Anchiornis

Name and Etymology

The generic name Anchiornis combines the Ancient Greek anchi (ἄγχι, near) and ornis (ὄρνις, bird), reflecting the animal's morphological proximity to avian body plans. The specific epithet huxleyi pays tribute to Thomas Henry Huxley, a contemporary of Charles Darwin and one of the earliest proponents of the hypothesis that birds descended from dinosaurs. The taxon was named and described by Xu Xing and colleagues in a paper published in Chinese Science Bulletin in 2009 (Xu et al., 2009).

Taxonomic Status

Anchiornis is a monotypic genus, with A. huxleyi as its sole valid species. Its phylogenetic placement has been highly contentious: it was originally assigned to the Avialae (Xu et al., 2009), then reclassified as a troodontid (Hu et al., 2009), subsequently placed in the Archaeopterygidae (Xu et al., 2011), back in the Avialae (Godefroit et al., 2013), and at the base of Paraves outside Eumaniraptora (Lefèvre et al., 2017). The current prevailing view, following Foth & Rauhut (2017) and Cau et al. (2017), places Anchiornis within the family Anchiornithidae—a clade of basal paravians that falls outside the Dromaeosauridae, Troodontidae, and Avialae. Additionally, Aurornis xui, Eosinopteryx brevipenna, and Serikornis sungei have been suggested as potential junior synonyms of Anchiornis, though this has not been formally resolved.

Summary

A crow-sized, four-winged feathered dinosaur from the Late Jurassic of China—the first Mesozoic dinosaur with nearly complete plumage coloration scientifically reconstructed, and a key taxon illuminating early stages in the evolutionary transition to birds.

Temporal Range

The temporal range of Anchiornis is constrained to the Late Jurassic Oxfordian stage. High-precision U-Pb zircon geochronology by Chu et al. (2016) dates the fossil-bearing horizons to approximately 160.89–160.25 Ma. This makes Anchiornis more than 10 million years older than Archaeopteryx (~150 Ma), establishing it as one of the oldest known feathered dinosaurs. The holotype was recovered from lacustrine sediments of the Tiaojishan Formation in the Yaolugou area of Jianchang County, Liaoning Province.

Formation and Lithology

All Anchiornis fossils derive from the Tiaojishan Formation, a volcano-sedimentary complex distributed across Hebei and Liaoning Provinces, China, spanning the Middle to Late Jurassic (Bathonian–Oxfordian, approximately 165–153 Ma). The formation is primarily composed of andesitic volcanic rocks interspersed with sandstone, shale, tuff, and lacustrine mudstone.

The tuffaceous composition of certain horizons indicates that the area experienced active volcanism with periodic heavy ashfalls, and the fine-grained lacustrine beds that yielded the exquisitely preserved fossils likely resulted from rapid burial by volcanic ash in anoxic lake-bottom conditions.

Palaeoenvironment

The depositional setting of the Tiaojishan Formation is interpreted as a lacustrine-fluvial complex in a volcanically active region. The fossil-bearing, finely laminated sediments represent quiet, deep-lake environments, surrounded by montane streams and dense gymnosperm forests (Tan et al., 2006). Palaeobotanical analyses by Wang et al. (2006) indicate a subtropical to warm-temperate, humid climate, with abundant ginkgoaleans (Ginkgo, Czekanowskia, Phoenicopsis), conifers (Pityophyllum, Podozamites), cycads (Anomozamites), and ferns (Todites, Coniopteris) forming gymnosperm-dominated woodland. Anchiornis shared this environment with a rich assemblage comprising the Yanliao Biota: salamanders, lizards, pterosaurs (Darwinopterus), other theropods (Xiaotingia, Eosinopteryx), and early mammaliaforms (Juramaia, Volaticotherium), among others.

Holotype and Key Specimens

Anchiornis is known from hundreds of specimens, some preserving feathers, skin, soft tissues, and even gut contents in exceptional detail:

As of 2010, the Shandong Tianyu Museum of Nature alone was reported to hold 255 Anchiornis specimens, indicating that the actual total is far higher than the number formally described.

Diagnosis

Synthesizing the original description (Xu et al., 2009), the supplementary description (Hu et al., 2009), and the revised diagnosis (Pei et al., 2017), the key diagnostic features that distinguish Anchiornis from other paravians include: (1) a triangular skull combining features shared with dromaeosaurids, troodontids, and basal avialans; (2) proportionally long forelimbs, measuring approximately 80% of hindlimb length; (3) a bird-like wrist including a semilunate carpal bone; (4) long pennaceous feathers attached to the metatarsus, forming hindwings; and (5) symmetrical wing feathers with rounded tips, longest at the wrist rather than at the wingtip (unlike other paravians).

Limitations of the Material

The holotype (IVPP V14378) lacks the skull, limiting the morphological information available at the time of the original description. The second specimen (LPM-B00169) and subsequent discoveries have substantially filled this gap, but only a small fraction of the hundreds of known specimens have been formally described in detail, and many remain in private collections or await preparation. Furthermore, Lindgren et al. (2015) raised the possibility that BMNHC PH828—the key specimen used for the 2010 colour reconstruction—might belong to a contemporaneous closely related species rather than Anchiornis itself.

Body Size

Anchiornis was an extremely small bipedal theropod, roughly crow-sized. The holotype (IVPP V14378), a subadult to young adult, measured approximately 34 cm in total length and an estimated 110 g in body mass (Xu et al., 2009). The largest known specimens (e.g., PKUP V1068) reached an estimated total length of 60–62 cm, a wingspan of approximately 57.4 cm, and a body mass of roughly 0.6–1 kg (Pei et al., 2017; Dececchi et al., 2020). Anchiornis ranks among the smallest known non-avian theropods.

Skull and Dentition

The skull was triangular and displayed a mosaic of features shared with dromaeosaurids, troodontids, and primitive avialans (Pei et al., 2017). Small teeth were present, tending toward small conical forms suitable for capturing small animal prey rather than the phylliform (leaf-shaped) teeth typical of herbivorous ornithischians.

Forewings

The forewings of Anchiornis comprised 11 primary feathers and 10 secondary feathers. The wing feathers were relatively short, symmetrical, and rounded at the tips—indicating poor aerodynamic capability. While in Microraptor and Archaeopteryx the longest feathers are positioned near the wingtip, creating a pointed wing profile, in Anchiornis the longest feathers were situated near the wrist, producing a wing broadest at mid-span that tapered toward the tip in a rounded, less flight-adapted shape (Hu et al., 2009). A propatagium (skin flap) connected the shoulder to the wrist, and covert feathers covered most of the wing surface in multiple layers. Unlike in modern birds, where coverts cover only the upper wing with the main flight feathers largely exposed below, in Anchiornis the coverts extended in several overlapping layers across much of the wing surface, presumably reinforcing the otherwise narrow and weak primary and secondary feathers (Longrich et al., 2012). Three clawed fingers were present in the wing, but the longest two were bound together by skin and tissue, making Anchiornis functionally two-fingered (Wang et al., 2017).

Hindwings

In addition to the forewings, Anchiornis bore long pennaceous feathers on the hindlimbs, qualifying it as a 'four-winged' dinosaur. The hindwings comprised 12–13 feathers anchored to the tibia (lower leg) and 10–11 to the tarsus (upper foot). These hindwing feathers were shorter than those of Microraptor, more symmetrical, and more curved, suggesting that their primary function was display rather than flight (O'Connor & Chang, 2015). The toes were almost entirely covered in short feathers, beneath which small scales were present (Wang et al., 2017).

Feather Microstructure and Texture

Saitta et al. (2018) demonstrated that Anchiornis body feathers consisted of short quills from which independent, flexible barbs projected at low angles on two opposing blades, giving each feather an overall forked shape. This produced a much coarser, 'shaggier' plumage texture than that of modern birds, likely affecting thermoregulation and water repellence while reducing aerodynamic efficiency. At the molecular level, Pan et al. (2019) showed that Anchiornis feathers contained both α-keratins and β-keratins, but with α-keratins predominating—in contrast to modern flight feathers, which are dominated by β-keratins. This composition rendered the feathers mechanically weaker than their modern avian counterparts.

Sternum and Flight Capability

No ossified sternum (breastbone) has been identified in Anchiornis, suggesting it was cartilaginous or absent (Zheng et al., 2014). The sternum is the primary attachment site for flight muscles in modern birds, and its underdevelopment strongly implies that powered flapping flight was highly limited or impossible. Dececchi et al. (2016) calculated that while juvenile specimens could potentially have used their wings to assist in running up inclines (wing-assisted incline running, WAIR) or even achieved weak flapping flight at very high wing-stroke angles, adult specimens were simply too heavy relative to their wing area to gain any aerodynamic benefit. Wing-flapping while running would have increased speed by only about 10%, and using the wings during leaps would have boosted height and distance by approximately 15–20%.

In 2025, Kiat et al. examined nine specimens and identified 20–28 primary feathers and three series of primary coverts—markedly different from the 9–11 primaries and two series of primary coverts found in modern volant birds. They further demonstrated that wing feather replacement (moult) in Anchiornis was asymmetrical and irregular, resembling the pattern seen in secondarily flightless birds rather than the sequential moult of modern volant birds and some non-avian pennaraptorans (e.g., Microraptor, confuciusornithiforms). This provides additional evidence that Anchiornis was flightless (Kiat et al., 2025).

Diet

Direct evidence of Anchiornis diet comes from gastric pellets. Zheng et al. (2018) surveyed over 230 Anchiornis specimens at the Shandong Tianyu Museum and discovered gastric pellets within or adjacent to the body cavities of several individuals. These pellets contained lightly acid-etched lizard bones and ptycholepid fish scales, demonstrating that Anchiornis expelled indigestible remains through regurgitation in a manner analogous to modern raptorial birds. This represents the oldest known record of avian-style gastric pellet production in a theropod. Taken together, Anchiornis is interpreted as an opportunistic small carnivore that fed on lizards, fish, and possibly other small vertebrates.

Ecological Niche

The Yanliao Biota in which Anchiornis lived included numerous small paravians—Xiaotingia, Eosinopteryx, Serikornis—that coexisted in the same formation and likely underwent ecological niche partitioning. Eosinopteryx, which lacked flight feathers on its tail and lower legs and possessed straight rear claws, appears to have been better adapted for cursorial (ground-running) habits (Godefroit et al., 2013), whereas Anchiornis, with its more extensive leg plumage and curved claws, may have been better suited for arboreal activity.

Locomotion

Anchiornis hindlimb proportions resemble those of more primitive theropods adapted for fast running, with elongate legs. However, the extensive feathering on the legs may have impeded running speed, suggesting that long legs could represent a vestigial trait from a cursorial ancestor (Hu et al., 2009). Powered flight was impossible or extremely limited in adults (Dececchi et al., 2016; Kiat et al., 2025). Tree-climbing (supported by claw curvature) and short-distance gliding may have served as supplementary locomotor modes.

Geographic Distribution

Anchiornis fossils are known exclusively from northeastern China (Liaoning and Hebei Provinces), all from the Tiaojishan Formation. The holotype locality is the Yaolugou area of Jianchang County, Liaoning, and the major collecting site at Daxishan (Linglongta) has also yielded numerous specimens.

Palaeogeographic Interpretation

During the Oxfordian, the palaeolatitude of the fossil localities was approximately 43°N and the palaeolongitude approximately 123°E, not substantially different from the present-day position. The climate was subtropical to warm-temperate and humid, in an inland region with active volcanism. The distribution of Anchiornithidae is currently restricted to northeastern China, suggesting this group may have been an endemic clade specialised to the Jurassic lacustrine-volcanic ecosystems of East Asia.

Classification History

The phylogenetic placement of Anchiornis has shifted repeatedly since its discovery:

Anchiornithidae

In 2017, Foth & Rauhut formally established the family Anchiornithidae during their re-evaluation of the Haarlem Archaeopteryx specimen, grouping Anchiornis with Eosinopteryx, Xiaotingia, Aurornis, Serikornis, and Pedopenna. The family is defined as "all maniraptoran theropods more closely related to Anchiornis huxleyi than to Passer domesticus, Archaeopteryx lithographica, Dromaeosaurus albertensis, Troodon formosus, or Oviraptor philoceratops." Under this framework, Anchiornithidae occupies a position as a basal paravian clade outside Eumaniraptora (the clade containing Dromaeosauridae, Troodontidae, and Avialae).

Potential Synonymy

Aurornis xui (Godefroit et al., 2013), Eosinopteryx brevipenna (Godefroit et al., 2013), and Serikornis sungei (Lefèvre et al., 2017) have all been suggested as potential junior synonyms of Anchiornis. All derive from the same Tiaojishan Formation and are osteologically very similar to Anchiornis, though they differ in plumage structure and certain skeletal details. This issue awaits formal taxonomic revision.

Established, Probable, and Hypothetical

Established: (1) Anchiornis is a small feathered dinosaur from the Late Jurassic (Oxfordian, ~160 Ma) Tiaojishan Formation of northeastern China, placed in Anchiornithidae. (2) It possessed a four-winged morphology with pennaceous feathers on both arms and legs. (3) Melanosome analysis has reconstructed a substantial portion of its plumage coloration. (4) Gastric pellets containing lizard bones and fish scales confirm a small-carnivore diet.

Probable: (1) Adults were incapable of powered flight (supported by wing morphology, absence of ossified sternum, feather structure, and moult pattern). (2) Anchiornithidae represents a basal paravian clade sister to or outside Eumaniraptora, documenting early experimental stages in the evolution toward avian flight. (3) A feathered crest was present on the head.

Hypothetical: (1) Whether the rufous crown coloration reconstructed for BMNHC PH828 is universal to Anchiornis or restricted to certain individuals/species remains unresolved. (2) The synonymy of Aurornis, Eosinopteryx, and Serikornis with Anchiornis is unconfirmed. (3) Whether the hindwings functioned exclusively for display or also provided some aerodynamic advantage remains debated. (4) Whether Anchiornis was primarily arboreal or terrestrial is uncertain.

Popular Media vs. Science

Anchiornis is frequently introduced in popular media as 'the first dinosaur whose colour was reconstructed', with the 2010 rufous-crested, black-and-white-winged restoration serving as the iconic image. However, this reconstruction is based on a single specimen (BMNHC PH828), and a 2015 study found inconsistent results in a different specimen. Additionally, Anchiornis is sometimes depicted as a 'flying dinosaur', whereas the current scientific consensus is that adults were flightless.

All members of Anchiornithidae derive from the Yanliao Biota of the same age and region, predating Archaeopteryx by approximately 10 million years. They display pre-flight feather and wing configurations that contrast with the more advanced four-winged gliding apparatus of Microraptor (~35 million years later) and the flight-adapted plumage of Archaeopteryx.