Because life on
Earth developed only once all organisms are interrelated, and birds are no exception. The closest living relatives of birds are crocodiles, alligators and caimans (Crocodilia). The group uniting
birds and crocodilians is referred to as Archosauria. Turtles and lizards are the next taxa most closely related to Archosauria, respectively.
Dating back to the year 1758 when the tenth edition of the
Systema Naturae of the famous Carl Linnaeus was published, birds are referred to by zoologists as Aves. There is, however, a dissent among
present-day scientists on the proper definition of the term. Personally, I prefer considering Aves as being represented by the last (i.e. the most recent) common ancestor of all extant birds and
all its descendants. According to this definition, the term Aves is synonymous to modern (or crown) birds. For historical reasons, however, many avian researchers still use the term Aves in a
broader sense to include the famous Archaeopteryx and other bird-like fossils; in this case modern birds are referred to as
Fossils not included in modern or crown birds but still closer to this group than to any other group of
living organisms are treated as members of the avian stem group. Stem-group fossils either pertain to the stem line itself, in which case they represent direct ancestors of the crown birds, or to
extinct side branches. Stem and crown birds together constitute the Pan-Aves, or total birds. Pan-Aves originated ~250 million years ago (mya), when the last common archosaurian ancestor split into two species,
one giving rise to the crocodilian and the other to the bird lineage.
Reconstructions of phylogenetic relationships among representatives of stem
groups are exclusively based on fossilized bones. Nuclear and mitochondrial DNA sequences, routinely used to reconstruct relationships among living groups, are usually not available. As a
consequence, inferring relationships among members of the avian stem-group is inherently problematic and the figure below depicts just one possible hypothesis. Despite this limitation, a
relatively coherent picture of stem avian evolution has developed over the last few decades:
Reconstruction of the evolutionary history of birds. Evolution from the earliest avian ancestors towards modern birds progresses along the avian stem-line (red) from Pan-Aves towards Aves. A number of stemline clades havn't been named yet (n.n.). For some clades the supposed age (in million years: Ma) is given in parentheses (green). Note that the popular Dinosauria clade comprises modern birds and almost all stem-birds (except Pterosauromorpha, Lagerpetonidae and Marasuchus). This means that dinosaurs didn't become extinct (a popular misconception), but are represented in the present-day fauna by modern birds. In fact, dinosaurs probably never were as diverse as they are today being reperesented by approximately 10.500 bird species.
The successive transformation into more and more bird-like
creatures started with the enlargement of the rear limbs. Initially, stem-birds possibly ran on their hind legs only for short periods, while moving on all four legs most of the time.
Bipedalism may have evolved in order to enhance the acceleratory phase when trying to escape from predators. In the course of evolution, stem-birds became lighter and
smaller, showing increased levels of activity and higher body temperatures. All living birds have body temperatures above 40° Celsius.
Along with increased body-temperature birds developed proto-feathers for better insulation and possibly display. Later, the forelimbs
became significantly longer than the hindlimbs, and the long bony tail disappeared.
Ultimately, complex contour feathers developed and birds learned to fly as an effective way of escaping from earth-bound predators. Considering the clumsy flight of extant tinamous and landfowl, avian flight almost
certainly evolved from the ground with a subsequent gliding phase.
Early dinosaurs had a global distribution. However,
since almost all non-avian fossils of the Eumaniraptora (with the remarkable exception of
Enantiornithes and Patagopteryx) were found in North America and Eurasia, it seems plausible to assume that eumaniraptorans evolved in the northern
hemisphere after the supercontinent Pangaea broke apart into a northern (Laurasia) and a southern (Gondwana) hemisphere.
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