''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 aviala ...

theropod Theropoda (; ), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three toes and claws on each limb. Theropods are generally classed as a group of saurischian dinosaurs. They were ancestrally c ...

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

from

China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...

that lived during the

Late Jurassic The Late Jurassic is the third epoch of the Jurassic Period, and it spans the geologic time from 163.5 ± 1.0 to 145.0 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic strata.Owen 1987. In European lithostratigraphy, the name ...

period.

Discovery and naming

At Gangou, Qinglong, in the north of

Hebei Hebei or , (; alternately Hopeh) is a northern province of China. Hebei is China's sixth most populous province, with over 75 million people. Shijiazhuang is the capital city. The province is 96% Han Chinese, 3% Manchu, 0.8% Hui, an ...

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 Jur ...

. 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 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 species that contains the biological type specimen ...

''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 bone is a small and fragile bone of the facial skeleton; it is roughly the size of the little fingernail. It is situated at the front part of the medial wall of the orbit. It has two surfaces and four borders. Several bony landmarks of ...

of the skull. The

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of several ...

, housed at the

Paleontological Museum of Liaoning Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossi ...

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 stages). It is known for its exceptionally preserved fossils, including those of ...

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 . 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 t ...

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

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 anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower tooth, teeth in place. The mandible sits beneath the maxilla. It is the only movabl ...

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 (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates with ...

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

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 term, ...

Paraves 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 avial ...

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

genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...

of the Yanliao Biota, such as ''

Eosinopteryx ''Eosinopteryx'' is an extinct genus of theropod dinosaurs known to the late Jurassic period of China. It contains a single species, ''Eosinopteryx brevipenna''. Discovery and naming ''E. brevipenna'' is known from a single fossil specimen recov ...

'' 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 skin ...

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 dinosaur 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 the pa ...

)'', 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 archosauriforms, first appearing during the Triassic Period. Among extant archosaurs, birds ...

, 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 (''archaīos''), 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 mammal has b ...

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 twenty-six tail vertebrae. The dorsal vertebrae of the back do not possess

pleurocoel Skeletal pneumaticity is the presence of air spaces within bones. It is generally produced during development by excavation of bone by pneumatic diverticula (air sacs) from an air-filled space, such as the lungs or nasal cavity. Pneumatization is h ...

s. 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 (; ) 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 roun ...

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

ulna The ulna (''pl''. ulnae or ulnas) is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. That is, the ulna is on the same side of t ...

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 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 ...

— 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 pubic region ( la, pubis) is the most forward-facing (ventral and anterior) of the three main regions making up the coxal bone. The left and right pubic regions are each made up of three sections, a superior ramus, inferior ra ...

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 gro ...

. 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, tyrann ...

: 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 eighty-two millimetres long. It is fused with the upper ankle bones into a tibiotarsus. The third

metatarsal The metatarsal bones, or metatarsus, are a group of five long bones in the foot, located between the tarsal bones of the hind- and mid-foot and the phalanges of the toes. Lacking individual names, the metatarsal bones are numbered from the med ...

has a length of forty-nine 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 photoprotection i ...

s, showing similarity to organelles that produce a black

iridescent Iridescence (also known as goniochromism) is the phenomenon of certain surfaces that appear to gradually change color as the angle of view or the angle of illumination changes. Examples of iridescence include soap bubbles, feathers, butterfl ...

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

platelet Platelets, also called thrombocytes (from Greek θρόμβος, "clot" and κύτος, "cell"), are a component of blood whose function (along with the coagulation factors) is to react to bleeding from blood vessel injury by clumping, thereby ini ...

-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 biological family Trochilidae. With about 361 species and 113 genera, they occur from Alaska to Tierra del Fuego, but the vast majority of the species are found in the tropics aro ...

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 ...

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

References