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Neognathae

Neognathae is a major clade of birds within the subclass Neornithes, encompassing approximately 11,000 extant species that represent nearly all modern birds except for the paleognaths, such as ostriches, emus, kiwis, and tinamous.^[1] This group is distinguished primarily by its neognathous palate, characterized by unfused palate bones, a reduced or absent vomer, and generally kinetic skulls that enable greater cranial flexibility compared to the more rigid structures in paleognaths.^[2] Neognathae originated during the Late Cretaceous period and underwent rapid diversification following the Cretaceous-Paleogene (K-Pg) mass extinction event approximately 66 million years ago, evolving into a highly diverse array of flying and secondarily flightless forms adapted to diverse ecological niches worldwide.^[3]^[4] Taxonomically, Neognathae is one of two primary infraclasses of Neornithes, the other being Palaeognathae, with the division based originally on differences in bony palate structure and jaw morphology.^[5] Within Neognathae, the clade is further subdivided into two main lineages: Galloanserae, which includes the orders Galliformes (e.g., chickens, pheasants) and Anseriformes (e.g., ducks, geese), and the larger Neoaves, which comprises the remaining orders, such as Passeriformes (perching birds, the most species-rich order with over 6,000 species), Columbiformes (pigeons), and Charadriiformes (shorebirds).^[1] This phylogenetic structure has been robustly supported by molecular and genomic studies, revealing deep evolutionary splits within Neognathae that predate the K-Pg boundary in some lineages.^[3] Evolutionarily, Neognathae represents the dominant radiation of crown-group birds, with fossil evidence indicating that early members coexisted with extinct Mesozoic avian lineages like enantiornithines before becoming the sole survivors post-extinction.^[5] Key innovations in Neognathae include enhanced flight capabilities in most taxa, advanced cranial kinesis for improved feeding efficiency, and fused metacarpals with an elongate third finger, adaptations that facilitated their ecological success across terrestrial, aquatic, and aerial environments.^[2] Today, Neognathae accounts for over 95% of all living bird diversity, underscoring its pivotal role in avian evolution and global biodiversity.^[6]

Overview

Definition and etymology

Neognathae is a monophyletic clade within the subclass Neornithes, serving as the sister group to Palaeognathae and including nearly all extant bird species.^[7] This diverse group encompasses approximately 11,000 living species as of 2025, representing over 99% of all modern birds and spanning a wide array of ecological niches from aquatic to terrestrial habitats.^[1] The term "Neognathae" originates from the Greek prefix "neo-" (νέος), meaning "new" or "modern," combined with "gnathae," derived from "gnathos" (γνάθος), meaning "jaw," in reference to the distinctive neognathous palate.^[8] This palatal structure features a flexible upper jaw, enabling greater cranial kinesis, and a typically reduced or rod-like vomer bone, contrasting with the more rigid configuration in palaeognaths.^[2] The clade was formally established by Thomas Henry Huxley in 1867, who introduced the division based on comparative anatomy of the avian palate to differentiate "modern" birds from those with more primitive jaw features.^[9] Huxley's classification highlighted the taxonomic significance of cranial bones, laying foundational insights into avian systematics that persist in contemporary phylogenetic frameworks.^[10]

Scope and membership

Neognathae comprises the vast majority of extant bird species, including all modern birds except those belonging to the sister clade Palaeognathae, which encompasses the flightless ratites—such as ostriches (Struthio), emus (Dromaius), cassowaries (Casuarius), rheas (Rhea), and kiwis (Apteryx)—along with the flight-capable tinamous (Tinamidae).^[8] This clade is phylogenetically defined as the minimum clade containing Gallus gallus (domestic chicken) and Passer domesticus (house sparrow), explicitly excluding Palaeognathae.^[8] Palaeognathae are excluded from Neognathae due to their retention of plesiomorphic (primitive) traits shared with stem-group avians, such as aspects of cranial morphology and reduced flight adaptations in ratites.^[11] In contrast, Neognathae exhibit derived features enabling greater diversification, including enhanced mobility in the palatal region. The clade is subdivided into two principal subgroups: Galloanserae, which includes the order Galliformes (landfowl like chickens, turkeys, and quail) and Anseriformes (waterfowl like ducks, geese, and swans); and Neoaves, a hyperdiverse assemblage encompassing all remaining neognaths, such as passerines (perching birds), raptors (e.g., hawks and eagles), shorebirds, and seabirds.^[8] Neognathae accounts for approximately 11,000 extant species as of 2025 across 39 orders, representing over 99% of global avian diversity and spanning diverse ecological niches from terrestrial to aquatic habitats.^[1]

Evolutionary history

Origins and timeline

Neognathae originated in the Late Cretaceous, approximately 80–100 million years ago, as part of the broader neornithine radiation within Ornithuromorpha, diverging from their sister clade Palaeognathae.^[12] This split marked the initial branching of modern bird lineages (Neornithes), with molecular clock estimates placing the event amid a period of increasing avian morphological and ecological diversity in Gondwanan and Laurasian landmasses.^[13] The earliest known neognath fossils date to around 70 million years ago, represented by specimens such as Vegavis iaai from the Maastrichtian of Antarctica, which exhibit key neognath features like a heterocoelous cervical vertebrae and advanced palatal kinesis.^[14] The Cretaceous-Paleogene (K-Pg) extinction event, approximately 66 million years ago, triggered a major diversification of Neognathae, as surviving lineages rapidly exploited vacant ecological niches left by the demise of non-avian dinosaurs and many enantiornithine birds.^[15] This post-extinction radiation was characterized by accelerated rates of speciation and morphological innovation, particularly within subclades like Galloanserae and Neoaves, allowing Neognathae to dominate aerial, terrestrial, and aquatic habitats.^[13] Diversification continued intensely through the Paleogene, with key evolutionary milestones in the Eocene and Oligocene featuring the emergence of major lineages such as passerines, raptors, and waterbirds, culminating in the establishment of crown groups for most modern orders by the Miocene.^[13] Environmental drivers, including climate fluctuations and continental drift, profoundly shaped these events; global cooling episodes fragmented habitats and boosted speciation rates, while the progressive separation of Gondwanan continents facilitated vicariance and biogeographic isolation that influenced early divergences.^[16]^[17]

Fossil record

The earliest undisputed fossils attributable to Neognathae date to the Late Cretaceous, with Vegavis iaai representing a key specimen from the Maastrichtian stage of Vega Island, Antarctica, approximately 68 million years ago (mya). This anseriform-like bird, preserved in marine sediments, provides definitive evidence of crown-group neognaths prior to the Cretaceous-Paleogene (K-Pg) boundary, featuring a mix of derived traits such as a keeled sternum and waterfowl affinities confirmed through phylogenetic analyses and histological data. A nearly complete skull, collected in 2011 and described in 2025, further elucidates its morphology, confirming its anseriform affinities through detailed cranial analysis including advanced palatal kinesis and other neognathous traits, and suggesting a piscivorous diet based on inferred ecological role in Antarctic coastal environments.^[18]^[14] The Paleogene fossil record of Neognathae expands significantly, revealing early diversification within major lineages. Presbyornithidae, an extinct family of primitive anseriforms, is well-documented from late Paleocene to Eocene deposits, with specimens exhibiting long-legged, wading adaptations akin to modern flamingos but aligned phylogenetically with waterfowl; notable finds include Presbyornis pervetus from North American sites and a substantial collection from the early Eocene of Mongolia. Early passerines are represented by zygodactylids such as Zygodactylus, stem-group perching birds from the Eocene, characterized by their zygodactyl foot structure and inferred arboreal habits, as evidenced by multiple skeletons preserving skeletal details indicative of songbird precursors.^[19] Key fossil sites have yielded exceptional neognath material, illuminating post-K-Pg radiation. The Green River Formation (early Eocene, ~52 mya) in the western United States, particularly the Fossil Butte Member in Wyoming, has produced a diverse array of neognath fossils, including well-preserved skeletons of zygodactylids, early charadriiforms, and other waterbirds, reflecting a thriving lacustrine ecosystem with over 200 bird species represented across lagerstätten conditions. Similarly, the Messel Pit (middle Eocene, ~47 mya) in Germany offers unparalleled preservation of soft tissues in neognaths, such as feathers, stomach contents, and pigmentation in specimens of early perching birds and galliforms, providing insights into coloration, diet, and ecology through its anoxic lake bottom deposits. Despite these rich Paleogene assemblages, the Cretaceous record remains sparse, largely due to taphonomic biases favoring marine over terrestrial deposits and the fragility of small-bodied neognaths in non-lagerstätten environments. This scarcity suggests potential underrepresentation of pre-K-Pg diversity, with implications for understanding the timing and extent of neognath origins and survival across the end-Cretaceous mass extinction.^[4]

Taxonomy and systematics

Historical classification

The concept of Neognathae emerged from early efforts to classify birds based on cranial morphology, particularly the structure of the palate. In 1867, Thomas Henry Huxley proposed a foundational division of birds into two major groups: Saururae, encompassing ratites with a paleognathous palate characterized by a broad vomer and separate vomers from pterygoids, and Carinatae, comprising all other birds with a neognathous palate featuring a narrower vomer fused to the pterygoids. Huxley's classification emphasized the taxonomic value of palatal features in distinguishing these lineages, marking the initial recognition of neognathous birds as a distinct assemblage within modern avian diversity. During the late 19th and early 20th centuries, ornithologists refined this framework through comparative anatomical studies. Henry Seebohm, in his 1890 monograph, explored relationships among carinate birds, particularly emphasizing affinities within passerines and their placement relative to other neognathous groups based on skeletal and plumage characters.^[20] Alexander Wetmore further advanced the taxonomy in the 1930s by formalizing the distinction between neognathous and paleognathous palates, integrating Neognathae into the subclass Neornithes alongside Palaeognathae to represent all post-Cretaceous birds. Wetmore's 1951 revised classification solidified Neognathae as a superorder, incorporating over 8,000 species and underscoring the prevalence of neognathous traits across flying and secondarily flightless forms. These developments faced challenges from incomplete fossil evidence and reliance on morphology alone. Early classifications often confused Archaeornithes—Huxley's term for primitive fossil birds like Archaeopteryx—with Saururae or basal Carinatae due to shared reptilian-like features such as teeth and long tails, complicating the delineation of neognathous lineages.^[21] Comparative anatomy served as the primary tool before genetic methods, with dissections of palatal bones revealing subtle variations that informed groupings but also led to debates over whether certain taxa, like tinamous, bridged paleognathous and neognathous forms.^[21]

Phylogenetic relationships

Neognathae represents the crown group of Neornithes, the clade encompassing all extant birds, and is positioned as the sister group to Palaeognathae within Aves.^[22] This relationship is strongly supported by whole-genome analyses, with the divergence between Neognathae and Palaeognathae estimated at approximately 87–100 million years ago during the Late Cretaceous, based on molecular clock calibrations using fossil constraints (e.g., Jarvis et al. 2014; Kuhl et al. 2024).^[22]^[13] Genomic data, including alignments of over 41 million base pairs from 48 representative species, provide 100% bootstrap support for this basal split in the avian tree of life. In 2022, Sangster et al. provided phylogenetic definitions under the PhyloCode for Neognathae as the least inclusive crown clade containing Gallus gallus and Anas platyrhynchos but not Struthio camelus, formalizing its monophyly.^[8] Internally, Neognathae exhibits a basal dichotomy into two major clades: Galloanserae, comprising galliforms (e.g., chickens, pheasants) and anseriforms (e.g., ducks, geese), which are the closest relatives to Palaeognathae among neognaths, and the more diverse Neoaves, which includes the vast majority of modern bird orders.^[22] Within Neoaves, phylogenomic evidence delineates key nodes, including the split into Columbea (encompassing columbiforms like pigeons and gruiform relatives such as mesites and buttonquails) and Passerea approximately 67–69 million years ago, shortly after the Cretaceous-Paleogene boundary.^[22] Passerea further divides into Aequornithia (waterbirds, including pelecaniforms and charadriiforms) and Telluraves (landbirds, such as passerines, parrots, and raptors), with Telluraves itself branching into Afroaves and Australaves; these relationships are resolved with high statistical support in genome-scale datasets.^[22] The phylogenetic framework for Neognathae relies on integrated evidence from mitochondrial and nuclear DNA sequences, with phylogenomic studies like Jarvis et al. (2014) utilizing concatenated alignments and coalescent methods to resolve the early branches in the tree of life of modern birds, achieving high bootstrap support (often 100%) for major clades such as Galloanserae and the basal splits within Neoaves, overcoming challenges from incomplete lineage sorting (ILS).^[22] Earlier mitochondrial genome analyses also corroborated the Galloanserae-Neoaves split but struggled with deeper Neoaves branches due to rapid radiations.^[23] However, controversies persist regarding the exact placement of certain basal Neoaves lineages, such as the hoatzin (Opisthocomus hoazin) and owls (Strigiformes), where conflicting signals arise from data type variations and high ILS, leading to alternative topologies in some datasets.^[24] Additionally, integrating fossil-calibrated molecular clocks into these trees remains debated, as short internodes at the Neoaves base amplify sensitivity to calibration choices and substitution models.^[25]

Morphological characteristics

Cranial and palatal features

The neognathous palate, from which Neognathae derives its name meaning "new jaws," is characterized by a reduced or absent vomer bone, which minimizes rigidity and promotes mobility in the palatal complex.^[2] In this arrangement, the vomer, when present, forms a narrower structure with mediolateral articulation to the maxilla and reduced contact with the parasphenoid rostrum, while the maxillopalatine bones fuse in a manner that allows sliding contacts.^[2] The palatine bones are enlarged, and the pterygoids are shortened and segmented, excluding the vomer from key articulations and enabling a flexible upper jaw distinct from the more fused, rigid palaeognathous condition.^[2]^[26] Cranial kinesis in Neognathae arises from these palatal modifications, creating flexion zones—such as the prokinetic joint at the base of the beak and mobile articulations between the palatine and ectethmoid—that permit independent rostral movement relative to the braincase.^[2] This kinetic skull contrasts sharply with the largely rigid cranium of Palaeognathae, where fused pterygoids and palatines limit such flexibility.^[26] Associated cranial features include a relatively large braincase proportional to body size, which supports expanded cerebral hemispheres and enhanced neural processing compared to the smaller brains typical of palaeognaths (with exceptions like kiwis). Advanced sensory adaptations in the neognathous cranium are evident in groups like procellariiform seabirds and cathartid vultures, where enlarged olfactory bulbs facilitate superior olfaction for detecting food over vast distances, an improvement over basal avian levels.^[27] These anatomical traits collectively enhance foraging versatility, enabling behaviors such as wide-gape prey capture, precise pecking, and probing in varied substrates, which contributed to the ecological success of Neognathae.^[2]

Skeletal and soft tissue adaptations

Neognathae exhibit distinctive postcranial skeletal features that enhance mobility and support diverse locomotor strategies. The cervical vertebrae are heterocoelous, featuring saddle-shaped articular surfaces that permit extensive flexion and rotation of the neck, facilitating behaviors such as preening and foraging in varied environments.^[28] Thoracic ribs bear uncinate processes—posteriorly directed bony projections—that overlap adjacent ribs, stabilizing the rib cage and providing attachment points for costal cartilages and flight muscles like the appendicocostal elevators, which are crucial for powering wingbeats during flight.^[29] In the forelimbs, the carpometacarpus forms through fusion of metacarpals II (minor) and III (major), creating a rigid, lightweight structure that strengthens the wing for aerodynamic efficiency and withstands stresses during flapping flight.^[30] This fusion is a defining neognath trait, contrasting with less integrated wrist elements in palaeognaths, and supports the high-speed, maneuverable flight typical of many neognaths. Hindlimb diversity includes specialized foot configurations, such as the zygodactyl arrangement in parrots (Psittaciformes), where toes II and III face forward and toes I and IV backward, enabling precise grasping of branches and manipulation of objects akin to a hand.^[31] Soft tissue adaptations in Neognathae optimize physiological demands of flight and vocal communication. The respiratory system features a unidirectional airflow through rigid, non-expandable lungs supplemented by nine air sacs (interclavicular, cervical, thoracic, and abdominal), which store and circulate air, minimizing respiratory dead space and maximizing oxygen extraction during sustained activity.^[32] Uncinate processes on the ribs integrate with this system by anchoring muscles that expand the sternum and ribs, facilitating bellows-like ventilation that maintains airflow even mid-wingbeat.^[29] The syrinx, a bipartite vocal organ at the tracheobronchial junction, enables complex sound production in many neognaths, with labia and intrinsic muscles allowing independent control of each bronchus for duetting or harmonic songs, particularly advanced in oscine passerines within Neoaves.^[33] These adaptations vary across Neognathae subgroups: Neoaves emphasize flight enhancements like elongated primaries and alula for lift and control, while Galloanserae, such as ducks and geese (Anseriformes), prioritize swimming with webbed feet, dense plumage for buoyancy, and lobate toes for propulsion in water.^[34]

Diversity and distribution

Major clades and orders

Neognathae is divided into two principal clades: the basal Galloanserae and the more speciose Neoaves, a division supported by phylogenomic analyses of nuclear genomes.^[12] Galloanserae represents the earliest divergence within Neognathae, serving as the sister group to Neoaves in the avian tree of life.^[12] Galloanserae encompasses two orders: Galliformes, which includes approximately 300 species such as pheasants, turkeys, and chickens, and Anseriformes, comprising about 180 species like ducks, geese, and swans.^[35] These orders together form a monophyletic group characterized by their early branching position. Neoaves constitutes the bulk of neognath diversity and is resolved into multiple major lineages through large-scale genomic sequencing, including the clades Columbea and Telluraves, alongside other neoavian groups such as waterbirds.^[12] Columbea unites pigeons, sandgrouse, and related forms, totaling around 350 species across several small orders.^[12]^[35] Telluraves forms a large, monophyletic assemblage of landbirds that includes raptors, woodpeckers, and passerines, encompassing approximately 7,000 species.^[12]^[35] Additional neoavian divisions, such as the waterbird clade Aequornithes, further diversify this radiation. Among neognath orders, Passeriformes is the largest, with approximately 6,500 species that account for about 60% of all extant birds, highlighting the dominance of perching birds in avian diversity.^[35] Recent taxonomic updates, such as the 2025 eBird revision, continue to refine these counts through species splits and lumps.^[36] Other prominent orders include Charadriiformes, with approximately 350 species of shorebirds and allies, and Apodiformes, featuring around 500 species of hummingbirds and swifts.^[35] In total, Neognathae includes 30 to 40 orders depending on taxonomic delimitations, with the monophyly of its major clades robustly corroborated by phylogenomic datasets.^[12]

Global distribution and habitats

Neognathae exhibit a near-cosmopolitan distribution, occurring across all continents and major landmasses except for the extreme interiors of polar regions, such as the central Antarctic plateau where breeding populations are absent due to harsh conditions, and certain remote oceanic islands lacking suitable breeding sites prior to human intervention.^[37]^[38] This clade, encompassing approximately 11,000 species as of 2025, achieves its highest diversity in tropical regions, reflecting the influence of stable climates and varied ecosystems on avian speciation and persistence.^[39]^[40] The habitat versatility of Neognathae spans a broad spectrum of environments, from dense tropical and temperate forests occupied by diverse passerines that forage in canopies and understories, to freshwater and coastal wetlands utilized by anseriforms like ducks and geese for nesting and feeding.^[41] Pelagic oceans serve as primary habitats for procellariiform seabirds, including albatrosses and petrels that undertake long migrations over vast marine expanses, while certain columbiforms, such as doves, thrive in arid deserts by exploiting scattered water sources and seed resources.^[41] This adaptability underscores the clade's success in exploiting both terrestrial and aquatic niches worldwide. Species richness hotspots for Neognathae are concentrated in the Neotropics, home to approximately 4,600 species, and the Indo-Malaya region with around 2,500 species, where complex topography, rainfall, and habitat heterogeneity drive elevated diversity.^[42]^[43]^[44] In these ecosystems, Neognathae play crucial roles as pollinators (e.g., hummingbirds in the Americas), seed dispersers (via frugivorous species), and predators (raptors controlling herbivore populations), thereby maintaining biodiversity and trophic balance.^[37] Human activities have significantly altered Neognathae distributions through the intentional and accidental introduction of species, enabling range expansions beyond native limits; for instance, the common starling (Sturnus vulgaris), native to Eurasia, has been introduced to North America, Australia, New Zealand, South Africa, and parts of South America, where it now occupies urban, agricultural, and natural habitats across these continents.^[45] Such introductions often lead to competition with native species and ecosystem disruptions, highlighting the ongoing anthropogenic influence on global avian biogeography.^[46]

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