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Knowledge of the evolution of birds is limited because their bones are thin and fragile, so it is difficult its preservation as fossils.
There is a consensus among scientists which states that Archaeopteryx lithographica (150 million years [M.Y.]), is the most primitive bird known, which has several characters shared with theropods dinosaurs (carnivores). It was found in 1861 in a limestone deposit of Bavaria (Germany). The wings had three separated fingers with claws, and their jaws had teeth. The tail was long and bony while its pelvis was dinosaur-like, with non-fused bones. Their feathers were equal to those of modern birds.
Already in 1870, the English naturalist Thomas Henry Huxley claimed that Archaeopteryx was a dinosaur with feathers and that birds had evolved from small theropods. Today this theory is widely accepted and supported by several lines of evidence.
Many Mesozoic bird lineages became extinct along with the dinosaurs 65 M.Y. ago. But, since then, they have diversified greatly, accounting for 30 current bird orders, which are grouped into around 200 families and 10,000 species, of which more than 280 are present in the Province of Neuquén.
This evolutionary process covers the last two periods of the Mesozoic Era: Jurassic (from 200 to 144 million years ago) and Cretaceous (from 144 to 65 million years ago), as well as the two periods of the Cenozoic Era: Tertiary (from 65 million years to 1,800,000 years ago) and the Quaternary (from 1,800,000 years ago to the present).
In a very synthetic way, it is possible to affirm that a group of theropod dinosaurs (from approximately 5 meters and a half) evolved into Archaeopteryx (50 centimeters approximately) and from them, a certain group towards the current birds, which make up the passerine order, such as the Dark-faced ground tyrant, of about 16 centimeters.
Today it is considered that birds are “living dinosaurs”. Proof of this is:
– The presence of reptilian-like scales on the legs of modern birds.
– The presence of feathers in carnivorous dinosaurs.
– The same type of eggshell present in fossil and modern birds.
Even there were found theropod dinosaurs covering eggs on the nest in an identical manner to the birds nowadays.
An ancestor of birds found in the region
90 M.Y. ago, in South America there existed a theropod dinosaur called Unenlagia. Its Mapuche name means «half bird» and its general appearance would have been similar to that of the famous Velociraptor. Two specimens of this dinosaur are known, both found in the center of the province of Neuquén, whose skeletons, (which would have been more than two meters long when complete), have several characteristics present in birds. For example, the ability to fold the arms backwards as if they were small wings stands out, and the bones of its pelvis have a constitution very similar to that present in Archeopteryx.