The hunt for the ancestors of living birds began with a specimen of Archaeopteryx, the first known bird, discovered in the early s. Like birds, it had feathers along its arms and tail, but unlike living birds, it also had teeth and a long bony tail. Furthermore, many of the bones in Archaeopteryx's hands, shoulder girdles, pelvis, and feet were distinct, not fused and reduced as they are in living birds.
Based on these characteristics, Archaeopteryx was recognized as an intermediate between birds and reptiles; but which reptiles? The Berlin specimen of Archaeopteryx lithographica.
Speech On: If I were a bird
In the s, paleontologists noticed that Archaeopteryx shared unique features with small carnivorous dinosaurs called theropods. All the dinosaur groups on this evogram, except the ornithischian dinosaurs, are theropods. Based on their shared features, scientists reasoned that perhaps the theropods were the ancestors of birds.
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When paleontologists built evolutionary trees to study the question, they were even more go here. The birds are simply a twig on the dinosaurs' branch of the tree of life. As birds evolved from these theropod dinosaurs, many of their features were modified. However, it's important to remember that the animals were not "trying" to be birds in any sense.
In fact, the more closely we look, the more obvious it is that the suite of features that characterize birds evolved through a complex series of steps and served different functions along the way.
Take feathers, for example. Small theropods related to Compsognathus e.
These short, hair-like feathers grew on their heads, necks, and bodies and provided insulation. The feathers seem to have had different color patterns as well, although whether these were for display, camouflage, species recognition, or another function is difficult to tell.
This fossil of Sinosauropteryx preserves evidence of hair-like feathers. In theropods even more closely related to birds, like the oviraptorosaurs, we find several new types of feathers.
One is branched and downy, as pictured below. Others have evolved a central stalk, with unstructured branches coming off it and its base. Still others like the dromaeosaurids and Archaeopteryx have a vane-like structure in which the barbs are well-organized and locked together by barbules.
This is identical to the feather structure of living birds. At right, asymmetrical flight feathers are present in a fossil of a dromaeosaurid that may have had the ability to glide. Another line of evidence comes from changes in the digits of the dinosaurs leading to birds.
The first theropod dinosaurs had hands with small fifth and fourth digits and a long second digit. As the evogram shows, in the theropod lineage that would eventually lead to birds, the fifth digit e. The wrist bones underlying the first and second digits consolidated and took on a semicircular form that allowed the hand to rotate sideways against the forearm. This eventually allowed birds' wing joints to move in a https://oops-essay.icu/blog20/5425-thesis-statement-on.php that creates thrust for flight.
This oviraptorid dinosaur, Citipati osmolskae, may have been protecting a nest of eggs. The functions of feathers as they evolved have long been debated. As we have seen, the first, simplest, hair-like feathers obviously served an insulatory function.
But in later theropods, such as some oviraptorosaurs, the feathers on the arms and hands are long, even though the forelimbs themselves are short. What did these animals do with long feathers on short arms? One suggestion comes from some remarkable fossils of oviraptorosaurs preserved in the Cretaceous sediments of the Gobi Desert. The skeleton of the animal is hunched up on a nest of eggs, like a brooding chicken.
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The hands are spread out over the eggs as if to shelter them. So perhaps these feathers served the function of warming the eggs and shielding them from harm. Birds after Archaeopteryx continued evolving in some of the same directions as their theropod ancestors.
Many of their bones were reduced and fused, which may have helped increase the efficiency of flight. Similarly, the bone walls became even thinner, and the feathers became longer and their vanes asymmetrical, probably also improving flight. The bony tail was reduced to a stump, and a.