Asteriornis

Asteriornis, meaning "Asteria's bird," is an extinct genus of bird from the Late Cretaceous of Belgium, represented by a single species, *Asteriornis maastrichtensis*. This small, long-legged bird lived along the coastline alongside more primitive birds like *Ichthyornis*. It is one of the oldest birds definitively belonging to Neornithes, the group that includes all modern birds, and may be closely related to living galloanserans such as chickens and ducks. Its skull shows features of both galliformes (chicken-like birds) and anseriformes (duck-like birds), suggesting it was a close relative of the last common ancestor of these two groups.

Asteriornis may help explain why Neornithes were the only dinosaurs to survive the Cretaceous–Paleogene extinction event. Its coexistence with non-neornithean birds like *Ichthyornis* implies competition was not the main reason those other birds went extinct, even though they resembled modern birds in most ways. Small size, a terrestrial lifestyle, and a generalist diet are thought to have been ecological advantages for early neornithes, allowing them to survive and diversify after the extinction. Asteriornis fits this profile, supporting that idea. It also challenges the hypothesis that modern birds originated in southern continents—a view supported by the discovery of *Vegavis* in Antarctica and certain phylogenetic reconstructions. Asteriornis's presence in Europe suggests modern birds may have been widespread in northern continents early in their evolution.

**Discovery and naming** The holotype, specimen NHMM 2013 008, is held at the Maastricht Natural History Museum. It consists of a nearly complete skull, leg bone fragments, and a radius. The fossil came from four sediment blocks collected at the CBR-Romontbos quarry near Eben-Emael in the Maastricht Formation of Belgium. Amateur paleontologist Maarten van Dinther first unearthed it in 2000 and donated it to the museum. This formation gives its name to the Maastrichtian stage, the last stage of the Cretaceous and the Mesozoic era. The fossil dates to around 66.8 to 66.7 million years old, less than a million years before the asteroid impact that triggered the Cretaceous–Paleogene extinction, killing all non-avian dinosaurs and many other organisms.

The genus name *Asteriornis* combines *ornis*, Greek for bird, with Asteria, a Greek goddess associated with falling stars who famously transformed into a quail. The "Asteri" part thus references the Chicxulub impactor (a "falling star") and quails, which are galloanserans. The species name *maastrichtensis* refers to the Maastricht Formation. The researchers who described the fossil nicknamed it "Wonderchicken," a term picked up by news outlets.

**Description** The holotype is estimated to have reached skeletal maturity or close to it before death. Its beak was slightly downcurved and lightweight, lacking specialized connections to the skull or a hooked tip, instead ending in a slightly rounded front. The skull was narrowest over the eye sockets, where the frontal bones were cut by a V-shaped part of the nasal bones. The jaw joint bones were very galloanseran-like: the quadrate bone connected to the skull roof via two pronounced knobs next to a smaller knob (the tuberculum subcapitulare). The lower jaw connected to the quadrate with two sockets, and its rear end had a large, hooked, rearward-facing retroarticular process and a smaller inward-facing medial process. These features are considered unique to or most common in galloanserans, though a 2025 study by Crane et al. could not confirm the retroarticular process based on a re-examination of the mandible.

Some skull traits resemble galliform birds like chickens and pheasants, such as unfused snout bones and nasal bones that fork in front of the eyes. Others resemble anseriform birds like ducks and geese, including a postorbital process that curves forward at its lower end. This mix illustrates how animals close to a common ancestor share similarities with both descendant groups.

The radius fragment flattens and widens toward the wrist, with a large hooked bump. The leg bones are elongated and slender, similar to modern ground-living birds. The femur has well-developed muscle ridges and a large, angular medial condyle. The tibiotarsus is widest near the knee, while the tarsometatarsus is thinner and covered with ridges.

**Classification** Phylogenetic analysis places Asteriornis near the base of Galloanserae, the superorder containing chickens, ducks, pheasants, and other fowl and gamebirds. Its exact position depends on the method used: parsimony analysis places it as the sister taxon to Galloanserae, meaning it was a distant relative of the last common ancestor of chickens and ducks, while Bayesian analysis places it within Galloanserae itself.