The link between dinosaurs and birds is a hotly contested issue, as is the whole field of human evolution itself. However, a great deal of scientific evidence suggests that birds are in fact avian dinosaurs. The similarities between birds and reptiles, considered the direct descendants of the dinosaurs, support this link. The connection is so strong that birds themselves can technically be considered as reptiles.
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Birds and reptiles both have scales. These scales are less evident on a bird, because they are confined to the feet and legs. Feathers, however, are also produced by tissues similar to those that produce scales, according to the University of California Museum of Paleontology website.
Birds and reptiles both lay shelled eggs. The formation of these eggs offers further similarities between these two classes of animal. According to Robert E. Ricklefs and J. Matthias Starck in "Avian Growth and Development," the eggs of birds and reptiles do not appear to differ in the rate of maturation, nor do they differ greatly in the energy efficiency of embryonic growth.
The internal organs of birds and reptiles show a number of similarities. Both have a cloaca, an orifice used to excrete urine and faeces. The cloaca is present in reptiles and birds but is not found in most mammals and fish, says the online Encyclopedia Britannica. According to Ricklefs and Starck, birds and reptiles share a number of characteristics within the respiratory and nervous systems. Sensory organs are similar, particularly the workings of the ear. Birds also have a circulatory system comparable to that of crocodiles.
Birds and reptiles both have nucleated red blood cells, says the Smithsonian National Zoological Park website. This blood is not dissimilar to human blood, but, unlike bird and reptile blood, the red corpuscles in human blood have no nucleus.
The internal skeletal structure, or endoskeleton, of birds and reptiles reveals many similarities. According to Professor R. L. Kotpal, author of "Modern Text Book of Zoology," both have a single occipital condyle, a bone at the back of the skull that attaches the head to the neck. Most mammals have a double occipital condyle. Other skeletal similarities include the structure of the jaw, comparable vertebral structure, and loosely related pectoral and pelvic girdles.
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- University of California Museum of Paleontology: Are Birds Really Dinosaurs?
- "Avian Growth and Development"; Robert E. Ricklefs and J. Matthias Starck; 1998
- Encyclopedia Britannica: Cloaca
- Smithsonian National Zoological Park: Reptiles to Robins
- "Modern Text Book of Zoology"; Professor R. L. Kotpal; 2009