Why do paleontologists classify birds as dinosaurs
Only in the 1990s was the fossil record in the evolutionary line from the theropod dinosaurs to the birds through a series of discoveries feathered dinosaur and earlier enriched birds in the People's Republic of China. Ever since the Archeopteryx in Solnhofen limestone in 1861 there is certainty that the birds descended from reptiles. However, until a few years ago, other fossils that show this development were extremely rare.
The primeval bird Archeopteryx represents a mosaic of features of both dinosaurs and birds. The dinosaur features include, for example, the long tail, the dinosaur-like chest girdle, the toothed jaws or the abdominal ribs. The wings with the feathers are particularly typical of birds, but also the legs with a special joint, the intertarsal joint, in the tarsus. This set of features confirmed the assumption about the descent of birds from the dinosaurs and thus represented an essential piece of evidence for Charles Darwin's theory of evolution, which was only three years old. Archeopteryx remained the only animal that represented a connection between birds and dinosaurs and so many theories, especially about the development of flight and the feathers designed for it, remained unproven.
Archeopteryx already had feathers that corresponded in structure and function to the feathers of today's birds. These so-called contour springs consist of a central shaft from which smaller side branches branch off. These twigs are hooked together and thus receive stability. In addition, they are asymmetrical and, together with neighboring springs, form a wing that becomes impermeable to air on impact and permeable to air on impact. Since the wings of the Archeopteryx were constructed in this way, today there is broad consensus about the flight ability of this primeval bird. How and whether these feathers could have developed from reptile scales, however, has so far only been the subject of speculation. See also: evolution of the bird's feather.
In 1996, researchers discovered a fossil in the sediments of the Jehol Group in Liaoning Province, China, which marked the beginning of a series of spectacular discoveries. This dinosaur fossil, named a theropod Sinosauropteryx, had a contour made of thread structures. Also later fossils of the same region as Caudipteryx, Sinornithosaurus and especially Beipiaosaurus showed similar body-enveloping fibers, which later turned out to be down-like structures made of several filaments with a common point of origin. In addition, elements with a central shaft and symmetrically arranged secondary branches were also found, especially on the forelegs and tail. However, these springs lacked reinforcement. Accordingly, they could not be used as flight instruments, they probably served more to insulate the body from heat. Based on finds of the Oviraptor in a clear breeding position on a clutch it was already known that this group of dinosaurs must have been warm-blooded. An insulating spring layer confirms this assumption.
This does not fully explain the evolution of feathers and bird flight, but the gaps have become much smaller than they were before 1996. But the Chinese fossils also have something to offer for understanding the further evolution of birds to their modern representatives. At the beginning of 2001 a bird fossil with the name was found Apsaravis in the magazine Nature reported. Since the discovery of the Ichthyornis In 1870, no more close relatives of recent birds were found.
Some American paleornithologists, such as Alan Feduccia, interpret the relationships between birds and dinosaurs differently; they think that the birds split off from the archosaurs before the dinosaurs.
So have Archeopteryx 150 million years ago already had contour feathers like today's birds, some much later species of theropods like Caudipteryx or Protarcheopteryxwhich are believed to be the ancestors of the birds, would have had much more primitive feathers. This can be better explained by the fact that these alleged theropods are descendants of flightless birds, for which they coined the expression "bird non-dinosaurs".
Their study from 2005 came to the conclusion that the fibers that envelop the body, which some of the theropods found in the Jehol group have, are not feathers, but rather connective tissue in the form of collagen fibers.
Finally, they object that the theropod's fingers developed from fingers I – III, whereas in birds they developed from fingers II – IV.
However, the majority of paleontologists do not share Feduccia's view.
- Zhou, Z. (2004): The origin and early evolution of birds: discoveries, disputes and perspectives from fossil evidence. - Natural Sciences 91: 455–471 (full text).
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