Dinosaurs in the context of "Cenozoic"

Birds are a group of warm-blooded theropod dinosaurs constituting the class Aves, characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton. Birds live worldwide and range in size from the 5.5 cm (2.2 in) bee hummingbird to the 2.8 m (9 ft 2 in) common ostrich. There are over 11,000 living species and they are split into 44 orders. More than half are passerine or "perching" birds. Birds have wings whose development varies according to species; the only known groups without wings are the extinct moa and elephant birds. Wings, which are modified forelimbs, gave birds the ability to fly, although further evolution has led to the loss of flight in some birds, including ratites, penguins, and diverse endemic island species. The digestive and respiratory systems of birds are also uniquely adapted for flight. Some bird species of aquatic environments, particularly seabirds and some waterbirds, have further evolved for swimming. The study of birds is called ornithology.

Birds evolved from earlier theropods, and thus constitute the only known living dinosaurs. 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. According to some estimates, modern birds (Neornithes) evolved in the Late Cretaceous or between the Early and Late Cretaceous (100 Ma) and diversified dramatically around the time of the Cretaceous–Paleogene extinction event 66 million years ago, which killed off the pterosaurs and all non-ornithuran dinosaurs.

Avialae ("bird wings") is a clade containing the only living dinosaurs, the birds, and their closest relatives. It is usually defined as all theropod dinosaurs more closely related to birds (Aves) than to deinonychosaurs, though alternative definitions are occasionally used (see below).

Archaeopteryx lithographica, from the late Jurassic Period Solnhofen Formation of Germany, is usually considered the earliest known avialan which may have had the capability of powered flight; a minority of studies have suggested that it might have been a deinonychosaur instead. Several older (but non flight-capable) possible avialans are known from the late Jurassic Tiaojishan Formation of China, dated to about 160 million years ago.

The Cenozoic Era (also known as the Caenozoic, Kainozoic, or Neozoic Era; /ˌsiːnəˈzoʊ.ɪk, ˌsɛn-/; SEE-nə-ZOH-ik, SEN-ə-; lit. 'new life') is Earth's current geological era, representing the last 66 million years of Earth's history. It is characterized by the dominance of mammals, insects, birds and angiosperms (flowering plants). It is the latest of three geological eras of the Phanerozoic Eon, preceded by the Mesozoic and Paleozoic. The Cenozoic started with the Cretaceous–Paleogene extinction event, when many species, including the non-avian dinosaurs, became extinct in an event attributed by most experts to the impact of a large asteroid or other celestial body, the Chicxulub impactor.

The Cenozoic is commonly known as the Age of Mammals because the terrestrial animals that dominated both hemispheres were mammals – the eutherians (placentals) in the Northern Hemisphere and the metatherians (marsupials, now mainly restricted to Australia and to some extent South America) in the Southern Hemisphere. The extinction of many groups allowed mammals and birds to greatly diversify so that large mammals and birds dominated life on Earth. The continents also moved into their current positions during this era.

A coracoid is a paired bone which is part of the shoulder assembly in all vertebrates except therian mammals (marsupials and placentals). In therian mammals (including humans), a coracoid process is present as part of the scapula, but this is not homologous with the coracoid bone of most other vertebrates.

In other tetrapods, it joins the scapula to the front end of the sternum and has a notch on the dorsal surface which, along with a similar notch on the ventral surface of the scapula, forms the socket in which the proximal end of the humerus (upper arm bone) is located. The acrocoracoid process is an expansion adjacent to this contact surface, to which the shoulderward end of the biceps brachii muscle attaches in these animals. In birds (and generally theropods and related animals), the entire unit is rigid and called scapulocoracoid. This plays a major role in bird flight. In other dinosaurs, the main bones of the pectoral girdle were the scapula (shoulder blade) and the coracoid, both of which directly articulated with the clavicle.

Pseudosuchia (from Ancient Greek ψεύδος (pseúdos), meaning "false", and Σοῦχος (Soûkhos), meaning "Sobek") is one of two major divisions of Archosauria, including living crocodilians and all archosaurs more closely related to crocodilians than to birds. Pseudosuchians are also informally known as "crocodilian-line archosaurs", in contrast to the "bird-line archosaurs" or Avemetatarsalia. Despite Pseudosuchia meaning "false crocodiles", the name is a misnomer as true crocodilians are now defined as a subset of the group.

The clade Pseudosuchia is potentially equivalent to another term, Crurotarsi, even though the latter has a different, node-based definition: "all taxa the least inclusive clade containing Rutiodon carolinensis (Emmons, 1856), and Crocodylus niloticus (Laurenti, 1768)." Many paleontologists of the late 20th century took this proposal for granted, using Crurotarsi as the term for crocodilian ancestors. In 2011, a major revision of Triassic archosaur relations proposed that Rutiodon's group, Phytosauria, was not as closely related to other traditional "crurotarsans", at least compared to avemetatarsalians such as pterosaurs and dinosaurs. Under that interpretation, Crurotarsi would be a much broader clade than Pseudosuchia. Other recent studies continue to support a more traditional phylogeny with phytosaurs as an early branch of Pseudosuchia. If the traditional interpretation is maintained, Pseudosuchia and Crurotarsi are roughly equivalent categories.

Saurischia (/sɔːˈrɪskiə/ saw-RIS-kee-ə, meaning "reptile-hipped" from the Greek sauros (σαῦρος) meaning 'lizard' and ischion (ἴσχιον) meaning 'hip joint') is one of the two basic divisions of dinosaurs (the other being Ornithischia), classified by their hip structure. Saurischia and Ornithischia were originally called orders by Harry Seeley in 1888, though today most paleontologists classify Saurischia as an unranked clade rather than an order.