Evolution of birds
For more than a century, the small theropod dinosaur Archaeopteryx lithographica from the Late Jurassic period was considered to have been the earliest bird.
According to the current consensus, Aves and a sister group, the order Crocodilia, together are the sole living members of an unranked reptile clade, the Archosauria.
Phylogenetically, Aves is usually defined as all descendants of the most recent common ancestor of a specific modern bird species (such as the house sparrow, Passer domesticus), and either Archaeopteryx,[2] or some prehistoric species closer to Neornithes (to avoid the problems caused by the unclear relationships of Archaeopteryx to other theropods).
[5]Discoveries in northeast China (Liaoning Province) demonstrate that many small theropod dinosaurs did indeed have feathers, among them the compsognathid Sinosauropteryx and the microraptorian dromaeosaurid Sinornithosaurus.
[6] Cryptovolans, a dromaeosaurid found in 2002 (which may be a junior synonym of Microraptor) was capable of powered flight, possessing a sternal keel and ribs with uncinate processes.
[10] The basal bird Archaeopteryx, from the Jurassic, is well known as one of the first "missing links" to be found in support of evolution in the late 19th century.
It may be predated by Protoavis texensis, though the fragmentary nature of this fossil leaves it open to considerable doubt whether this was a bird ancestor.
The skeleton of all early bird candidates is basically that of a small theropod dinosaur with long, clawed hands, though the exquisite preservation of the Solnhofen Plattenkalk shows Archaeopteryx was covered in feathers and had wings.
The loss of a long tail was followed by a rapid evolution of their legs which evolved to become highly versatile and adaptable tools that opened up new ecological niches.
The ratites are a paraphyletic (artificial) grouping because tinamous are part of their evolutionary clade and they have likely lost the ability to fly independently, becoming an example of convergent evolution.
In contrast, most major radiations of seabirds and shorebirds (as well as in paleognaths, despite their ancient origins) were found to have only occurred after the K-Pg extinction event, and primarily after the Paleocene–Eocene Thermal Maximum.
Evidence from modern bird anatomy, fossils and DNA have all been brought to bear on the problem but no strong consensus has emerged.
Structural characteristics and fossil records have historically provided enough data for systematists to form hypotheses regarding the phylogenetic relationships between birds.
Imprecisions within these methods is the main factor for why a lack of exact knowledge with regards to the orders and families of birds exists.
For example, the many hybrid hummingbirds found in northwest South America may represent a threat to the conservation of the distinct species involved.
In some birds this is limited to color variations, while others are bred for larger egg or meat production, for flightlessness or other characteristics.
In December 2019 the results of a joint study by Chicago's Field Museum and the University of Michigan into changes in the morphology of birds were published in Ecology Letters.
The study shows that the length of birds' lower leg bones (an indicator of body sizes) shortened by an average of 2.4% and their wings lengthened by 1.3%.
