was originally proposed by Alexander Wetmore, who attempted to link fossil birds with the presence of teeth, specifically of the orders Hesperornithiformes and Ichthyornithiformes...
.
According to Romer
Romer
A Reference Card or "Romer" is a device for increasing the accuracy when reading a grid reference from a map. Made from transparent plastic, paper or other materials, they are also found on most baseplate compasses. Essentially, it is a specially marked-out ruler which matches the scale of the map...
, the Archaornithes are charachterised by having clawed wings, a reptilian style ribcage without a large carina and the presence of a long, bony tail. The known members of the group by the time of it's erection was Archaeopteryx
Archaeopteryx
Archaeopteryx , sometimes referred to by its German name Urvogel , is a genus of theropod dinosaur that is closely related to birds. The name derives from the Ancient Greek meaning "ancient", and , meaning "feather" or "wing"...
and Archaeornis. The two are now thought to represent a single species, Archaeopteryx lithographica, the Archaeornis being the Berlin specimen of Archaeopteryx.
The Confuciusornithidae
Confuciusornithidae
Confuciusornithidae is a family of primitive birds from the early Cretaceous Period of China. It consists of four genera. About half of all the confuciusornithid specimens, including reprersentatives of all species, that have preserved feathers possess a pair of distinctive ribbon-like tail...
and Enantiornithes
Enantiornithes
Enantiornithes is an extinct group of primitive birds. They were the most abundant and diverse avialans of the Mesozoic. Almost all retained teeth and clawed fingers on each wing, but otherwise looked much like modern birds externally. Over 50 species of Enantiornithines have been named, but some...
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 the ploughshare
Ploughshare
A ploughshare or plowshare is a component of a plough or plow.Ploughshare or Plowshare may also refer to:* Operation Plowshare, United States project to use nuclear explosives for civilian purposes* vomer, facial bone in the human skull...
-shaped one in modern birds) and the presence of a small carina.
While rarely used by palaenthologists today, the term was revived by the ornithologists
Ornithology
Ornithology is a branch of zoology that concerns the study of birds. Several aspects of ornithology differ from related disciplines, due partly to the high visibility and the aesthetic appeal of birds...
Livezey and Zusi in 2007, for a group comprising Archaeopterygidae
Archaeopterygidae
The Archaeopterygidae is a group of maniraptoran dinosaurs that lived during the late Jurassic and period.-Taxonomy:The order Archaeopterygiformes was coined by Max Fürbringer in 1888 to contain the single family Archaeopterygidae and genus Archaeopteryx...
and the Confuciusornithidae
Confuciusornithidae
Confuciusornithidae is a family of primitive birds from the early Cretaceous Period of China. It consists of four genera. About half of all the confuciusornithid specimens, including reprersentatives of all species, that have preserved feathers possess a pair of distinctive ribbon-like tail...
.