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Ornithurae
Ornithurae
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Ornithurans
Temporal range:
Early Cretaceous - Present, 130–0 Ma
Cast skeleton of Ichthyornis dispar, Rocky Mountain Dinosaur Resource Center
Australian brushturkey (Alectura lathami)
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Dinosauria
Clade: Saurischia
Clade: Theropoda
Clade: Avialae
Clade: Ornithuromorpha
Clade: Ornithurae
Haeckel, 1866
Subgroups

Ornithurae (meaning "bird tails" in Greek) is a natural group that includes modern birds and their very close relatives such as the ichthyornithines and the hesperornithines. This clade is defined in the PhyloCode by Juan Benito and colleagues in 2022 as "the smallest clade containing Ichthyornis dispar, Hesperornis regalis, and Vultur gryphus".[2]

Classification

[edit]

Ernst Haeckel coined the name in 1866 and included in the group all "true birds" with the "characteristic tail morphology of all extant birds" (translation by Jacques Gauthier). This distinguishes the group from Archaeopteryx, which Haeckel placed in another new group called Sauriurae. Said simply, modern birds have short, fused pygostyle tails, while Archaeopteryx retained the long tail characteristic of non-avian theropod dinosaurs.[3]

Gauthier converted Ornithurae into a clade, giving it a branch-based definition: "extant birds and all other taxa, such as Ichthyornis and Hesperornithes, that are closer to extant birds than is Archaeopteryx". Later he and de Queiroz redefined it as an apomorphy-based clade more in keeping with Haeckel's original usage, including the first pan-avian with a "bird tail" homologous with that of Vultur gryphus, and all of its descendants.[4] They defined "bird tail" as a tail that is shorter than the femur, with a pygostyle that is a ploughshare-shaped, compressed element, with the bones fused in the adult, composed of less than six caudal vertebrae, and shorter than the free part of the tail, which itself is composed of less than eight caudal vertebrae. They included Aves (which they defined as the "crown group" of modern birds), Ichthyornis, Hesperornithes, and Apsaravis in Ornithurae.

Neornithes was originally proposed as a replacement for Ornithurae by Gadow in 1892 and 1893. Gauthier and de Queiroz, therefore, consider Neornithes a junior synonym of Ornithurae,[4] though many other scientists use Neornithes to refer to the much more restrictive crown group consisting only of modern birds (a group for which Gauthier uses the name Aves). Alternately, some researchers have used Ornithurae to refer to a more restrictive node-based clade, anchored on Hesperornis and modern birds.[5]

Relationships

[edit]

The cladogram below is the result of a 2017 analysis by McLachlan and colleagues.[6]

Ornithurae

†Hesperornithes

†Ichthyornithes

†Ichthyornis dispar

†Ornithurine D

†Iaceornis marshi

†Vegaviidae

†Cimolopteryx minima

†Cimolopteryx petra

†Cimolopteryx rara

†Ornithurine A

†Ornithurine E

†Ceramornis major

†Cimolopteryx maxima

†Ornithurine B

†Ornithurine C

†Ornithurine F

Aves

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Ornithurae

View on Grokipedia
from Grokipedia
Ornithurae is a clade of avialan dinosaurs within Aves that encompasses the crown group of modern birds (Neornithes) and their closest stem-group relatives, defined phylogenetically as the last common ancestor of taxa such as Ichthyornis and Aves and all of its descendants. This group is distinguished from more basal avialans like Enantiornithes by key synapomorphies, including a short tail terminating in a pygostyle, heterocoelous cervical vertebrae, a keeled sternum with a midline pneumatic foramen, and advanced forelimb features such as a large bicipital tubercle on the ulna. Originating in the Early Cretaceous around 130 million years ago, as evidenced by fossils like Archaeornithura meemannae from China's Huajiying Formation, Ornithurae represents a critical evolutionary lineage that survived the end-Cretaceous extinction event, giving rise to the diverse radiation of extant birds. Notable Mesozoic members include flight-capable taxa such as Ichthyornis dispar and Hesperornis regalis from the Late Cretaceous of North America, as well as a rich assemblage of Early Cretaceous ornithurines from the Jehol Biota in China, including Yixianornis grabaui, Yanornis martini, and Gansus yumenensis, which exhibit early innovations in rostral morphology and flight adaptations. These fossils illuminate the transition from toothed, long-tailed basal birds to the edentulous, short-tailed forms characteristic of modern avian diversity, highlighting Ornithurae's role in the stepwise evolution of powered flight and ecological specialization among early birds.

Definition and Etymology

Definition

Ornithurae is a within the larger group that encompasses all extant birds, collectively known as Neornithes or crown-group Aves, along with their closest extinct relatives, but excludes the diverse lineage . Ornithurae is phylogenetically defined as the most inclusive containing modern birds (Neornithes) but not , or equivalently, the stemming from the last common ancestor of and Aves and all of its descendants. This natural group represents the lineage leading to modern avian diversity, characterized by shared derived features that distinguish it from more basal avialans. The scope of Ornithurae includes several key subgroups, such as (often referred to as "true birds" due to their advanced flight adaptations and modern-like skeletal structure), as well as the extinct orders Ichthyornithiformes (seagull-like flying birds) and Hesperornithiformes ( with reduced wings). These subgroups highlight the ecological breadth of Ornithurae, ranging from aerial to aquatic specialists during the era. Diagnostic apomorphies of Ornithurae include skeletal modifications that support powered flight and efficient locomotion, such as heterocoelous , a keeled with a midline pneumatic , and advanced features including a large bicipital on the . Additional traits include a long nasal process of the approaching the frontal, presence of a procoracoid process on the , and proximal plantar displacement of metatarsal III relative to metatarsals II and IV. These traits collectively define the clade's and underscore its evolutionary progression toward the avian body plan. The term Ornithurae was originally coined by in 1866 to denote "true birds" distinguished by modern skeletal attributes, particularly the reduced tail with a fused , separating them from archaic forms like . This early conceptualization laid the foundation for recognizing Ornithurae as a distinct group, later formalized in cladistic frameworks as a stem-based comprising all avialans more closely related to Neornithes than to . Subsequent phylogenetic analyses have refined its boundaries, confirming its position as the sister group to within Ornithothoraces while emphasizing its role as the stem to all living birds.

Etymology

The term Ornithurae was coined by in 1866 in his Generelle Morphologie der Organismen, derived from ornis (ὄρνις, "bird") and oura (οὐρά, "tail"), literally meaning "bird tails." This nomenclature highlights the characteristic short, fused pygostyle at the tail's end, which differentiates members of the group from those with longer, unfused tails. Haeckel originally applied Ornithurae to encompass all "true s" exhibiting modern morphological features, such as those seen in extant . In 1986, Jacques Gauthier redefined the term within a cladistic framework, establishing it as a monophyletic supported by shared derived traits, including modifications to the caudal vertebrae forming the . The prefix "ornith-" is a standard element in avian taxonomy, appearing in names like Ornithischia (bird-hipped dinosaurs) and Ornithopoda (bird-footed dinosaurs), denoting bird-like attributes. The suffix "-urae" specifically pertains to tail structure, underscoring the morphological focus of Haeckel's coinage.

Evolutionary History

Origins in the Early Cretaceous

The Ornithurae, a clade encompassing modern birds and their closest Mesozoic relatives, first appeared in the fossil record during the Early Cretaceous, with the earliest definitive evidence dating to approximately 131 million years ago in the Hauterivian-Barremian stages. This temporal range spans the Hauterivian to Aptian stages (roughly 131–113 Ma), marking the initial radiation of ornithurines from more basal avialan ancestors. These early forms are primarily known from lacustrine deposits that exceptionally preserved skeletal and soft tissue details, providing insights into the clade's basal morphology, including a reduced tail culminating in a pygostyle—a fused structure of the last few caudal vertebrae that supported tail feathers for flight control. Key early fossils of Ornithurae have been recovered from the Jehol Biota in northeastern China, particularly from the Huajiying, Yixian, and Jiufotang formations. One of the earliest known members is Archaeornithura meemannae from the Huajiying Formation (~131 Ma), followed by Gansus yumenensis, represented by well-preserved specimens from the Xiagou Formation (Aptian, ~109 Ma) in the Changma Basin, Gansu Province, exhibiting advanced ornithurine features such as a keeled sternum and heterodactyl feet adapted for perching. Similarly, Schizooura lii, from the Jiufotang Formation (Aptian, ~120 Ma) in Liaoning Province, is a basal ornithurine notable for its toothless beak and elongated hindlimbs, indicating early adaptations for terrestrial and possibly arboreal locomotion. Another significant specimen, Archaeorhynchus spathula, from the Yixian Formation (Barremian–Aptian), represents one of the earliest beaked ornithurines, with a spatulate rostrum suggesting a diet focused on soft prey. These fossils highlight the rapid evolution of cranial and postcranial traits distinguishing Ornithurae from contemporaneous enantiornithines. The origins of Ornithurae are closely tied to the environmental conditions of the , a diverse ecosystem characterized by volcanic activity, , and extensive lake systems in what is now western and surrounding regions of . These lacustrine settings facilitated the exceptional preservation of feathered specimens, revealing integumentary structures akin to those in modern birds. The clade likely emerged in amid a broader diversification of avialans, transitioning from Archaeopteryx-like ancestors with long bony tails to more derived forms with shortened tails and enhanced flight capabilities. Evolutionary drivers included niche specialization in insectivorous and piscivorous feeding strategies, enabled by refinements in structure and limb proportions that improved efficiency in forested and aquatic habitats. This initial radiation laid the groundwork for Ornithurae's ecological success, though much of the group's early diversity remains underrepresented due to the localized nature of Jehol deposits.

Diversification During the Cretaceous

During the mid-, particularly in the and stages (approximately 100–90 million years ago), Ornithurae underwent significant proliferation, marking an early phase of . Fossils from this period indicate a broadening global distribution, with records in , such as a cf. specimen from the of central , and in , including an -like distal tibiotarsus from the middle of Saratov Province, . Additional evidence comes from high-latitude sites, like the (~92 Ma) and of Tingmiatornis arctica from in the Canadian , suggesting ornithurines occupied diverse environments from subtropical marine settings to polar freshwater systems during a time of greenhouse climates. This expansion reflects the clade's ability to exploit varied aquatic and coastal niches as it radiated alongside other avian groups like , though ornithurines exhibited more derived perching feet and enhanced flight capabilities akin to modern birds. Ecological diversity within Ornithurae increased notably during this interval, with the development of specialized forms adapted to specific strategies. Piscivorous taxa like , a flying seabird-like ornithurine, demonstrated advanced aerial locomotion, enabling sustained flapping flight and aerial similar to modern terns, likely targeting and possibly over marine waters. In contrast, members of the Hesperornithiformes, such as , evolved as foot-propelled diving swimmers, with robust tarsometatarsi and reduced wings suited for underwater propulsion in pursuit of and aquatic prey, representing one of the earliest avian radiations into fully aquatic lifestyles. Early evidence also points to proto-shorebird and aerial morphologies among basal ornithurines, inferred from limb proportions and skeletal robusticity in Cenomanian-Turonian specimens, allowing exploitation of intertidal zones and airborne . The rise of angiosperms during the may have indirectly influenced ornithurine diets by expanding populations and resources, though direct evidence remains sparse for these early forms. Key fossil hotspots, such as the Smoky Hill Chalk Member of the in (approximately 84 Ma, late Coniacian-early ), have preserved multiple ichthyornithiform ornithurines, including well-articulated skeletons of and Apatornis, highlighting the clade's abundance in Western Interior Seaway marine deposits. These assemblages, alongside isolated elements from Eurasian sites, underscore the mid-Cretaceous as a pivotal era for ornithurine ecological expansion, setting the stage for their dominance post-K-Pg boundary.

Survival Through the K-Pg Extinction

The Cretaceous–Paleogene (K–Pg) extinction event, dated to approximately 66 million years ago and primarily triggered by the Chicxulub asteroid impact off the , eradicated non-avian dinosaurs and the vast majority of contemporaneous avian groups, including nearly all enantiornithines. Ornithurae, however, persisted through this crisis, representing one of the few avian clades to cross the boundary intact, with fossil evidence indicating that few pre-extinction avian lineages, primarily basal Ornithurae, survived into the . This selective survival marked a pivotal transition, as the event cleared ecological space previously occupied by larger reptiles and archaic birds. Key attributes of likely facilitated their amid the global devastation, which included widespread wildfires, a prolonged "," and disrupted food webs. Their predominantly small body sizes minimized caloric demands in a resource-scarce environment, while elevated metabolic rates supported quick recovery and population rebound. Dietary versatility, encompassing omnivory and specialized seed-eating, proved advantageous, as seeds offered a durable, post-impact food source resilient to environmental upheaval. Potential behavioral adaptations, such as burrowing or nocturnal , may have shielded them from immediate threats like heat and fallout, and the —a fused structure—enhanced flight efficiency for evasion, migration, and accessing scattered resources. The extensive diversification of Ornithurae during the likely served as a precursor, fostering the ecological flexibility that bolstered resilience. In the immediate aftermath during the , surviving ornithurines underwent rapid diversification, evolving into the crown group Neornithes and radiating to exploit vacant niches. Basal forms such as , discovered in deposits dated to approximately 69–68 million years ago (as of 2025), exemplify this early transition, featuring near-modern ankle bones and pelvic structures indicative of advanced aquatic or semi-aquatic adaptations within the . This swift evolutionary burst capitalized on the collapse of and enantiornithine-dominated aerial ecosystems. The long-term consequences of ornithurine survival reshaped vertebrate evolution, propelling birds to dominance in skies and terrestrial habitats. By filling roles vacated by extinct flying reptiles and early avian competitors, Neornithes diversified into thousands of , establishing modern avian ecological supremacy that persists today. This post-K-Pg trajectory underscores how a narrow escape enabled profound .

Anatomy and Morphology

Skeletal Characteristics

Ornithurae is characterized by several distinctive osteological features that distinguish it from more basal stem-group avialans, such as enantiornithines and anchiornithines, reflecting adaptations toward modern avian morphology. These include modifications in the cranium for enhanced mobility, a shortened with a fused terminal element, robust pectoral elements supporting flight, and specialized pelvic and structures facilitating diverse locomotion. These traits evolved progressively within the , with basal members like retaining primitive features such as , while derived forms approach the neornithine condition. The cranium of ornithuran taxa exhibits a kinetic skull, enabled by a flexible quadrate bone that articulates with the braincase via a double ball-and-socket joint, allowing independent movement of the upper jaw relative to the lower jaw and neurocranium. This kinesis is facilitated by an open frontoparietal suture and prokinetic mechanisms, where the premaxilla contacts the frontal anterior to the orbit. Basal ornithurans, such as Ichthyornis, retain teeth in the premaxilla, maxilla, and dentary, with conical, unserrated teeth suited for grasping prey; these are lost in the crown-group Neornithes, resulting in an edentulous rostrum. The quadrate features a single large pneumatic foramen on its anteromedial surface near the pterygoid articulation, enhancing lightness and airflow. In the , ornithurans display a reduced tail, with fewer than 10 free caudal vertebrae—typically five to eight—contrasting with the 20 or more elongated caudals in outgroups like . The terminal five to six caudal vertebrae fuse to form a , a plowshare-shaped, mediolaterally compressed element that supports tail feather rectrices and musculature, shorter in length than four free caudals. are heterocoelous anteriorly, with elongate mid-series elements (fifth through eighth) promoting neck flexibility; thoracic are amphicoelous with large lateral pneumatic fenestrae. The pectoral girdle forms a robust scapulocoracoid complex, where the and , though unfused in basal forms like , articulate via a deep subcircular cotyla on the , creating a stable yet flexible for movement. The bears a procoracoid process and a large supracoracoideus for passage, while the has a short, pointed and elongate blade. In derived ornithurans, the develops a prominent for attachment of flight muscles such as the pectoralis, along with a midline pneumatic , crossed coracoidal sulci, and five costal processes; early members show raised intermuscular ridges parallel to the . Pelvic and hindlimb osteology in Ornithurae features anterior heterocoelous for enhanced neck flexibility and amphicoelous , differing from the more rigid amphicoelous or opisthocoelous conditions in outgroups. The pubis is retroverted, slender, and rod-like, curving ventrally without a pectineal process, and often forms an elongate with the while remaining unfused distally. The foot is anisodactyl, with digits II–IV directed anteriorly and the hallux (digit I) reversed posteriorly for perching; the is fully fused and mediolaterally compressed, with metatarsal III proximally displaced plantarly relative to II and IV, and pedal phalanges featuring deep flexor pits and unguals shorter than their penultimate phalanges. Recent analyses of multiple specimens confirm at least 11 and extensive vertebral pneumatization, supporting these adaptations.

Flight and Locomotion Adaptations

Ornithurae exhibited advanced structures that enhanced aerodynamic efficiency, particularly in flight-capable members like those within . Vaned featured interlocking barbules along the barbs, forming cohesive, flat vanes that provided structural integrity and resistance to air flow during wingbeats. In , these feathers developed asymmetric vanes, with the leading edge narrower than the trailing edge, generating lift and reducing drag essential for powered flight. This , observed in basal forms such as Yanornis, marked a key innovation for sustained aerial locomotion beyond the more symmetric feathers of earlier avialans. Wing morphology in Ornithurae included specialized features for maneuverability and stability. The , a cluster of feathers on the thumb-like digit, protruded during low-speed flight to create a leading-edge vortex, preventing separation and stall at high angles of attack. This adaptation, preserved in early ornithuromorphs like Archaeornithura, allowed precise control during . Later Ornithurae developed elongated primary feathers, facilitating soaring by increasing wing and lift-to-drag efficiency in species adapted to open environments. Locomotion in Ornithurae displayed remarkable diversity, reflecting ecological specialization. Aerial forms like demonstrated powered flight capabilities comparable to modern terns, with strong evidence for flapping and soaring based on sternal morphology and skeletal proportions that supported high maneuverability over water. In contrast, aquatic specialists such as relied on underwater propulsion via lobed feet, similar to grebes, where the feet generated thrust through paddling motions while reduced wings minimized drag. This foot-based , enabled by robust hindlimbs, highlighted the clade's versatility beyond aerial dominance. Muscular and physiological adaptations underpinned these locomotor modes, with the pectoralis muscle enlarged to power the wing downstroke, providing the primary force for lift and in flying taxa. In basal avialans leading to Ornithurae, this muscle's development supported a transition toward sustained , contrasting with gliding-dominated locomotion in more primitive forms, though skeletal supports like the aided tail stabilization in both.

Classification and Phylogeny

Position Within Avialae

Ornithurae represents an unranked clade within Ornithothoraces, where it serves as the to , collectively forming a major subgroup of that excludes more basal paravians. This positioning situates Ornithurae as part of the advanced avialan radiation, distinct from earlier diverging forms like , with Ornithurae encompassing lineages that share derived traits such as a pygostyle-fused . The clade stems from anchiornithine paravians, basal members of that include taxa like as outgroups, marking the transition from non-ornithothoracine avialans to more derived bird-like forms. Ornithurae is nested within , the broader group characterized by a short, fused tail, as a derived . Ornithurae was first formalized in modern by (1986), who defined it as a stem-based comprising all taxa more closely related to extant birds (Neornithes) than to , effectively placing it crownward of in the avian tree. Subsequent morphological analyses have consistently supported its , with recent phylogenetic studies reinforcing this through comprehensive character matrices of Mesozoic avialans. Spanning from approximately 125 million years ago in the to the present, Ornithurae includes the radiation leading to roughly 10,000 extant species, all belonging to the crown-group Neornithes.

Key Relationships

Within Ornithothoraces, Ornithurae forms the to , with their basal divergence estimated at approximately 130 million years ago during the . This split marks a key bifurcation in early avian evolution, separating the predominantly lineage from the Ornithurae stem leading to modern birds. The internal phylogeny of Ornithurae reveals a structured , with representing the crownward that includes Neornithes and several extinct branches adapted to diverse ecologies. Ichthyornithiformes occupies a basal position as the sister taxon to , characterized by transitional features such as amphicoelous and a pneumatic . Hesperornithiformes, in turn, nests within as a specialized, flightless diving group more closely related to Neornithes than to Ichthyornithiformes. Phylogenetic analyses consistently recover a consensus topology for Ornithurae as Ichthyornithiformes + (Hesperornithiformes + Neornithes), supported by synapomorphies including crossed sulci and heterocoelous vertebrae in more derived members. This arrangement underscores the stepwise acquisition of neornithine traits, from the aerial piscivory of Ichthyornis-like forms to the fully aquatic adaptations of and the aerial diversification of crown birds. Debates persist regarding the precise placement of certain Late Cretaceous taxa, such as Vegavis iaai, which some analyses position as a basal neornithine within while others suggest a more stemward affinity near the base of Neornithes. Recent integrated morphological and molecular studies from the , including large-scale genomic phylogenies of neoavians, reinforce the of by aligning fossil-calibrated timelines with divergence estimates for crown bird clades.

Included Taxa

Ornithurae comprises a diverse array of clades, encompassing both extinct stem-group representatives and the lineage leading to modern birds. The major subgroups include the primarily flying Ichthyornithiformes, the flightless diving Hesperornithiformes, and the more advanced Euornithes, which gave rise to all post-Cretaceous avian diversity. These clades highlight the ecological breadth of Ornithurae during the Cretaceous, from aerial and piscivorous forms to fully aquatic specialists. The Ichthyornithiformes, known from Late Cretaceous marine deposits primarily in North America, represent early ornithurines adapted for flight and fish-eating. Key genera include Ichthyornis, with the type species I. dispar from Cenomanian to Campanian strata, characterized by toothed jaws and a keeled sternum; Apatornis celer, distinguished by fewer sacral vertebrae; Iaceornis marshi, featuring pneumatic foramina on the sternum; and Guildavis tener, a smaller taxon with similar skeletal traits. These taxa, totaling around five recognized genera, exhibit morphologies bridging enantiornithines and more derived birds, with fossils often preserving articulated skeletons that reveal growth patterns distinct from crown-group avians. Hesperornithiformes, a of foot-propelled restricted to Campanian-Maastrichtian marine environments across the , includes at least eleven genera, all extinct by the end-Cretaceous. Prominent examples are Hesperornis regalis, a large up to 2 meters long from chalks; Parahesperornis alexandrae, smaller and more gracile; Baptornis advenus, known from the ; and Asian forms like Chupkaornis kera, the oldest hesperornithiform at approximately 92 million years old. These birds lacked flight capabilities in adults but retained teeth and robust hindlimbs for underwater propulsion, with over 20 species described across the group. Euornithes forms the largest subclade within Ornithurae, including basal ornithurines and the crown group Neornithes, with early representatives appearing in the . basal euornithines from the in , such as Gansus yumenensis, Yanornis martini, and Yixianornis grabaui, represent transitional forms with mixed primitive and derived traits. Basal euornithines, such as Gansus yumenensis from Barremian Xiagou Formation deposits in , exhibit a transitional mix of primitive and modern traits like a fully fused and carpometacarpus. Other early forms include Limenavis patagonica from Campanian-Maastrichtian strata , known from partial elements suggesting aerial capabilities near the avian crown. The crown group Neornithes encompasses all extant birds, comprising approximately 250 families across diverse orders like and , with transitional fossils such as Asteriornis maastrichtensis from (~67 Ma), a basal galliform-like bird providing evidence of early neornithine diversification in . Overall, extinct Ornithurae diversity is represented by dozens of known genera, primarily from marine, freshwater, and coastal environments, showcasing a range of ecologies from piscivory and diving to aerial and possibly terrestrial adaptations, with definitive terrestrial herbivory emerging in neornithines.

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