Avialae in the context of "Bird"

Theropoda (/θɪəˈrɒpədə/; from ancient Greek θηρίο- ποδός [θηρίον, (therion) "wild beast"; πούς, ποδός (pous, podos) "foot"]) is one of the three major clades of dinosaur, alongside Ornithischia and Sauropodomorpha. Theropods, both extant and extinct, are characterized by hollow bones and three toes and claws on each limb. They are generally classed as a group of saurischian dinosaurs, placing them closer to sauropodomorphs than to ornithischians. They were ancestrally carnivorous, although a number of theropod groups evolved to become herbivores and omnivores. Members of the subgroup Coelurosauria were most likely all covered with feathers, and it is possible that they were also present in other theropods. In the Jurassic, birds evolved from small specialized coelurosaurian theropods, and are currently represented by about 11,000 living species, making theropods the only group of dinosaurs alive today.

Theropods first appeared during the Carnian age of the Late Triassic period 231.4 million years ago (Ma) and included the majority of large terrestrial carnivores from the Early Jurassic until the end of the Cretaceous, about 66 Ma, including the largest terrestrial carnivorous animals ever, such as Tyrannosaurus and Giganotosaurus, though non-avian theropods exhibited considerable size diversity, with some non-avian theropods like scansoriopterygids being no bigger than small birds.

Archaeopteryx (/ˌɑːrkiːˈɒptərɪks/ ; lit. 'ancient wing'), sometimes referred to by its German name, "Urvogel " (lit. 'Primeval Bird') is a genus of bird-like dinosaurs. The genus name derives from the Ancient Greek ἀρχαῖος (archaîos), meaning 'ancient', and πτέρυξ (ptérux), meaning 'feather, wing'. Between the late 19th century and the early 21st century, Archaeopteryx was generally accepted by palaeontologists and popular reference books as the oldest known bird (member of the group Avialae). Older potential avialans have since been identified, including Anchiornis, Xiaotingia, Aurornis, and Baminornis.

Archaeopteryx lived in the Late Jurassic around 150 million years ago, in what is now southern Germany, during a time when Europe was an archipelago of islands in a shallow warm tropical sea, much closer to the equator than it is now. Similar in size to a Eurasian magpie, with the largest individuals possibly attaining the size of a raven, the largest species of Archaeopteryx could grow to about 50 cm (20 in) in length. Despite their small size, broad wings, and inferred ability to fly or glide, Archaeopteryx had more in common with other small Mesozoic dinosaurs than with modern birds. In particular, they shared the following features with the dromaeosaurids and troodontids: jaws with sharp teeth, three fingers with claws, a long bony tail, hyperextensible second toes ("killing claw"), feathers (which also suggest warm-bloodedness), and various features of the skeleton.

Maniraptora is a clade of coelurosaurian dinosaurs which includes birds and the non-avian dinosaurs that were more closely related to them than to Ornithomimus velox. It contains the major subgroups Avialae, Dromaeosauridae, Troodontidae, Oviraptorosauria, and Therizinosauria. Ornitholestes and the Alvarezsauroidea are also often included. Together with the next closest sister group, the Ornithomimosauria, Maniraptora comprises the more inclusive clade Maniraptoriformes. Maniraptorans first appear in the fossil record during the Jurassic Period (see Eshanosaurus), and survive today as living birds.

Paraves are a widespread group of theropod dinosaurs that originated in the Middle Jurassic period. In addition to the extinct dromaeosaurids, troodontids, anchiornithids, and possibly the scansoriopterygids, the group also contains the avialans, which include diverse extinct taxa as well as the over 11,000 species of living birds.

Basal members of Paraves are well known for the possession of an enlarged claw on the second digit of the foot, which was held off the ground when walking in some species. A number of differing scientific interpretations of the relationships between paravian taxa exist. New fossil discoveries and analyses make the classification of Paraves an active subject of research.