Birds (class Aves) are winged, bipedal, endothermic (warm-blooded), egg-laying, vertebrate animals. There are around 10,000 living species, making them the most numerous tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from the 5 cm (2 in) Bee Hummingbird to the 3 m (10 ft) Ostrich. The fossil record indicates that birds evolved from theropod dinosaurs during the Jurassic period, around 150200 Ma (million years ago), and the earliest known bird is the Late Jurassic Archaeopteryx, c 150145 Ma. Most paleontologists regard birds as the only clade of dinosaurs to have survived the CretaceousTertiary extinction event approximately 65.5 Ma.
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