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Aves (birds) See text for extinct groups. Reptiles, as commonly defined, are a group of tetrapods with an ectothermic ('cold-blooded') metabolism and amniotic development. Living reptiles comprise four orders: Testudines (turtles), Crocodilia (crocodilians), Squamata (lizards and snakes), and Rhynchocephalia (the tuatara).
Contents. Evidence of common descent. Evidence of common descent of living organisms has been discovered by scientists researching in a variety of disciplines over many decades, demonstrating that all life on Earth comes from a single ancestor. This forms an important part of the evidence on which evolutionary theory rests, demonstrates that ...
Likewise, birds are considered reptiles in the modern cladistic sense of the term, and their closest living relatives are the crocodilians. Birds are descendants of the primitive avialans (whose members include Archaeopteryx ) which first appeared during the Late Jurassic .
The harder beta-keratins (β-keratins) are found only in the sauropsids, that is all living reptiles and birds. They are found in the nails, scales, and claws of reptiles, in some reptile shells (Testudines, such as tortoise, turtle, terrapin), and in the feathers, beaks, and claws of birds. [10] These keratins are formed primarily in beta sheets.
Ossea Batsch, 1788[2] Vertebrates (/ ˈvɜːrtəbrɪts, - ˌbreɪts /) [3] are deuterostomal animals with bony or cartilaginous axial endoskeleton — known as the vertebral column, spine or backbone — around and along the spinal cord, including all fish, amphibians, reptiles, birds and mammals.
Archosauria (lit. 'ruling reptiles') or archosaurs (/ ˈɑːrkəˌsɔːr / [3]) is a clade of diapsid sauropsid tetrapods, with birds and crocodilians being the only extant representatives. Although broadly classified as reptiles, which traditionally exclude birds, the cladistic sense of the term includes all living and extinct relatives of ...
The scientific question of within which larger group of animals birds evolved has traditionally been called the " origin of birds ". The present scientific consensus is that birds are a group of maniraptoran theropod dinosaurs that originated during the Mesozoic Era. A close relationship between birds and dinosaurs was first proposed in the ...
Gnathostomata is traditionally a infraphylum, broken into three top-level groupings: Chondrichthyes, or the cartilaginous fish; Placodermi, an extinct grade of armored fish; and Teleostomi, which includes the familiar classes of bony fish, birds, mammals, reptiles, and amphibians. Some classification systems have used the term Amphirhina.