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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 Dinosaur 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
The genus name, Asteriornis, was constructed from ornis, the Greek language word for bird,. and from Asteria, a Greek gods who was associated with falling stars, and about whom there is a famous myth in which she transforms herself into a quail. The Asteri part of genus name thus alludes to the Chicxulub impactor (a "falling star"), and also alludes to quails which are members of the galloanserans. The species name A. maastrichtensis is named after the Maastricht Formation. The researchers who discovered and described the fossil gave Asteriornis the nickname "Wonderchicken", which was picked up by various news outlets.
The beak was slightly downcurved and lightweight. Unlike galloanserans, the beak did not have any specialized connections to the rest of the skull, nor a hooked tip. Instead its front tip was slightly rounded. The skull was narrowest over the orbits (eye sockets), where the were incised by a V-shaped part of the . The bones forming the jaw joint were very galloanseran-like. The quadrate bone (the cranium's contribution to the jaw joint) connected to the skull roof via two pronounced knobs, which were adjacent to a third smaller knob, the tuberculum subcapitulare. The mandible (lower jaw) connected to the quadrate with a pair of sockets, and the rear end of the lower jaw had a large hooked rearward-facing retroarticular process as well as a smaller inward-facing medial process. All of these characteristics are considered unique to (or at least most common in) galloanserans.
In some respects the skull seems more similar to galliform birds such as chickens and pheasants. These include unfused snout bones and nasal bones which fork in front of the eyes. In other respects it resembles anseriform birds such as ducks and geese. Such features include the hooked retroarticular process of the jaw and a postorbital process (the portion of bone forming the rear edge of the eye socket) which curves forward at its lower extent. These demonstrate a principle of evolution that animals close to the common ancestor of two groups share some similarities with each group.
The radius fragment flattens and widens towards the wrist, where it possesses a large hooked bump. Leg bones are elongated and slender, similar in proportions and structure to modern ground-living birds. The femur has well-developed muscle ridges and a large, angular medial condyle. The tibiotarsus is widest towards the knee, while the tarsometatarsus is thinner and covered with ridges.
Classifying Asteriornis as a relative of chickens and ducks means that it is unequivocally a Bird. This is important because Neornithes originated at the last common ancestor of all living birds, and corresponds to the term "bird" as it refers to modern-day animals. Pre-neornithean birds such as Ichthyornis, enantiornitheans, or Archaeopteryx generally resemble modern birds but retain primitive features such as teeth or wing claws. Neornithean fossils are extremely rare from the Mesozoic age, and are generally fragmentary or poorly described. One of the oldest known neognath, specifically a stem-anseriform, is the Presbyornithidae Teviornis from the Nemegt Formation of Mongolia around . The taxonomic identity of Vegavis from the Late Cretaceous (~69.2–68.4 Ma) of Antarctica was debated among paleontologists until a nearly complete skull was described in 2025, which confirmed its identity as a crown group anseriform. Asteriornis is based on diagnostic and well-preserved skull material and its status is less unstable, so it can be considered among the oldest known undisputed fossil of a modern-style neornithean bird.
At least two studies in 2021 and 2024 recovered Asteriornis as a paleognath, sister to Lithornithidae and Tinamou, albeit "with limited support". Other researchers still support Asterornis as a neornithean closely related to or within Galloanserae based on morphometric and phylogenetic analyses. In 2025, the phylogenetic analyses by Torres and colleagues recovered Asteriornis within Neognathae, a sister taxon of Palaeognathae, or as a sister taxon of Galliformes. In the same year, Crane and colleagues placed Asteriornis within the crown-group Galloanserae based on their phylogenetic analyses, specifically as a Pangalliformes, though reexaminations of its mandible suggested that the taxon lacked a key galloanserine feature.
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