Ornithuromorpha

Ornithuromorpha is a clade of birds within Avialae that includes all descendants of the most recent common ancestor of modern birds (Aves) and their close extinct relatives, excluding the Enantiornithes. In this sense, it represents the lineage leading to the crown group Aves, or Neornithes, and all of its extinct members. The name reflects a focus on “bird-form” lineage within the broader early bird radiation.

The fossil record of Ornithuromorpha spans the Late Jurassic to the Cretaceous, with many well-known examples appearing in Early Cretaceous faunas such as the Jehol Biota. Members range from early, sometimes toothed forms to later, beaked birds that show increasingly specialized flight and skeletal adaptations. Some ornithuromorphs, including groups such as Ichthyornithiformes and Hesperornithiformes in various analyses, are discussed as having close links to modern birds, while other work treats them as more basal relatives within the clade.

Key morphological trends within Ornithuromorpha include a shift toward a more derived, rigid skeletal structure, adaptations for flight in many lineages, and, in many descendants, the reduction or loss of teeth in favor of beaks. The sternum often becomes more robust with a pronounced keel in flight-capable members, and the wrist, hand, and pelvis show reorganizations that support diverse ecological niches.

Systematics within Ornithuromorpha are subject to ongoing study. Importantly, modern birds (Neornithes) are nested within this group, making Ornithuromorpha a crucial framework for understanding the origin and evolution of all living birds, as well as many extinct relatives.