|C. sanctus fossil preserving long wing and tail feathers.|
Hou et al., 1995
Hou et al., 1995
Confuciusornis is a genus of primitive crow-sized birds from the Early Cretaceous Period of the Yixian and Jiufotang Formations of China, dating from 125 to 120 million years ago. Like modern birds, Confuciusornis had a toothless beak, but close relatives of modern birds such as Hesperornis and Ichthyornis were toothed, indicating that the loss of teeth occurred convergently in Confuciusornis and living birds. It is the oldest known bird to have a beak. It was named after the Chinese moral philosopher Confucius (551–479 BC). Confuciusornis is one of the most abundant vertebrates found in the Yixian Formation, and several hundred complete, articulated specimens have been found.
Confuciusornis was about the size of a modern pigeon, with a total length of 50 centimetres (1.6 feet) and a wingspan of up to 70 cm (2.3 ft). Its body weight has been estimated to have been as much as 1.5 kilograms (3.3 lb), or less than 0.2 kg (0.44 lb). C. feducciai was about a third longer than average specimens of C. sanctus.
Confuciusornis shows a mix of basal and derived traits. It was more "advanced" or derived than Archaeopteryx in possessing a short tail with a pygostyle (a bone formed from a series of short, fused tail vertebrae) and a bony sternum (breastbone) , but more basal or "primitive" than modern birds in retaining large claws on the forelimbs, having a primitive skull with a closed eye-socket, and a relatively small breastbone. At first the number of basal characteristics was exaggerated: Hou assumed in 1995 that a long tail was present and mistook grooves in the jaw bones for small degenerated teeth.
The skull of Confuciusornis was equipped with a pointed toothless beak. It was relatively heavy-built and immobile, incapable of the kinesis of modern birds that can raise the snout relative to the back of the skull. This immobility was caused by the presence of a triradiate postorbital separating the eye socket from the lower temporal opening, as with more basal theropod dinosaurs, and the premaxillae of the snout reaching all the way to the frontals, forcing the nasals to the sides of the snout.
The various specimens seem to have a variable number of neck vertebrae, some showing eight, others nine. The first vertebra, the atlas, bore a faint keel on the underside. The next, the axis, had an expanded spinal process on the top and its side was excavated by an elongated groove in the side. The remaining neck vertebrae all had rather low spinal processes. There is no clear evidence of a pneumatisation, in the form of internal air spaces, of the vertebral bodies of the neck. The front articulation facets of the neck vertebrae were saddle-shaped. Their undersides were pinched.
There were at least twelve back vertebrae. They were amphiplatian, flat at both ends, and had rather small intervertebral foramina, the spaces between the vertebral body and the neural arch. Their spinal processes were tall and narrow in side view. Their side processes projected horizontally and were deeply excavated at the rear underside. The sides of the back vertebrae also had deep oval excavations.
Seven sacral vertebrae were fused into a synsacrum. The front sacral vertebra had a round and concave front articulation facet. The vertebral bodies of the front half of the synsacrum were excavated at their sides, comparable to the back vertebrae. Robust side processes connected the synsacrum to the ilia of the pelvis.
Although earlier descriptions had counted four or five "free", not fused, tail vertebrae, Chiappe e.a. in 1999 reported seven of them. These had round and somewhat concave front articulation facets. Their spinal processes were high and transversely compressed. The side processes were robust and stick out horizontally to the side. Their articulation processes were rather long. The last of these vertebrae had a rectangular profile. Its neural arch had short processes pointing obliquely to above and sideways. The tail ended in a pygostyle, a complete fusion of the last vertebrae. Their number is uncertain. The pygostyle was about 40% longer than the first part of the tail. At its underside the pygostyle bore a well-developed keel, running from front to rear. Its top was incised by a long groove between prominent ridges.
Confuciusornis had an exceptionally large humerus (upper arm bone). Near its shoulder-end this was equipped with a prominent deltopectoral crest. Characteristically this crista deltopectoralis was with Confuciusornis pierced by an oval hole which may have reduced the bone's weight or enlarged the attachment area of the flight muscles. The furcula or wishbone, like that of Archaeopteryx, was a simple curved bar lacking a pointed process at the back, a hypocleidum. The sternum was relatively broad and had a low keel which was raised at the back end. This bony keel may or may not have anchored a larger, cartilaginous, keel for enlarged pectoral muscles. The scapulae (shoulder blades) were fused to the strut-like coracoid bones and may have formed a solid base for the attachment of wing muscles. The orientation of the shoulder joint was sideways, instead of angled upward as in modern birds; this means that Confuciusornis was unable to lift its wings high above its back. According to a study by Phil Senter in 2006, the joint was even pointed largely downwards meaning that the humerus could not be lifted above the horizontal. This would make Confuciusornis incapable of the upstroke required for flapping flight; the same would have been true for Archaeopteryx.
The wrist of Confuciusornis shows fusion, forming a carpometacarpus. The second and third metacarpals were also partially fused, but the first was unfused, and the fingers could freely move relative to each other. The second metacarpal, which supported the flight feathers, was very heavily built; its finger carries a small claw. The claw of the first finger to the contrary was very large and curved. The stub-like third metacarpal, which supported the calami of the feathers, was probably enclosed in the flesh of the hand. The formula of the finger phalanges was 2-3-4-0-0.
The pelvis was connected to a sacrum formed by seven sacral vertebrae. The pubis was strongly pointing backwards. The left and right ischia were not fused. The femur was straight; the tibia only slightly longer. The metatarsals of the foot were relatively short and fused to each other and to the lower ankle bones, forming a tarsometatarsus. A rudimentary fifth metatarsal is present. The first metatarsal was attached to the lower shaft of the second and supported a first toe or hallux, pointing to the back. The formula of the toe phalanges was 2-3-4-5-0. The proportions of the toes suggest that they were used for both walking and perching, while the large claws of the thumb and third finger were probably used for climbing.
Feathers and soft tissue
The wing feathers of Confuciusornis were long and modern in appearance. The primary wing feathers of a 0.5-kilogram individual reached 20.7 centimetres in length. The five longest primary feathers (remiges primarii) were more than 3½ times the length of the hand and relatively longer than those of any living bird, while the secondary feathers of the lower arm were rather short by comparison. The outermost primary was much shorter than the second outermost primary, creating a relatively round, broad wing. Its wing shape does not specifically match any particular shape found among living birds. The primary feathers were asymmetrical to varying degrees, and especially so in the outermost primaries. It is unclear whether the upper arm carried tertiaries. Covert feathers are preserved covering the upper part of the wing feathers in some specimens, and some specimens have preserved the contour feathers of the body.
Unlike some more advanced birds, Confuciusornis lacked an alula, or "bastard wing". In modern birds this is formed by feathers anchored to the first digit of the hand, but this digit appears to have been free of feathers and independent of the body of the wing in Confuciusornis. According to Dieter Stefan Peters to compensate for the lack of an alula, the third finger might have formed a separate winglet below the main wing, functioning like the flap of an aircraft. Despite the relatively advanced and long wing feathers, the forearm bones lacked any indication of quill knobs (papillae ulnares), or bony attachment points for the feather ligaments.
Many specimens preserve a pair of long, narrow tail feathers, which grew longer than the entire length of the rest of the body. Unlike the feathers of most modern birds, these feathers were not differentiated into a central quill and barbs for most of their length. Rather, most of the feather formed a ribbon-like sheet, about six millimetres wide. Only at the last one quarter of the feather, towards the rounded tip, does the feather become differentiated into a central shaft with interlocking barbs. Many individuals of Confuciusornis lacked even these two tail feathers, possibly due to sexual dimorphism. The rest of the tail around the pygostyle was covered in short, non-aerodynamic feather tufts similar to the contour feathers of the body, rather than the familiar feather fan of modern bird tails.
Laser fluorescence of two Confuciusornis specimens revealed additional details of their soft-tissue anatomy. The propatagium of Confuciusornis was large, likely relatively thick, and extended from the shoulder to the wrist, as in modern birds; the extent of the postpatagium is also similar to modern birds. Reticulate scales covered the underside of the foot, and the phalanges and metatarsals supported large, fleshy pads, although the interphalangeal pads were either small or entirely absent.
In early 2010, a group of scientists led by Zhang Fucheng examined fossils with preserved melanosomes (organelles which contain colors). By studying such fossils with an electron microscope, they found melanosomes preserved in a fossil Confuciusornis specimen, IVPP V13171. They reported the presence of melanosomes were of two types: eumelanosomes and pheomelanosomes. This indicated that Confuciusornis had hues of grey, red/brown and black, possibly something like the modern zebra finch. It was also the first time an early bird fossil has been shown to contain preserved pheomelanosomes. However, a second research team failed to find these reported traces of pheomelanosomes. Their 2011 study also found a link between the presence of certain metals, like copper, and preserved melanin. Using a combination of fossil impressions of melanosomes and the presence of metals in the feathers, the second team of scientists reconstructed Confuciusornis with darkly colored body feathers and upper wing feathers, but found no trace of either melanosomes or metals in the majority of the wing feathers. They suggested that the wings of Confuciusornis would have been white or, possibly, colored with carotenoid pigments. The long tail feathers of male specimens would have also been dark in color along their entire length.
A 2018 study of the specimen CUGB P1401 indicated the presence of heavy spotting on the wings, throat, and crest of Confuciusornis.
Discovery and naming
In November 1993 Chinese paleontologists Zhou Zhonge, Hu Yoaming and Hou Lianhai, of the Institute of Vertebrate Paleontology and Paleoanthropology at Beijing, were presented a bird fossil bought at a flea market in Sihetun by amateur paleontologist Zhang He. It showed a partial skeleton of a bird new to science of which even some feather remains had been preserved. In December 1993 two further specimens were acquired from a farmer, Yang Yushan. Soon afterwards, it was discovered that local farmers were in the process of collecting hundreds of specimens that were provisionally prepared by them to be illegally sold to commercial fossil dealers. Many hundreds have since also been added to the official collections of Chinese institutions. In 2010 the Shandong Tianyu Museum of Nature alone possessed 536 specimens. The fossils are compressed but otherwise typically very complete often showing the entire feathering of the animal.
Based on the abundant material, six species have been formally named and described: C. sanctus (the type species), C. dui, C. feducciai, C. jianchangensis, C. chuonzhous and C. suniae. The latter two are usually considered synonymous with C. sanctus. Most species lived in the early Aptian 125-million-year-old Jianshangou Beds of the Yixian Formation, though C. jianchangnsis is found in the later (120 Ma) middle Aptian Jiufotang Formation. C. sanctus is known from the former, and is one of the most common vertebrate species found in the Yixian, often present in dense concentrations. At one time forty were discovered on a surface of about 100 m2. This has been explained as the result of entire flocks of birds being simultaneously killed by ash, heat or poisonous gas following the volcanic eruptions that caused the tuff stone in which the fossils were found to be deposited as lake sediments.
In 1995 Zhou, Hou, Zhang and Gu Youcai named Confuciusornis sanctus. The generic name combines the philosopher Confucius with a Greek ὄρνις, (ornis), "bird". The specific name means "holy one" in Latin and is a translation of Chinese 圣贤, shèngxián, "sage", again in reference to Confucius. The holotype specimen is IVPP V10918. In 1997 Hou named two species. The first was Confuciusornis chuonzhous, based on specimen IVPP V10919, originally a paratype of C. sanctus. The specific name refers to Chuanzhou, an ancient name for Beipiao. The second species was Confuciusornis suniae, based on specimen IVPP V11308. The specific name honours madam Sun, the wife of Shikuan Liang who donated the fossil to the IVPP. In 1999 Hou, Zhou, Zhang, Larry Martin and Alan Feduccia named Confuciusornis dui, based on specimen IVPP V11553. The specific name again honours a donating collector: Du Wengya. In 2009 Zhang Fucheng et al. named Confuciusornis feducciai, based on specimen D2454, the specific name honouring Feduccia. In 2010 Li Li, Wang Jingqi and Hou Shilin named Confuciusornis jianchangensis, based on specimen PMOL-AB00114 found at Toudaoyingzi.
In 2002 Hou named the genus Jinzhouornis, but Chiappe et al. (2018) and Wang et al. (2018) showed that this genus is a junior synonym of Confuciusornis based on morphometry and examination of known confuciusornithiform specimens.
Hou assigned Confuciusornis to the Confuciusornithidae in 1995. At first he assumed it was a member of the Enantiornithes and the sister taxon of Gobipteryx. Later he understood that Confuciusornis was not an enantiornithean but concluded it was the sister taxon of the Enantiornithes, within a larger Sauriurae. This was heavily criticised by Chiappe who regarded Sauriurae to be paraphyletic as there were insufficient shared traits that indicated that the Confuciusornithidae and the Enantiornithes were closely related. In 2001, Ji Qiang suggested an alternative position as the sister taxon of the Ornithothoraces.
In 2002 Ji's hypothesis was confirmed by a cladistic analysis by Chiappe, who defined a new group: the Pygostylia of which Confuciusornis is by definition the most basal member. Several traits of Confuciusornis illustrate its position in the tree of life; it has a more "primitive" skull than Archaeopteryx, but it is the first known bird to have lost the long tail of Archaeopteryx and develop fused tail vertebrae, a pygostyle. One controversial study concluded that Confuciusornis may be more closely related to Microraptor and other dromaeosaurids than to Archaeopteryx, but this study was criticized on methodological grounds.
A close relative, the confuciusornithid Changchengornis hengdaoziensis, also lived in the Yixian Formation. Changchengornis also possessed the paired, long tail feathers, as did several more advanced enantiornith birds. True, mobile tail fans only appeared in ornithuromorph birds, and possibly in the enantiornithine Shanweiniao.
The large, fleshy phalangeal foot pads, the small interphalangeal foot pads, the presence of only reticulate scales on the underside of the foot (which increases flexibility), and curved foot claws of Confuciusornis are all traits shared with modern tree-dwelling, perching birds, suggesting that Confuciusornis itself may have had a similar lifestyle.
Confuciusornis has traditionally been assumed to have been a competent flier based on its extremely long wings with strongly asymmetrical feathers. Other adaptations for improved flight capabilities include: a fused wrist, a short tail, an ossified sternum with a central keel, a strut-like coracoid, a large deltopectoral crest, a strong ulna (forearm bone) and an enlarged second metacarpal. The sternal keel and deltopectoral crest (which provides a more powerful upstroke) are adaptations to flapping flight in modern birds, indicating that Confuciusornis may have been capable of the same. However, it may have had a different flight stroke due to being incapable of rotating its arm behind the body, and its relatively smaller sternal keel indicates that it likely was not capable of flight for extended periods of time.
Several contrary claims have been made against that the flight capabilities of Confuciusornis. The first of these regarded problems to attain a steep flight path due to a limited wing amplitude. In Senter's interpretation of the position of the shoulder joint, a normal upstroke would be impossible precluding flapping flight entirely. Less radical is the assessment that due to the lack of a keeled sternum and a high acrocoracoid, the musculus pectoralis minor could not serve as a M. supracoracoideus lifting the humerus via a tendon running through a foramen triosseum. This, coupled with a limited upstroke caused by a lateral position of the shoulder joint, would have made it difficult to gain altitude. Some authors therefore proposed that Confuciusornis used its large thumb claws to climb tree trunks. Martin assumed that it could raise its torso almost vertically like a squirrel. Daniel Hembree, however, while acknowledging that tree climbing was likely, pointed out that the rump was apparently not lifted more than 25° relative to the femur in vertical position, as shown by the location of the antitrochanter in the hip joint. Dieter S. Peters considered it very unlikely that Confuciusornis climbed trunks as turning the thumb claw inwards would stretch the very long wing forwards, right in the path of obstructing branches. Peters sees Confuciusornis as capable of flapping flight but specialised in soaring flight.
A second problem is the strength of the feathers. In 2010, Robert Nudds and Gareth Dyke published a study arguing that in both Confuciusornis and Archaeopteryx, the raches (central shafts) of the primary feathers were too thin and weak to have remained rigid during the power stroke required for true flight. They argued that Confuciusornis would at most have employed gliding flight, which is also consistent with the unusual adaptations seen in its upper arm bones, and more likely used its wings for mere parachuting, limiting fall speed if it dropped from a tree. Gregory S. Paul, however, disagreed with their study. He argued that Nudds and Dyke had overestimated the weights of these early birds, and that more accurate weight estimates allowed powered flight even with relatively narrow raches. Nudds and Dyke assumed a weight of 1.5 kilograms for Confuciusornis, as heavy as the modern teal. Paul argued that a more reasonable body weight estimate is about 180 grams (6.3 oz), less than that of a pigeon. Paul also noted that Confuciusornis is commonly found as large assemblages in lake bottom sediments with little to no evidence of extensive postmortem transport, and that it would be highly unusual for gliding animals to be found in such large numbers in deep water. Rather, this evidence suggests that Confuciusornis traveled in large flocks over the lake surfaces, a habitat consistent with a flying animal. A number of researchers have questioned the correctness of the rachis measurements, stating that the specimens they had studied showed a shaft thickness of 2.1–2.3 millimetres (0.083–0.091 in), compared to 1.2 mm (0.047 in) as reported by Nudds and Dyke. Nudd and Dyke replied that, apart from the weight aspect, such greater shaft thickness alone would make flapping flight possible; however, they allowed for the possibility of two species being present in the Chinese fossil material with a differing rachis diameter.
In 2016, Falk et al. argued in favor of flight capabilities for Confuciusornis using evidence from laser fluorescence of two soft tissue-preserving specimens. They found that, contrary to Nudds and Dyke's assertions, the raches of Confuciusornis were relatively robust, with a maximum width of over 1.5 mm (0.059 in). The wing shape is consistent with either birds that live in dense forests or gliding birds; the former is consistent with its environment being densely forested, and requiring more maneuverability and stability than speed. The substantial propatagium would have produced a generous amount of lift, while the likewise large postpatagium would have provided a large attachment area for the calami of the feathers, which would have kept them as a straight airfoil. This collectively is strongly indicative that Confuciusornis was capable of powered flight, if not only for short periods of time.
Reproduction and growth
In 2007, Gary Kaiser mentioned a Confuciusornis skeleton preserving an egg near its right foot – the first possible egg referable to the genus. The skeleton is from the short-tailed form and thus might represent a female. The egg might have fallen out of the body after the death of the presumed female, although it cannot be excluded that this adult-egg association was only by chance. The egg is roundish in shape and measures 17 mm in diameter, slightly smaller than the head of the animal; according to Kaiser, it would have fitted precisely through the pelvic canal of the bird.:244–245 In dinosaurs and Mesozoic birds, the width of the pelvic canal was restricted due to connection of the lower ends of the pubic bones, resulting in a V-shaped bony aperture through which eggs must fit. In modern birds, this connection of the pubes is lost, presumably allowing for larger eggs. In a 2010 paper, Gareth Dyke and Kaiser showed that the breath of the Confuciusornis egg was indeed smaller than what would be expected for a modern bird of similar size. A 2018 study by Charles Deeming and Gerald Mayr measured the size of the pelvic canal of various Mesozoic birds including Confuciusornis to estimate egg size, concluding that eggs would have been small in proportion to body mass for Mesozoic birds in general. These researchers further posit that an avian-style contact incubation (sitting on eggs for breeding) was not possible for non-avian dinosaurs and Mesozoic birds, as these animals would have been too heavy in relation to the size of their eggs. Kaiser, in 2007, argued that Confuciusornis likely did not brood in an open nest but might have used crevices in trees for protection, and that the small size of the only known egg indicates large clutch sizes.:246 In contrast, a 2016 review by David Varricchio and Frankie Jackson argued that nesting above the ground evolved only at a much later stage, within Neornithes, and that Mesozoic birds would have buried their eggs on the ground, either fully or partially, as seen in non-avian dinosaurs.
Growth speed can be estimated based on the inner bone structure, which may preserve growth lines. In 1998 Zhang determined that growth was fast, leading to fibrolamellar bone. Comparison with the bone structure of an alligator indicated that Confuciusornis had a high metabolism comparable to that of modern birds. The growth speed of Confuciusornis would have been similar to that of Beipiaosaurus. A study by Armand de Ricqlès in 2003 indicated that growth was essentially modern, Confuciusornis reaching its maximum size in at most twenty weeks, perhaps as few as thirteen. Basal birds would have limited their size, not by slowing growth but by limiting the period of growth. A 2013 study found medullary bone in a short tailed specimen, confirming that it was female.
Many specimens of Confuciusornis preserve a single pair of long, streamer-like tail feathers. However, most do not, even specimens which otherwise have exquisitely preserved feathers on the rest of the body; their predominance has been estimated to be as high as 90–95%. This difference has been suggested to indicate sexual dimorphism, with one gender (likely the males) using the streamer-like feathers in courtship displays. However, while sexual variation is the most obvious explanation for the presence or absence of long tail feathers, other factors cannot be ruled out. For example, it is possible that some individuals lack tail feathers because they were fossilized during molting. As in modern birds, molting individuals may have been present alongside non-molting individuals, and males and females may have molted at different times during the year.
Due to the large number of specimens, statistical analysis can be used to investigate this problem. In 2008 Chiappe et alii published a morphometrical study of 106 fossils. The population showed a clear bimodal distribution of the size of the animals with two weight classes. However, there was no correlation between size and the possession of the long tail feathers. From this it was concluded that either the sexes did not differ in size or both sexes had the long feathers. The first case was deemed most likely which left the size distribution to be explained. It was hypothesised that the smaller animals consisted of very young individuals, that the large animals were adults and that the rarity of individuals with an intermediate size was caused by Confuciusornis experiencing a growth spurt just prior to reaching adulthood, the shortness of which would have prevented many becoming fossilised during this phase. This interpretation was criticised by Dieter S. Peters who pointed out that, as the smaller animals were not neonates, the young of Confuciusornis had to grow very fast immediately after hatching regardless. This would then imply two growth spurts, which assumption was not very parsimonious. He thought the data would be more simply explained by assuming that both sexes had the long feathers and one of them was the largest. He favoured the possibility that these were the females, having observed such a pattern with the modern pheasant-tailed jacana (Hydrophasianus chirurgus), a water-bird in which both sexes have long tails and the female sex is the heaviest. He accounted for the fact that most specimens show no tails at all by the possibility that Confuciusornis shed the feathers as a defence mechanism, a method used by several extant species, which would have been triggered by the stress induced by the very volcanic explosion that buried the animals.
There are immature specimens known, and from the analysis of bone growth patterns of young adults it has been estimated that Confuciusornis reached maturity somewhat slower than extant small birds, but faster than advanced dinosaurs, which might indicate an omnivorous diet similar to modern crows.
It has been hypothesized that Confuciusornis fed on plant materials due to its toothless beak, but no gastroliths or stomach contents had been reported (Zhou & Zhang, 2003). Dalsätt and colleagues (2006) described a specimen, IVPP V13313 found in the Jiufotang Beds, that preserves seven to nine vertebrae and several ribs of a small fish, probably Jinanichthys. These fish bones are formed into a tight cluster about six millimetres across, and the cluster is in contact with the seventh and eighth cervical vertebrae of the bird. In this position it was likely in the crop of the bird, which may have been preparing to regurgitate a pellet when it died. No other fish remains are present in the slab. Andrzej Elżanowski already in 2002 had predicted that Confuciusornis was a predator. Dieter S. Peters has hypothesized that, although no remains of toe webs have been conserved, it caught its prey swimming using its rather soft bill to search for prey below the waterline. Andrei Zinoviev assumed it caught fish on the wing.
Several extant bird species have been presented as modern analogues of Confuciusornis providing insight into its possible lifestyle. Dieter S. Peters thought that it could be best compared with the white-tailed tropicbird (Phaeton lepturus), a fisher that too has a long tail and narrow wings—and even often nests in the neighbourhood of volcanoes. The paradise kingfishers (genus Tanysiptera) of modern Australia and New Guinea have elongated "racket plumes" as their central tail feathers, giving them a superficial resemblance to Confuciusornis. These kingfishers also take fish as prey, but they are not specialized fishers. They take many insects and other small prey from their forest habitat.
Confuciusornis was discovered in the Yixian Formation and is a member of the Jehol Biota. Tuff makes up a considerable amount of the rock composition due to frequent volcanic eruptions. This tuff has allowed detailed dating of the formation by using 40Ar-39Ar isotopes. This results in an age of approximately 125 to 120 million years ago for the whole formation. The fossils were buried as a result of flooding and volcanic debris. This method of preservation resulted in fossils that are very flat, almost two-dimensional. The volcanic strata have allowed the preservation of various soft tissues, such as detailed feather impressions. Using oxygen isotopes in reptile bones found in the formation, a 2010 study determined that many formations from East Asia, including the Yixian, had a cool temperate climate. The mean air temperature was estimated at 10 °C ± 4 °C. Fossils of Xenoxylon, a type of wood known from temperate areas of the time, have been found throughout the region. Additionally, reptiles needing heat, such as crocodilians, are absent. Flora in the Yixian were represented by most groups of mesozoic plants, including mosses, clubmosses, horsetails, ferns, seed ferns, Czekanowskiales, gingko trees, cycadeoids, Gnetales, conifers, and a small number of flowering plants. Fauna that were present in the Jehol Biota include ostracods, gastropods, bivalves, insects, fish, salamanders, mammals, lizards, pterosaurs, and an exceptionally large number of birds. Other dinosaurs were also present in the Jehol Group. These include the theropod Sinocalliopteryx gigas, a specimen of which was discovered with Confuciusornis bones in its abdominal contents.
- Ivanov, M., Hrdlickova, S. & Gregorova, R. (2001) The Complete Encyclopedia of Fossils. Rebo Publishers, Netherlands. pp. 312
- Xu, X.; Norell, M.A. (2006). "Non-Avian dinosaur fossils from the Lower Cretaceous Jehol Group of western Liaoning, China". Geological Journal. 41 (3–4): 419–437. doi:10.1002/gj.1044.
- Holtz, Thomas R. Jr.; Rey, Luis V. (2007). Dinosaurs: the most complete, up-to-date encyclopedia for dinosaur lovers of all ages (PDF). New York: Random House. ISBN 978-0-375-82419-7.
- Nudds, R.L.; Dyke, G.J (2010). "Narrow primary feather rachises in Confuciusornis and Archaeopteryx suggest poor flight ability". Science. 328 (5980): 887–9. Bibcode:2010Sci...328..887N. doi:10.1126/science.1188895. PMID 20466930.
- Paul, G.S. (2010). "Comment on 'Narrow Primary Feather Rachises in Confuciusornis and Archaeopteryx Suggest Poor Flight Ability.'". Science. 330 (6002): 320. Bibcode:2010Sci...330..320P. doi:10.1126/science.1192963. PMID 20947747.
- Zhang, Z.; Gao, C.; Meng, Q.; Liu, J.; Hou, L.; Zheng, G. (2009). "Diversification in an Early Cretaceous avian genus: evidence from a new species of Confuciusornis from China". Journal of Ornithology. 150 (4): 783–790. doi:10.1007/s10336-009-0399-x.
- Martin, L.D.; Zhou, Z.; Hou, L.; Feduccia, A. (1998). "Confuciusornis sanctus compared to Archaeopteryx lithographica". Naturwissenschaften. 85 (6): 286–289. Bibcode:1998NW.....85..286M. doi:10.1007/s001140050501.
- Chiappe, Luis M., Shu-An, Ji, Qiang, Ji, Norell, Mark A. (1999) "Anatomy and systematics of the Confuciusornithidae (Theropoda:Aves) from the Late Mesozoic of northeastern China" "Bulletin of the American museum of Natural History no.242 89pp.
- Senter, P. (2006). "Scapular orientation in theropods and basal birds, and the origin of flapping flight" (PDF). Acta Palaeontologica Polonica. 51 (2): 305–313.
- Falk, A.R.; Kaye, T.G.; Zhou, Z.; Burnham, D.A. (2016). "Laser Fluorescence Illuminates the Soft Tissue and Life Habits of the Early Cretaceous Bird Confuciusornis". PLoS ONE. 11 (12): e0167284. Bibcode:2016PLoSO..1167284F. doi:10.1371/journal.pone.0167284. PMC 5156344. PMID 27973609.
- Peters, D.S.; Ji, Q. (1999). "Muβte Confuciusornis klettern?". J. Ornithol. 140 (1): 41–50.
- Zhang, Fucheng; Kearns, Stuart L.; Orr, Patrick J.; Benton, Michael J.; Zhou, Zhonghe; Johnson, Diane; Xu, Xing; Wang, Xiaolin (2010). "Fossilized melanosomes and the colour of Cretaceous dinosaurs and birds". Nature. 463 (7284): 1075–8. Bibcode:2010Natur.463.1075Z. doi:10.1038/nature08740. PMID 20107440.
- Wogelius, R.A.; Manning, P.L.; Barden, H.E.; Edwards, N.P.; Webb, S.M.; Sellers, W.I.; Taylor, K.G.; Larson, P.L.; Dodson, P.; You, H.; Da-qing, L.; Bergmann, U. (2011). "Trace metals as biomarkers for eumelanin pigment in the fossil record". Science. 333 (6049): 1622–1626. Bibcode:2011Sci...333.1622W. doi:10.1126/science.1205748. PMID 21719643.
- Li, Quanguo; Clarke, Julia A.; Gao, Ke-Qin; Peteya, Jennifer A.; Shawkey, Matthew D. (2018-11-02). "Elaborate plumage patterning in a Cretaceous bird". PeerJ. 6: e5831. doi:10.7717/peerj.5831. ISSN 2167-8359. PMC 6216952. PMID 30405969.
- Zhou, Z. (1995). "The discovery of Early Cretaceous birds in China". Acta Palaeornithologica. 181: 9–22.
- Zheng, X; Xu, X; Zhou, Z; Miao, D; Zhang, F (2010). "Comment on "Narrow Primary Feather Rachises in Confuciusornis and Archaeopteryx Suggest Poor Flight Ability". Science. 330 (6002): 320, author reply 320. Bibcode:2010Sci...330..320Z. doi:10.1126/science.1193223. PMID 20947746.
- Li, D.; Sulliven, C.; Zhou, Z.; Zhang, Z. (2010). "Basal birds from China: a brief review". Chinese Birds. 1 (2): 83–96. doi:10.5122/cbirds.2010.0002.
- Wang X., Zhang F,, Xu X., Wang Y., Gu G. (2000). "Taphonomy and mass mortality of Confuciusornis and feathered dinosaurs at the Sihetun and Zhangjiagou sites in western Liaoning, China". Vertebrata PalAsiatica. 38 (supp): 32.
- Hou, L.; Zhou, Z.; Gu, Y.; Zhang, H. (1995). "Confuciusornis sanctus, a new Late Jurassic sauriurine bird from China". Chinese Science Bulletin. 40 (18): 1545–1551.
- Hou L., 1997, [Chinees] [Mesozoic Birds of China. Phoenix Valley Provincial Aviary of Taiwan]. Taiwan
- Hou, L.; Martin, L.D.; Zhou, Z.; Feduccia, A.; Zhang, F. (1999). "A diapsid skull in a new species of the primitive bird Confuciusornis". Nature. 399 (6737): 679–682. Bibcode:1999Natur.399..679H. doi:10.1038/21411.
- Li, L. (2010). "[Chinese] [A new species of Confuciusornis from Lower Cretaceous of Jianchung, Liaoning, China]". Global Geology. 29 (2): 183–187.
- Chiappe, Marugan-Lobon, Ji and Zhou, 2008. Life history of a basal bird: morphometrics of the Early Cretaceous Confuciusornis. Biology Letters. 4(6), 719-723.
- Wang M, O’Connor J, Zhou Z H, in press. A taxonomical revision of the Confuciusornithiformes (Aves: Pygostylia). Vertebrata PalAsiatica, DOI: 10.19615/ j.cnki.1000-3118.18053
- Zhou, Z; Hou, L. (1998). "Confuciusornis and the early evolution of birds". Vertebrata PalAsiatica. 36 (2): 136–146.
- Chiappe, L. (1997). "The Chinese early bird Confuciusornis and the paraphyletic status of Sauriurae". Journal of Vertebrate Paleontology. 17 (3): 1–93. doi:10.1080/02724634.1997.10011028.
- Ji, Q. (2001). "[Chinees] "New advances in the study of the primitive bird Confuciusornis". Geological Science and Technology Information. 20: 30–34.
- L. M. Chiappe (2002) "Basal bird phylogeny: problems and solutions". In: L. M. Chiappe and L. M. Witmer (eds.), Mesozoic Birds: Above the Heads of Dinosaurs. University of California Press, Berkeley pp. 448–472
- Clarke,,Julia. A. , Norell, Mark. A. (2002) "The Morphology and Phylogenetic Position of Apsaravis ukhaana from the Late Cretaceous of Mongolia". American Museum Novitates, No. 3387, American Museum of Natural History, New York, NY 10024.
- Mayr, G.; Pohl, B; Peters, D. S. (2005). "A well-preserved Archaeopteryx specimen with theropod features". Science. 310 (5753): 1483–6. Bibcode:2005Sci...310.1483M. doi:10.1126/science.1120331. PMID 16322455.
- Clarke, J. A.; Zhou, Z; Zhang, F (2006). "Insight into the evolution of avian flight from a new clade of Early Cretaceous ornithurines from China and the morphology of Yixianornis grabaui". Journal of Anatomy. 208 (3): 287–308. doi:10.1111/j.1469-7580.2006.00534.x. PMC 2100246. PMID 16533313.
- O'Connor, Jingmai K.; Wang, Xuri; Chiappe, Luis M.; Gao, Chunling; Meng, Qingjin; Cheng, Xiaodong; Liu, Jinyuan (2009). "Phylogenetic support for a specialized clade of Cretaceous enantiornithine birds with information from a new species". Journal of Vertebrate Paleontology. 29 (1): 188–204. doi:10.1080/02724634.2009.10010371. JSTOR 20491078.
- Schmitz, L.; Motani, R. (2011). "Nocturnality in Dinosaurs Inferred from Scleral Ring and Orbit Morphology". Science. 332 (6030): 705–8. Bibcode:2011Sci...332..705S. doi:10.1126/science.1200043. PMID 21493820.
- Zhou Z. and Farlow, J.O. (2001) "Flight capability and habits of Confuciusornis". In: Gauthier and Gall (eds). New perspectives on the origin and early evolution of birds: proceedings of the international symposium in honor of John H. Ostrom. Peabody Museum of Natural History. Yale University, New Haven. pp. 237–254
- Hembree, D. (1999). "Re-evaluation of the posture and claws of Confuciusornis". Journal of Vertebrate Paleontology. 19: 50A. doi:10.1080/02724634.1999.10011202.
- Nudds, R.L.; Dyke G. (2010). "Response to comments on "Narrow Primary Feather Rachises in Confuciusornis and Archaeopteryx Suggest Poor Flight Ability". Science. 330 (6002): 320. Bibcode:2010Sci...330..320N. doi:10.1126/science.1193474.
- Zhou, Z.; Barrett, P.M.; Hilton, J. (2003). "An exceptionally preserved Lower Cretaceous ecosystem". Nature. 421 (6925): 807–814. Bibcode:2003Natur.421..807Z. doi:10.1038/nature01420. PMID 12594504.
- Kaiser, Gary W. (2010). The inner bird: anatomy and evolution. UBC Press. ISBN 9780774813440.
- Dyke, Gareth J.; Kaiser, Gary W. (2010). "Cracking a developmental constraint: egg size and bird evolution". Records of the Australian Museum. 62 (1): 207–216.
- Charles Deeming, D.; Mayr, Gerald (2018). "Pelvis morphology suggests that early Mesozoic birds were too heavy to contact incubate their eggs". Journal of evolutionary biology. 31 (5): 701–709.
- Varricchio, David J.; Jackson, Frankie D. (2016). "Reproduction in Mesozoic birds and evolution of the modern avian reproductive mode". The Auk. 133 (4): 654–684.
- Zhang, F.; Hou, L.; Ouyang, L. (1998). "Osteological microstructure of Confuciusornis: preliminary report". Vertebrata PalAsiatica. 36: 126–135.
- Zhang, F.-C., Xu X. & Lü J., 1999, "Some microstructure difference among Confuciusornis, Alligator and a small theropod dinosaur, and its implications", Paleoworld, December 1999: 296–308
- Zhang, F.-C., et al. (2000) "Microstructures of Confuciusornis and Beipiaosaurus and their physiological implications", Vertebrata Pal-Asiatica 38 (suppl. Abstracts 5th Society of Avian Paleontology and Evolution Meeting and Jehol Biota Symposium, Beijing 2000): 36
- Ricqlès, De; Padian, K.; Horner, J.R.; Alamm, E.T.; Myhrvold, N. (2003). "Osteohistology of Confuciusornis sanctus (Theropoda: Aves)". Journal of Vertebrate Paleontology. 23 (2): 373–386. doi:10.1671/0272-4634(2003)023[0373:oocsta]2.0.co;2.
- Chinsamy, A.; Chiappe, L. M.; Marugán-Lobón, J. S.; Chunling, G.; Fengjiao, Z. (2013). "Gender identification of the Mesozoic bird Confuciusornis sanctus". Nature Communications. 4: 1381. Bibcode:2013NatCo...4E1381C. doi:10.1038/ncomms2377. PMID 23340421.
- Chiappe, L.M.; Marugan-Lobon, J.; Ji, S.; Zhou, Z. (2008). "Life history of a basal bird: morphometrics of the Early Cretaceous Confuciusornis". Biology Letters. 4 (6): 719–723. doi:10.1098/rsbl.2008.0409. PMC 2614169. PMID 18832054.
- Peters, Winfried S.; Dieter Stefan, Peters (2009). "Life history, sexual dimorphism and 'ornamental' feathers in the mesozoic bird Confuciusornis sanctus". Biology Letters. 5 (6): 817–820. doi:10.1098/rsbl.2009.0574. PMC 2828012. PMID 19776067.
- Dalsätt, J.; Zhou, Z.; Zhang, F.; Ericson, P. G. P. (2006). "Food remains in Confuciusornis sanctus suggest a fish diet". Naturwissenschaften. 93 (9): 444–6. Bibcode:2006NW.....93..444D. doi:10.1007/s00114-006-0125-y. PMID 16741705.
- Elzanowski, A. (2002) "Biology of basal birds and the origin of avian flight". In: Zhou Z., Zhang F. (eds) Proceedings of the 5th Symposium of the Society of Avian Paleontology and Evolution, Beijing, 1–4 June 2000. Science, Beijing, pp 211–226
- Zinoviev, A. (2009). "Опыт реконструкции экологического облика конфуциусорнитид (Aves, Confuciusornithiformes, Confuciusornithidae)". Палеонтологический журнал. 4: 83–91.
- Zinoviev, A.V. (2009). "An attempt to reconstruct the lifestyle of confuciusornithids (Aves, Confuciusornithiformes)". Paleontological Journal. 43 (4): 444–452. doi:10.1134/S0031030109040145.
- Bleiwess, Robert (1987). "Development and evolution of avian racket plumes: Fine structure and serial homology of the wire". Journal of Morphology. 194 (1): 23–39. doi:10.1002/jmor.1051940103. PMID 29907008.
- Zhou, Z. (2006). "Evolutionary radiation of the Jehol Biota: chronological and ecological perspectives". Geological Journal. 41 (3–4): 377–393. doi:10.1002/gj.1045.
- Rogers, Christopher S.; Hone, David W.E.; McNamara, Maria E.; Zhao, Qi; Orr, Ptrick J.; Kearns, Stuart L.; Benton, J. Michael. (2015) The Chinese Pompeii? Death and destruction of dinosaurs in the Early Cretaceous of Lujiatun, NE China. Palaeogeography, Palaeoclimatology, Palaeoecology 427: 89-99. doi:10.1016/j.palaeo.2015.03.037
- Amiot, R., Wang, X., Zhou, Z., Xiaolin Wang, X., Buffetaut, E., Lécuyer, C., Ding, Z., Fluteau, F., Hibino, T., Kusuhashi, N., Mo, J., Suteethorn, V., Yuanqing Wang, Y., Xu, X., and Zhang, F. (2011). "Oxygen isotopes of East Asian dinosaurs reveal exceptionally cold Early Cretaceous climates." Proceedings of the National Academy of Sciences, 108(13): 5179-5183. doi: 10.1073/pnas.1011369108
- Xing L, Bell PR, Persons WS IV, Ji S, Miyashita T, Burns ME, et al. (2012) Abdominal Contents from Two Large Early Cretaceous Compsognathids (Dinosauria: Theropoda) Demonstrate Feeding on Confuciusornithids and Dromaeosaurids. PLoS ONE 7(8): e44012. doi: 10.1371/journal.pone.0044012
- Well preserved fossil attributed to Confuciusornis from the Jiufotang Formation