The Fallen Avian Dynasty of the Cretaceous – Understanding Enantiornithes

Phylogenetic tree of ornithothoracines（圖片來源：Min Wang，採用 CC BY 4.0 授權）

During the Cretaceous period, bird evolution produced two major lineages. One was the Euornithes (Ornithuromorpha), the lineage from which all living birds ultimately descend. The other lineage was the Enantiornithes, a diverse group that disappeared completely at the end of the Cretaceous. The name “Enantiornithes” meaning “opposite birds” refers to the reversed articulation between the scapula and coracoid compared with modern birds: in living birds the coracoid bears a concave surface receiving a convex scapular articulation, whereas in enantiornithines the configuration is reversed.

Because many birds are small and possess hollow bones, their chances of fossilization are relatively low. Bird fossils are therefore most commonly discovered in exceptional preservation deposits known as Lagerstätten, where fossilization conditions are unusually favorable.

The group Enantiornithes was first formally recognized by paleontologist Cyril Walker, who studied unusual bird fossils from the Late Cretaceous Lecho Formation in northwestern Argentina. These fossils differed markedly from those of modern-type birds, leading Walker to describe a new species, Enantiornis leali, and subsequently define the broader clade Enantiornithes based on similar skeletal characteristics.

In addition to the distinctive scapula-coracoid articulation, enantiornithines typically possessed a relatively small sternal keel, teeth in the beak, and claws on the wings. Fossil evidence suggests that these birds may have used their forelimbs to help climb among branches. A comparable feature occurs in the chicks of the modern hoatzin, whose wing claws assist in climbing.

Over time, convergent evolution brought some enantiornithines to resemble more advanced birds. For instance, the genus Neuquenornis developed a keel similar to that of ornithuromorph birds, and Yuornis evolved a toothless beak—features that otherwise characterize birds more closely related to modern species.

Cuspirostrisornis houi（圖片來源：TinyLongwing，採用 CC BY-SA 4.0 授權）

During the Cretaceous, enantiornithines were at least as diverse as ornithuromorph birds and may even have been more abundant. Fossils of enantiornithines have been discovered on every continent except Antarctica. One of the most important sources of information about their diversity comes from the Jehol Biota of northeastern China, where numerous exquisitely preserved bird fossils have been uncovered.

These fossils reveal a remarkable range of species, from small birds comparable in size to sparrows, such as Longirostravis, to somewhat larger forms belonging to families such as Bohaiornithidae and Pengornithidae. Many of these birds appear to have been adapted to life in dense forests. Their foot structure—featuring a reversed first toe and long curved claws—was ideal for grasping branches, suggesting that most enantiornithines were arboreal.

Because forest environments provided shelter from predators, sexual selection may have favored species with striking plumage and ornamental tail feathers. Many enantiornithines likely exhibited sexual dimorphism, with males bearing especially elaborate feathers, including elongated decorative tail plumes similar to those seen in many modern tropical birds.

Nevertheless, enantiornithines were not limited to a single ecological lifestyle. Some species likely occupied niches comparable to modern ground birds, water birds, and birds of prey. In contrast, some researchers suggest that early ornithuromorph birds were more often associated with open or aquatic environments, where greater flight capability may have been advantageous.

Fossils of eggs, embryos, and juvenile individuals indicate that enantiornithines were highly precocial. Newly hatched chicks already possessed well-ossified skeletons and fully developed feathers, allowing them to fly and forage within a short time after hatching. This implies that juvenile enantiornithines depended relatively little on parental care.

However, despite their early independence, enantiornithines grew slowly. Bone microstructure indicates that they required several years to reach full adult size. This growth pattern contrasts strongly with that of modern birds, which typically grow rapidly and achieve adult size within weeks or months.

Falcatakely forsterae（圖片來源：Nix Illustration，採用 CC BY-NC 4.0 授權）

Throughout the Mesozoic, enantiornithines were among the most successful avian groups. Yet, at the end of the Cretaceous, they vanished together with all other non-avian dinosaurs. Only the ornithuromorph lineage survived and ultimately gave rise to modern birds.

The reasons for the extinction of enantiornithines remain uncertain, but several clues have emerged. Their slow growth and delayed reproductive maturity may have limited their ability to recover from catastrophic population declines. In addition, none of the numerous enantiornithine fossils preserves gastroliths, stones used by many birds to grind hard foods such as seeds. This absence suggests that enantiornithines may have specialized in softer foods and lacked the dietary flexibility that characterizes modern birds.

When global ecosystems collapsed at the end of the Cretaceous, these biological limitations may have prevented enantiornithines from adapting to rapidly changing environmental conditions. Meanwhile, the lineage leading to modern birds—capable of faster reproduction and broader diets—managed to survive.

Once vibrant inhabitants of Cretaceous forests, enantiornithines are now known only from their fossils, leaving scientists to reconstruct the lives of these extraordinary birds that once dominated ancient skies.

Author: Shui Ye-You

Reference:

 O'Connor, J. (2022). Enantiornithes. Current Biology.