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Confuciusornithidae is an extinct family of pygostylian
avialans Avialae ("bird wings") is a clade containing the only living dinosaurs, the birds. It is usually defined as all theropod dinosaurs more closely related to birds (Aves) than to deinonychosaurs, though alternative definitions are occasionally used ...
known from the
Early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145& ...
, found in northern China. They are commonly placed as a sister group to Ornithothoraces, a group that contains all extant birds along with their closest extinct relatives. Confuciusornithidae contains four genera, possessing both shafted and non-shafted (downy) feathers. They are also noted for their distinctive pair of ribbon-like tail feathers of disputed function. The wing anatomy of confuciusornithids suggests an unusual flight behavior, due to anatomy that implies conflicting abilities. They possessed feathers similar to those of fast-flapping birds, which rely on quick flapping of their wings to stay aloft. At the same time, their wing anatomy also suggests a lack of flapping ability. Confuciusornithids are also noted for their beak and lack of teeth, similar to modern birds. Both predators and prey, confuciusornithid fossils have been observed with fish remains in their digestive systems and have themselves been found in the abdominal cavities of ''
Sinocalliopteryx ''Sinocalliopteryx'' (meaning 'Chinese beautiful feather') is a genus of carnivorous compsognathid theropod dinosaurs from the Lower Cretaceous Yixian Formation of China (Jianshangou Beds, dating to 124.6 Ma). While similar to the related ' ...
'', a
compsognathid Compsognathidae is a family of coelurosaurian theropod dinosaurs. Compsognathids were small carnivores, generally conservative in form, hailing from the Jurassic and Cretaceous Periods. The bird-like features of these species, along with other d ...
predator.


Classification

Confuciusornithidae was first named by Hou ''et al.'' in 1995 to contain the type genus, ''Confuciusornis'', and assigned to the monotypic clade Confuciusornithiformes within the
class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differently ...
Aves Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...
. The group was given a
phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups ...
definition by Chiappe, in 1999, who defined a node-based
clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English ter ...
Confuciusornithidae to include only ''Changchengornis'' and ''Confuciusornis''.Chiappe, Luis M. et al. (1999) There are a number of features that define the clade. The most significant is the presence of a toothless jaw, which shows a more birdlike adaptation compared to ''Archaeopteryx''. The other defining features are as follows, according to Chiappe ''et al.'' (1999): * There is a fork in the
rostral Rostral may refer to: Anatomy * Rostral (anatomical term), situated toward the oral or nasal region * Rostral bone, in ceratopsian dinosaurs * Rostral organ, of certain fish * Rostral scale, in snakes and scaled reptiles Other uses * Rostral ...
(front) part of the
mandibular symphysis In human anatomy, the facial skeleton of the skull the external surface of the mandible is marked in the median line by a faint ridge, indicating the mandibular symphysis (Latin: ''symphysis menti'') or line of junction where the two lateral halve ...
, the area where the mandibles fuse together. * The presence of a distinct maxillary fenestra (breathing hole) in the
maxilla The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. T ...
's ascending ramus. * The deltopectoral crest of the
humerus The humerus (; ) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius and ulna, and consists of three sections. The humeral upper extremity consists of a r ...
is prominent, which allows for increased muscle attachment for adductor muscles. * The first
metacarpal In human anatomy, the metacarpal bones or metacarpus form the intermediate part of the skeletal hand located between the phalanges of the fingers and the carpal bones of the wrist, which forms the connection to the forearm. The metacarpal bones ar ...
is not attached by bone to the other co-ossified metacarpals. * The claw of the second manual digit is much smaller than the other claws. * A V-shape in the
sternum The sternum or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major blood vessels from injury. Sha ...
in the
caudal Caudal may refer to: Anatomy * Caudal (anatomical term) (from Latin ''cauda''; tail), used to describe how close something is to the trailing end of an organism * Caudal artery, the portion of the dorsal aorta of a vertebrate that passes into the ...
end. * The
proximal Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position ...
phalanx The phalanx ( grc, φάλαγξ; plural phalanxes or phalanges, , ) was a rectangular mass military formation, usually composed entirely of heavy infantry armed with spears, pikes, sarissas, or similar pole weapons. The term is particularly ...
of the third digit is much smaller than the other phalanges. Confuciusornithidae is the most basal group of the clade Pygostylia, whose members possess a
pygostyle Pygostyle describes a skeletal condition in which the final few caudal vertebrae are fused into a single ossification, supporting the tail feathers and musculature. In modern birds, the rectrices attach to these. The pygostyle is the main compone ...
, a fused set of caudal vertebrae at the end of the tail. The pygostyle replaced the longer, unfused tail found in more primitive avialans such as ''Archaeopteryx'', and may have served to improve flight. Pygostylia includes all modern birds, the only living members of the clade. Additional members have been added to Confuciusornitidae since 1999. ''Jinzhouornis'' was added by Hou, Zhou, and Zhang in 2002, and in 2008, Zhang, Zhou and Benton assigned the newly described genus ''Eoconfuciusornis'' to the family.


Biogeography

Most confuciusornithids are known from the upper
Jehol group The Jehol Biota includes all the living organisms – the ecosystem – of northeastern China between 133 and 120 million years ago. This is the Lower Cretaceous ecosystem which left fossils in the Yixian Formation and Jiufotang Formation. These ...
, the
Yixian Formation The Yixian Formation (; formerly transcribed as Yihsien Formation) is a geological formation in Jinzhou, Liaoning, People's Republic of China, that spans the late Barremian and early Aptian stages of the Early Cretaceous. It is known for its exq ...
and
Jiufotang Formation The Jiufotang Formation ( Chinese: 九佛堂组, pinyin: ''jiǔfótáng zǔ'') is an Early Cretaceous geological formation in Chaoyang, Liaoning which has yielded fossils of feathered dinosaurs, primitive birds, pterosaurs, and other organisms (see ...
, dating from 125 to 120 million years ago. ''Eoconfuciusornis'', however, predated the other confuciusornithids by 6 million years, dating to 131 Ma ago.


Anatomy

The entire body of confuciusornithids was covered in contour feathers, except for the foot, base of beak, and the tarsometatarsus, the bone directly attached to the foot. It appears that they may also have had down feathers. The beaks of confuciusornithids show development of modern birdlike characteristics, such as a large beak and lack of teeth. The premaxilla and dentary are larger than those of ''Archaeopteryx''. The anterior of these bones shows evidence of vasculature and innervation, implying the presence of a beak. The lack of recovery of this structure indicates that the beak had a soft horny sheath. The softness of the beak along with the innervation suggest that the beak was sensitive, making it useful for searching for prey. Much of their anatomy resembles that of ''Archaeopteryx'', especially the pectoral girdle and forelimbs. They were better adapted for flight than ''Archaeopteryx'', due to the elimination of two thoracic vertebrae. The development of a pygostyle also shows better adaptation for flight, as this replaces the long tails present in earlier avialans. Similarly to ''Archaeopteryx'', confuciusornithids possessed a large first digit with a hook-like claw. The digit implies a climbing lifestyle, as it serves to allow for hooking onto the grooves of trees. A similar anatomy and function is seen in the nestlings of the
hoatzin The hoatzin ( ) or hoactzin ( ), (''Opisthocomus hoazin''), is the only species in the order Opisthocomiformes. It is a species of tropical bird found in swamps, riparian forests, and mangroves of the Amazon and the Orinoco basins in South Ameri ...
, an extant South American bird. The biomechanics of the wing itself are quite contentious due to a combination of traits that imply different modes of flight. Confuciusornithids possessed long primary feathers similar to those of modern fast-flapping birds, as opposed to gliding birds which have short primaries relative to their size. However, the narrowness of the wings of confuciusornithids along with the lack of upstroke ability during flapping motion seem to preclude the ability to flap their wings quickly. Thus, they may have relied upon a flight method that no longer exists in modern birds. The hindlimbs of confuciusornithids did not resemble those of living birds. They were bad runners, with feet curved in a way that implies they did not move on the ground. The long feathers of the tail (central
rectrices Flight feathers (''Pennae volatus'') are the long, stiff, asymmetrically shaped, but symmetrically paired pennaceous feathers on the wings or tail of a bird; those on the wings are called remiges (), singular remex (), while those on the tai ...
) of confuciusornithids are of disputed function. Sexual dimorphism is an explanation, with males presumed to use the feathers in mating displays. However, it has been argued that the long rectrices were instead used as a defense against predators, as many birds shed feathers to protect themselves. The observation that less than 10% of confuciusornithid fossils possess these feathers supports this, as they may have been shed either in response to predators, or to the stress of the sudden death that produced the fossils.


Paleoecology

Confuciusornithids were first thought to be herbivorous due to the lack of teeth. However, their anatomy was not adapted for plant consumption, as gastroliths have never been found, nor did the weak rhamphotheca of the beak allow for grinding. Instead, the beak appears to have been sensitive enough to assist in food acquisition and capable of holding potential prey. This beak type is well suited for skimming prey off of the top of a body of water. Large numbers of fossils appear to originate from the tops of freshwater lakes, further supporting the water feeding connection. The remains of fish have been found in fossils of ''C. sanctus''. Confuciusornithids appear to have been unable to take off from water and lacked the adaptations necessary to live aquatically. Thus, it appears that they flew along the surface of the water, using their beak to search for fish. Confuciusornithid remains have been found in the abdominal contents of ''Sinocalliopteryx gigas'', a compsognathid predator. Multiple confuciusornithids were present in the remains, implying that they were all captured in a short time. Confuciusornithids appear to have been social animals, as concurrently buried fossils are often found in close proximity.


Reproduction

A 2018 study suggests that confuciusornithids could not have incubated their eggs like modern birds do. Other paravians (including '' Deinonychus'') and pterosaurs are known to be superprecocial and able to fly soon after birth, but for now there are no unambiguous confuciusornithid juveniles to attest this.


References

{{Taxonbar, from=Q135442 Early Cretaceous dinosaurs of Asia Hauterivian first appearances Early Cretaceous extinctions Prehistoric dinosaur families