Birds are feathered, winged, bipedal, endothermic, egg-laying, vertebrate animals. There are around 10,000 living species, making them the most speciose class of tetra-pod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from the 5 cm. Bee Hummingbird to the 2.75 m (9 ft) Ostrich. The fossil record indicates that birds evolved from tetra-pod dinosaurs during the Jurassic period, around 150–200 Ma (million years ago), and the earliest known bird is the Late Jurassic Archaeopteryx, c 150–145 Ma. Most paleontologists regard birds as the only clade of dinosaurs to have survived the Cretaceous–Tertiary extinction event approximately 65.5 Ma.
Modern birds are characterized by feathers, a beak with no teeth, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a lightweight but strong skeleton.
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