''Caihong'' ( zh, c=彩虹, p=cǎihóng, l=rainbow) is a genus of small

paravian Paraves are a widespread group of theropod dinosaurs that originated in the Middle Jurassic period. In addition to the extinct dromaeosaurids, troodontids, anchiornithids, and possibly the scansoriopterygids, the group also contains the av ...

theropod Theropoda (; from ancient Greek , (''therion'') "wild beast"; , (''pous, podos'') "foot"">wiktionary:ποδός"> (''pous, podos'') "foot" is one of the three major groups (clades) of dinosaurs, alongside Ornithischia and Sauropodom ...

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...

from

China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...

that lived during the

Late Jurassic The Late Jurassic is the third Epoch (geology), epoch of the Jurassic Period, and it spans the geologic time scale, geologic time from 161.5 ± 1.0 to 143.1 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic stratum, strata.Owen ...

period.

Discovery and naming

At Gangou, Qinglong, in the north of

Hebei Hebei is a Provinces of China, province in North China. It is China's List of Chinese administrative divisions by population, sixth-most populous province, with a population of over 75 million people. Shijiazhuang is the capital city. It bor ...

province, peasant Yang Jun discovered in a quarry near the village of Nanshimenzi the skeleton of a small theropod, belonging to the

Yanliao Biota The Yanliao Biota is the name given to an assembly of fossils preserved in northeastern China from the Middle to Late Jurassic.Xu, X., Zhou, Z., Sullivan, C. and Wang, Y., 2017. The Yanliao Biota: a trove of exceptionally preserved Middle-Late Ju ...

. It was in February 2014 acquired by the Paleontological Museum of Liaoning. The fossil was subsequently, and for the first time, prepared by Ding Xiaoqing and Matthew Brown. In 2018, the

type species In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...

''Caihong juji'' was named and described by Hu Dongyu, Julia A. Clarke, Chad M. Eliason, Qiu Rui, Li Quanguo, Matthew D. Shawke, Zhao Cuilin, Liliana D'Alba, Jiang Jinkai and Xu Xing. The generic name is the Mandarin ''caihong'', 彩虹, "rainbow", in reference to the splendor of the fossil and the spectrum of new scientific insights it offers. The

specific name Specific name may refer to: * in Database management systems, a system-assigned name that is unique within a particular database In taxonomy, either of these two meanings, each with its own set of rules: * Specific name (botany), the two-part (bino ...

is the Chinese ''ju ji'', "big crest", in reference to the large crest on the

lacrimal bone The lacrimal bones are two small and fragile bones of the facial skeleton; they are roughly the size of the little fingernail and situated at the front part of the medial wall of the orbit. They each have two surfaces and four borders. Several bon ...

of the skull. The

holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

, housed at the Paleontological Museum of Liaoning with the accession number PMoL-B00175, was found in a layer of the

Tiaojishan Formation The Tiaojishan Formation is a geological formation in Hebei and Liaoning, People's Republic of China, dating to the middle-late Jurassic period (Bathonian-Oxfordian (stage), Oxfordian stages). It is known for its Lagerstätte, exceptionally preser ...

dating to the Oxfordian, about 161 million years old. It consists of a complete articulated skeleton, compressed on a slab and counterslab. Soft tissues and extensive remains of the plumage have been preserved. It represents an adult individual.

Description

Size and distinguishing traits

''Caihong'' was a rather small dinosaur. Its length was estimated at , and its weight at . It is estimated to have a wingspan of approximately , given that the wingspan is about 2.1 times the wing length which is about . The describing authors indicated a number of unique derived traits,

autapomorphies In phylogenetics, an autapomorphy is a distinctive feature, known as a derived trait, that is unique to a given taxon. That is, it is found only in one taxon, but not found in any others or outgroup taxa, not even those most closely related to ...

. In addition to the antorbital, maxillary and promaxillary fenestrae (skull openings) present within the antorbital fossa, the

maxilla In vertebrates, the maxilla (: maxillae ) 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. The two maxil ...

is also pierced by an accessory fenestra which opens behind and below the promaxillary fenestra. The lacrimal bone has a crest strongly projecting to above and sideways. In the lower jaw, the

dentary In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone ...

is robust with the front tip higher than the middle section. In the pelvis, the ilium is short with a length less than half that of the

femur The femur (; : femurs or femora ), or thigh bone is the only long bone, bone in the thigh — the region of the lower limb between the hip and the knee. In many quadrupeds, four-legged animals the femur is the upper bone of the hindleg. The Femo ...

(thighbone); in all other theropods the ilium is more than half as long. All these traits are unique within the

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

Paraves Paraves are a widespread group of theropod dinosaurs that originated in the Middle Jurassic period. In addition to the extinct dromaeosauridae, dromaeosaurids, troodontidae, troodontids, Anchiornithidae, anchiornithids, and possibly the scansor ...

. Besides, ''Caihong'' was especially distinguished from closely related

genera Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...

of the Yanliao Biota, such as ''

Eosinopteryx ''Eosinopteryx'' is an extinct genus of theropod dinosaurs known to the Late Jurassic epoch of China. It contains a single species, ''Eosinopteryx brevipenna''. Some researchers consider it a junior synonym of ''Anchiornis.'' Discovery and namin ...

'' and ''

Anchiornis ''Anchiornis'' is a genus of small, four-winged Paraves, paravian dinosaurs, with only one known species, the type species ''Anchiornis huxleyi'', named for its similarity to modern birds. The Latin name ''Anchiornis'' derives from a Greek word m ...

'', by the presence of prominent lacrimal crests, longer

pennaceous feather The pennaceous feather is a type of feather present in most modern birds and in some other species of maniraptoriform dinosaurs. Description A pennaceous feather has a stalk or quill. Its basal part, called a ''calamus'', is embedded in the sk ...

s on the forelimbs and hindlimbs and tail feathers with large asymmetrical vanes.

Skeleton

The skull of ''Caihong'' has a length of 67.6 millimetres. It is low and elongated (superficially similar to that of ''

Velociraptor ''Velociraptor'' (; ) is a genus of small dromaeosaurid dinosaurs that lived in Asia during the Late Cretaceous epoch, about 75 million to 71 million years ago. Two species are currently recognized, although others have been assigned in th ...

)'', only slightly shorter than the femur. The nostril is large and oval-shaped. The elliptical maxillary fenestra, an opening in front of the

antorbital fenestra An antorbital fenestra (plural: fenestrae) is an opening in the skull that is in front of the eye sockets. This skull character is largely associated with Archosauriformes, archosauriforms, first appearing during the Triassic Period. Among Extant ...

, is large and located in the middle of the antorbital fossa. The antorbital fenestra itself only occupies 40% of the length of this depression. These fenestrae are augmented by a rather large promaxillary fenestra at the front of the antorbital fossa, similar to that of ''

Archaeopteryx ''Archaeopteryx'' (; ), sometimes referred to by its German name, "" ( ''Primeval Bird'') is a genus of bird-like dinosaurs. The name derives from the ancient Greek (''archaîos''), meaning "ancient", and (''ptéryx''), meaning "feather" ...

'', as well as an additional smaller opening (an accessory fenestrae) slightly lower than the promaxillary fenestra, between it and the maxillary fenestra. The strut separating the maxillary fenestra from the antorbital fenestra is pierced by a connecting channel, a typical troödontid trait. The eye socket is longer than high. In front of this orbit, the

has a long and robust horn-like process, oriented sideways at its base and gradually curving upwards at its tip. The generally recurved teeth are more slender and tightly packed in the front of the jaws, such as in the

praemaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammals has ...

and the anterior part of the dentary in the lower jaw. Further back the teeth become larger and widely spaced. The tooth row in the upper jaw is exceptionally long, reaching to a position below the front of the eye socket. The front teeth, including the first maxillary teeth, do not have serrations. Teeth in the middle of the dentary are more strongly recurved and possess serrated rear edges. At the rear of the dentary, the teeth are short and stout with serrations on both, mesial and distal, edges. ''Caihong'' probably has ten neck vertebrae, thirteen back vertebrae, five sacral vertebrae and 26 tail vertebrae. The dorsal vertebrae of the back do not possess pleurocoels. The tail is short, with a length of 178 millimetres. The transition to the middle tail, defined as the point behind which transverse processes are completely lacking, is between the seventh and eighth vertebra. Vertebrae more to the rear gradually become more elongated. The arm is relatively short, equal to 60% of the hindlimb length. Especially the

humerus The humerus (; : humeri) 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 (bone), radius and ulna, and consists of three sections. The humeral upper extrem ...

is short, 42.1 millimetres long, with 60% of thighbone length, compared to a ratio of 100% with ''Anchiornis''. The

ulna The ulna or ulnar bone (: ulnae or ulnas) is a long bone in the forearm stretching from the elbow to the wrist. It is on the same side of the forearm as the little finger, running parallel to the Radius (bone), radius, the forearm's other long ...

is longer than the upper arm, a trait which in theropods is generally limited to flying birds. The ulna has a length of 47.2 millimetres and is only lightly bowed. In the hand the first

metacarpal In human anatomy, the metacarpal bones or metacarpus, also known as the "palm bones", are the appendicular bones that form the intermediate part of the hand between the phalanges (fingers) and the carpal bones ( wrist bones), which articulate ...

—applying the traditional numbering in which the three digits are the first, second and third—as well as the first finger are long, the third metacarpal is robust and the third phalanx of the third finger is longer than the first and second phalanges combined, which are all typical troödontid proportions. In the pelvis, the blade of the ilium is long before the position of the hip joint, but short and slightly curved downwards behind it. The

pubic bone In vertebrates, the pubis or pubic bone () forms the lower and anterior part of each side of the hip bone. The pubis is the most forward-facing (ventral and anterior) of the three bones that make up the hip bone. The left and right pubic bones ar ...

is slightly directed to the rear. It ends in a hook-shaped "foot", as with the

Unenlagiidae Unenlagiidae is a proposed family of eumaniraptoran paravians that includes the subfamilies Unenlagiinae and possibly Halszkaraptorinae. Fossils of both subfamilies have been found in both Gondwanan and Laurasian deposits. The biology of the g ...

. The ischium has a length of 20.5 millimetres. It is relatively short and wide in side view, forming a flat strap. Its obturator process on the front edge, has a rectangular profile, which is rare among the

Coelurosauria Coelurosauria (; from Greek, meaning "hollow-tailed lizards") is the clade containing all theropod dinosaurs more closely related to birds than to carnosaurs. Coelurosauria is a subgroup of theropod dinosaurs that includes compsognathids, tyra ...

: except ''Anchiornis'' and ''Archaeopteryx'' all other coelurosaurs possess a triangular process. The hindlimb is very long, 3.1 times as long as the torso. The thighbone is elongated and has a length of 70.9 millimetres. The shinbone is approximately 82 millimetres long. It is fused with the upper ankle bones into a tibiotarsus. The third

metatarsal The metatarsal bones or metatarsus (: metatarsi) are a group of five long bones in the midfoot, located between the tarsal bones (which form the heel and the ankle) and the phalanges ( toes). Lacking individual names, the metatarsal bones are ...

has a length of 49 millimetres and is at the top somewhat pinched by the second and fourth metatarsal. The first toe is relatively short. The second toe bears a small retractable sickle claw.

Feathering and coloration

The fossil is surrounded by a wide zone of feathers, only the snout and claws being uncovered. These feathers are not impressions but remains of the original plumage. The splitting of slab and counterslab revealed a complex layering of the plumage. The density of visible shafts and barbs is so extreme that it is difficult to identify individual feathers. The way they are presently preserved may not represent their original condition and position. The body contour feathers of ''Caihong'' are relatively longer than those of other non-avian dinosaurs. Skull and neck show two types of them: the first is stiff, two centimetres long and elongated; the other has a length of one centimetre and is more wavy. On the breast and limbs, four centimetres long feathers are visible with parallel straight and thick barbs. None of these contour feathers could be definitely identified as being pennaceous, with closed vanes. Caihong juji life restoration 3

The fossilized feathers of ''Caihong'' possessed nanostructures which were analyzed and interpreted as

melanosome A melanosome is an organelle found in animal cells and is the site for synthesis, storage and transport of melanin, the most common light-absorbing pigment found in the animal kingdom. Melanosomes are responsible for color and photoprotectio ...

s, showing similarity to organelles that produce a black

iridescent Iridescence (also known as goniochromism) is the phenomenon of certain surfaces that appear gradually to change colour as the angle of view or the angle of illumination changes. Iridescence is caused by wave interference of light in microstruc ...

color in extant birds. Other feathers found on the head, chest, and the base of the tail preserve flattened sheets of

platelet Platelets or thrombocytes () are a part of blood whose function (along with the coagulation#Coagulation factors, coagulation factors) is to react to bleeding from blood vessel injury by clumping to form a thrombus, blood clot. Platelets have no ...

-like melanosomes very similar in shape to those which create brightly colored iridescent hues in the feathers of modern

hummingbird Hummingbirds are birds native to the Americas and comprise the Family (biology), biological family Trochilidae. With approximately 366 species and 113 genus, genera, they occur from Alaska to Tierra del Fuego, but most species are found in Cen ...

s. However, these structures are seemingly solid and lack air bubbles, and thus are internally more akin to the melanosomes in

trumpeters The trumpet is a brass instrument commonly used in classical and jazz ensembles. The trumpet group ranges from the piccolo trumpet—with the highest register in the brass family—to the bass trumpet, pitched one octave below the standard B ...

than hummingbirds. ''Caihong'' represents the oldest-known evidence of platelet-like melanosomes.

References