Neornithes in the context of Oviparity

The evolution of birds began in the Jurassic Period, with the earliest birds derived from a clade of theropod dinosaurs named Paraves. Birds are categorized as a biological class, Aves. For more than a century, the small theropod dinosaur Archaeopteryx lithographica from the Late Jurassic period was considered to have been the earliest bird. Modern phylogenies place birds in the dinosaur clade Theropoda. According to the current consensus, Aves and a sister group, the order Crocodilia, together are the sole living members of an unranked reptile clade, the Archosauria. Four distinct lineages of bird survived the Cretaceous–Paleogene extinction event 66 million years ago, giving rise to ostriches and relatives (Palaeognathae), waterfowl (Anseriformes), ground-living fowl (Galliformes), and "modern birds" (Neoaves).

Phylogenetically, Aves is usually defined as all descendants of the most recent common ancestor of a specific modern bird species (such as the house sparrow, Passer domesticus), and either Archaeopteryx, or some prehistoric species closer to Neornithes (to avoid the problems caused by the unclear relationships of Archaeopteryx to other theropods). If the latter classification is used then the larger group is termed Avialae. Currently, the relationship between non-avian dinosaurs, Archaeopteryx, and modern birds is still under debate.

In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. It is thus a way of defining a clade, a group consisting of a species and all its extant or extinct descendants. For example, Neornithes (birds) can be defined as a crown group, which includes the most recent common ancestor of all modern birds, and all of its extant or extinct descendants.

The concept was developed by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms relative to their extinct relatives in his "Die Stammesgeschichte der Insekten",and the "crown" and "stem" group terminology was coined by R. P. S. Jefferies in 1979. Though formulated in the 1970s, the term was not commonly used until its reintroduction in 2000 by Graham Budd and Sören Jensen.

Asteriornis ("Asteria's bird") is an extinct genus of bird from the Late Cretaceous of Belgium which is known from a single species, Asteriornis maastrichtensis. It was a small, long-legged bird that lived near the coastline and co-existed with more "primitive" types of birds such as Ichthyornis. Asteriornis is one of the oldest-known birds irrefutably belonging to the group Neornithes, which encompasses all modern birds, and is possibly closely related to birds of the extant superorder Galloanserae such as chickens and ducks. It possesses characteristics of both galliformes (chicken-like birds) and anseriformes (duck-like birds), indicating its position as a close relative of the last common ancestor for both groups.

Asteriornis may shed light on why Neornithes were the only dinosaurs to survive the Cretaceous–Paleogene extinction event. Its coexistence with non-neornithean birds such as Ichthyornis implies that competition was not a primary factor for the extinction of non-neornitheans, which resembled modern birds in most respects but died out with other non-avian dinosaurs. Small size, a terrestrial lifestyle, and a generalist diet have all been inferred as ecological advantages possessed by early neornithes, allowing them to survive and diversify in the wake of the extinction. Asteriornis fulfills these qualities, suggesting that such suspicions were justified. Asteriornis is also evidence against a different hypothesis stating that modern birds originated from southern continents. This was supported by biogeographic ancestral reconstructions using phylogenies and the discovery of Vegavis (a possible neornithean from Antarctica), but Asteriornis's presence in Europe suggests that modern birds may have been widespread in northern continents in their early evolution.

Janavis (from the Roman god Janus and the Latin avis for bird) is an extinct toothed bird belonging to the Ichthyornithes from the Late Cretaceous of Belgium. The genus has one named species, Janavis finalidens (from Latin finalis, meaning ending or final, and dens, for tooth) that was discovered in the 1990s, reported in 2002, and described in 2022. Recovered almost simultaneously from the same area and age as Asteriornis maastrichtensis, then the oldest known modern bird, it provides information on the evolution and divergence of basal and modern birds, especially on the evolutionary modifications of bird skulls.