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''Jianianhualong'' (meaning "Jianianhua dragon") is a
genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
of
troodontid Troodontidae is a clade of bird-like theropod dinosaurs from the Late Jurassic to Late Cretaceous. During most of the 20th century, troodontid fossils were few and incomplete and they have therefore been allied, at various times, with many dinos ...
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 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 143.1 ...
of
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 ...
. It contains a single species, ''Jianianhualong tengi'', named in 2017 by Xu Xing and colleagues based on an articulated skeleton preserving feathers. The feathers at the middle of the tail of ''Jianianhualong'' are asymmetric, being the first record of asymmetrical feathers among the troodontids. Despite
aerodynamic Aerodynamics () is the study of the motion of atmosphere of Earth, air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dynamics and its subfield of gas dynamics, and is an ...
differences from the flight feathers of modern birds, the feathers in the tail vane of ''Jianianhualong'' could have functioned in
drag Drag or The Drag may refer to: Places * Drag, Norway, a village in Tysfjord municipality, Nordland, Norway * ''Drág'', the Hungarian name for Dragu Commune in Sălaj County, Romania * Drag (Austin, Texas), the portion of Guadalupe Street a ...
reduction whilst the animal was moving. The discovery of ''Jianianhualong'' supports the notion that asymmetrical feathers appeared early in the evolutionary history of the
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 ...
. ''Jianianhualong'' possesses a combination of traits seen in basal as well as traits seen in derived troodontids. This is consistent with its
phylogenetically In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical data ...
intermediate position among the troodontids. This mixture of traits shows a distinct spatial organization, with basal traits being present in the forelimbs and pelvis, and derived traits being present in the skull and hindlimbs. This may represent a case of
mosaic evolution Mosaic evolution (or modular evolution) is the concept, mainly from palaeontology, that evolutionary change takes place in some body parts or systems without simultaneous changes in other parts. Another definition is the "evolution of characters ...
, where
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the Heredity, heritable traits characteristic of a population over generation ...
acts upon the form of the body in a modular way. Similar transitional patterns of traits are seen in the troodontid ''
Sinusonasus ''Sinusonasus'' is a genus of dinosaurs from the Early Cretaceous Period, recovered from the Yixian Formation. It lived in what is now the Liaoning Province of China. ''Sinusonasus'' was a theropod, specifically a troodontid dinosaur. The type ...
'', a close relative of ''Jianianhualong''. Ecologically speaking, ''Jianianhualong'' and ''Sinusonasus'' are part of the considerable diversity of troodontids that is present within the
Yixian Formation The Yixian Formation (; formerly Romanization of Chinese, transcribed as Yihsien Formation or Yixiang Formation) is a geological formation in Jinzhou, Liaoning, People's Republic of China, that spans the Barremian stage of the Early Cretaceous. I ...
.


Discovery and naming

''Jianianhualong'' is known from the
type Type may refer to: Science and technology Computing * Typing, producing text via a keyboard, typewriter, etc. * Data type, collection of values used for computations. * File type * TYPE (DOS command), a command to display contents of a file. * ...
and only specimen, a nearly complete skeleton preserving feathers that is missing only the end of the tail. It is articulated and compressed on a stone plate. This
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 ...
specimen is stored under the collection number DLXH 1218 in the Dalian Xinghai Museum,
Liaoning Province ) , image_skyline = , image_alt = , image_caption = Clockwise: Mukden Palace in Shenyang, Xinghai Square in Dalian, Dalian coast, Yalu River at Dandong , image_map = Liaoning in China (+all claims hatched).svg , ...
,
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 ...
. It was excavated using mechanical tools (
geologist's hammer A geologist's hammer, also known as rock hammer, rock pick, geological pick, or geo pick, is a specialized hammer used for splitting and breaking rocks. In field geology, it is employed to expose fresh rock surfaces, as weathered surfaces may ...
s and
chisel A chisel is a hand tool with a characteristic Wedge, wedge-shaped cutting edge on the end of its blade. A chisel is useful for carving or cutting a hard material such as woodworking, wood, lapidary, stone, or metalworking, metal. Using a chi ...
s) from
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 143.1 ...
rocks of the
Yixian Formation The Yixian Formation (; formerly Romanization of Chinese, transcribed as Yihsien Formation or Yixiang Formation) is a geological formation in Jinzhou, Liaoning, People's Republic of China, that spans the Barremian stage of the Early Cretaceous. I ...
in the locality of Baicai Gou ( zh, c=白菜溝), located in the Yixian County of Liaoning. The Dakangpu Bed (equivalent to the Dawangzhangzi, Daxinfangzi, Hejiaxin, Sichakou, and Senjiru Beds), which Baicai Gou is a part of, is situated in the middle of the formation. The specific portion of the Beds from which ''Jianianhualong'' originates was dated in 2006 and 2008 to approximately 124.4 million years ago, which corresponds to the early
Aptian The Aptian is an age (geology), age in the geologic timescale or a stage (stratigraphy), stage in the stratigraphic column. It is a subdivision of the Early Cretaceous, Early or Lower Cretaceous epoch (geology), Epoch or series (stratigraphy), S ...
. In 2017, ''Jianianhualong'' was named and described by a team of researchers including Xu Xing, Philip J. Currie, Michael Pittman, Xing Lida, Meng Qingjin,
Lü Junchang Lü Junchang (; 1965 – 9 October 2018) was a Chinese palaeontologist and professor at the Institute of Geology, Chinese Academy of Geological Sciences. An expert on Mesozoic reptiles, he described and named dozens of dinosaur and pterosaur taxa ...
, Hu Dongyu, and Yu Congyu, in a research paper published in ''
Nature Communications ''Nature Communications'' is a peer-reviewed, open access, scientific journal published by Nature Portfolio since 2010. It is a multidisciplinary journal that covers the natural sciences, including physics, chemistry, earth sciences, medic ...
''. In order to investigate the preserved plumage of the specimen, Xu and colleagues made use of
laser-stimulated fluorescence Laser-induced fluorescence (LIF) or laser-stimulated fluorescence (LSF) is a spectroscopic method in which an atom or molecule is excited to a higher energy level by the absorption of laser light followed by spontaneous emission of light. It was fi ...
using a 405 nm laser. The
genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
name combines the name of a company that sponsored the research of this taxon, ''Jianianhua'' ( zh, c=嘉年華), and the suffix ''long'' ( zh, c=龍), meaning "dragon". 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 ...
, ''tengi'', honours Ms. Teng Fangfang who acquired the specimen and made it available for study.


Description

The type specimen of ''Jianianhualong'' measures approximately as preserved; since the back of the tail is missing, the animal would probably have been around long in life. Scaling from a femoral length of , Xu ''et al.'' found that it weighed . Despite its small size, the specimen was mature, as indicated by the fusion of the sutures in the vertebrae.


Skull

The skull of ''Jianianhualong'' is small and sub-triangular, with a short snout and a wide skull roof like '' Mei''.The frontmost-preserved bone of the snout is 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 ...
, the main body of which is taller than long. This contrasts ''Jianianhualong'' from all other members of the
Troodontidae Troodontidae is a clade of bird-like theropod dinosaurs from the Late Jurassic to Late Cretaceous. During most of the 20th century, troodontid fossils were few and incomplete and they have therefore been allied, at various times, with many dinos ...
, along with the top margin of the bone forming a large angle of 45° with the bottom margin. The rear branch of the bone is also deep, like derived troodontids but unlike ''
Sinovenator ''Sinovenator'' (meaning "Chinese hunter") is a genus of troodontid dinosaur from China. It is from the early Cretaceous Period (geology), Period. Discovery and naming Two specimens of a troodontidae, troodontid were described in 2002 in paleo ...
''. Behind the maxilla, the indentation known as the antorbital fossa bears two large openings (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 ...
and the elongated, enlarged maxillary fenestra) that stretch from the top margin to the bottom margin of the fossa; ''Sinovenator'', ''
Sinornithoides ''Sinornithoides'' (meaning "Chinese bird form") is a genus of troodontid theropod dinosaurs containing the single species ''Sinornithoides youngi''. ''S. youngi'' lived during the Early Cretaceous (Aptian/Albian stage, around 113 million years ...
'', and ''
Sinusonasus ''Sinusonasus'' is a genus of dinosaurs from the Early Cretaceous Period, recovered from the Yixian Formation. It lived in what is now the Liaoning Province of China. ''Sinusonasus'' was a theropod, specifically a troodontid dinosaur. The type ...
'' have one additional opening known as the promaxillary opening. The bar of bone separating the fenestrae is narrow, like in ''Sinovenator''. 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 ...
, encircling the fossa from above and behind, has a long forward-projecting branch much like other troodontids. However, the descending branch of the bone is also nearly the same length, and the descending branch also bears a unique ridge on its front portion. The
nasal bone The nasal bones are two small oblong bones, varying in size and form in different individuals; they are placed side by side at the middle and upper part of the face and by their junction, form the bridge of the upper one third of the nose. Eac ...
is short and broad like ''Mei''. The inner surface of the frontal suggests details about the
braincase In human anatomy, the neurocranium, also known as the braincase, brainpan, brain-pan, or brainbox, is the upper and back part of the skull, which forms a protective case around the brain. In the human skull, the neurocranium includes the calv ...
, including the widened
olfactory tract The olfactory tract (olfactory peduncle or olfactory stalk) is a bilateral bundle of afferent nerve fibers from the mitral and tufted cells of the olfactory bulb that connects to several target regions in the brain, including the piriform cort ...
and back of the
cerebrum The cerebrum (: cerebra), telencephalon or endbrain is the largest part of the brain, containing the cerebral cortex (of the two cerebral hemispheres) as well as several subcortical structures, including the hippocampus, basal ganglia, and olfac ...
. Compared to ''Sinovenator'', the forward branch of the
postorbital bone The ''postorbital'' is one of the bones in vertebrate skulls which forms a portion of the dermal skull roof and, sometimes, a ring about the orbit. Generally, it is located behind the postfrontal and posteriorly to the orbital fenestra. In some ...
is not as long. Overall, the lower jaw is subtriangular when viewed from the side, with a straight top margin and a curved bottom margin; it is not downturned, unlike ''Sinovenator''. The
dentary bone 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 i ...
is long (occupying half of the jaw's length), shallow, and triangular, and it bears a groove that widens towards the back of the bone. A distinct fossa on the top surface of the surangular bone separates ''Jianianhualong'' from other troodontids. The
angular bone The angular is a large bone in the lower jaw (mandible) of amphibians and reptiles (birds included), which is connected to all other lower jaw bones: the dentary (which is the entire lower jaw in mammals), the splenial, the suprangular, and the ...
projects upwards behind the dentary, like ''Sinovenator'' and various other members of the
Deinonychosauria Deinonychosauria is a clade of paravian dinosaurs which lived from the Late Jurassic to the Late Cretaceous periods. Fossils have been found across the globe in North America, Europe, Africa, Asia, South America, and Antarctica,Case, J.A., Marti ...
. The maxilla of ''Jianianhualong'' bore 21 teeth on each side, while the dentary bore 25 on each side. Like other troodontids, the teeth are short-crowned, strongly recurved, and unevenly distributed. The teeth at the front of the jaws are more closely packed than the rear teeth, which also have fine serrations on their rear edges. These serrations are fine, as in ''Sinovenator'', instead of robust as in derived troodontids. Unusually, the first several teeth in the dentary appear to be angled forwards, or procumbent. This condition is present in several contemporary dromaeosaurids.


Axial skeleton

The vertebral column of ''Jianianhualong'' is nearly complete. However, individual vertebrae are not always possible to discern. The neck (i.e. the
cervical vertebrae In tetrapods, cervical vertebrae (: vertebra) are the vertebrae of the neck, immediately below the skull. Truncal vertebrae (divided into thoracic and lumbar vertebrae in mammals) lie caudal (toward the tail) of cervical vertebrae. In saurop ...
) is long, the torso (i.e. the
dorsal vertebrae In vertebrates, thoracic vertebrae compose the middle segment of the vertebral column, between the cervical vertebrae and the lumbar vertebrae. In humans, there are twelve thoracic vertebra (anatomy), vertebrae of intermediate size between the ce ...
) is long, and the tail (i.e. the caudal vertebrae) is long. Overall, there may have been 31 caudal vertebrae in total. Uniquely among troodontids, the
neural spine Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spinal ...
of the
axis An axis (: axes) may refer to: Mathematics *A specific line (often a directed line) that plays an important role in some contexts. In particular: ** Coordinate axis of a coordinate system *** ''x''-axis, ''y''-axis, ''z''-axis, common names ...
, the second cervical vertebra, has a convex top margin. Also uniquely, the front rim of the neural spine is thickened, while the rear portion is strongly expanded at the top. The length of the cervicals increases until the fifth to seventh cervicals, which have the greatest length. Further back, the remaining cervicals are significantly shorter. The front also dorsal vertebrae are relatively short, with the middle and back dorsals being longer. The dorsal
rib In vertebrate anatomy, ribs () are the long curved bones which form the rib cage, part of the axial skeleton. In most tetrapods, ribs surround the thoracic cavity, enabling the lungs to expand and thus facilitate breathing by expanding the ...
s lack uncinate process, and the
sternum The sternum (: sternums or sterna) 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 bl ...
is also unossified, like other troodontids. Like other theropods, the first pair of the
gastralia Gastralia (: gastralium) are dermal bones found in the ventral body wall of modern crocodilians and tuatara, and many prehistoric tetrapods. They are found between the sternum and pelvis, and do not articulate with the vertebrae. In these reptil ...
near the front of the torso is noticeably more robust, particularly in the central portion. Similar to other
paravians 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 avi ...
, the caudal vertebrae of ''Jianianhualong'' become increasingly longer towards the back of the tail; for instance, the fifteenth caudal is about twice as long as the sixth. From the twenty-third caudal backwards, the sides of the caudals become compressed such that they are sub-triangular. Towards the back of the tail, the forward and rear projections of the
chevrons Chevron (often relating to V-shaped patterns) may refer to: Science and technology * Chevron (aerospace), sawtooth patterns on some jet engines * Chevron (anatomy), a bone * '' Eulithis testata'', a moth * Chevron (geology), a fold in rock la ...
are roughly equal in length. Unlike ''Sinusonasus'', where chevrons come into contact with each other, there are gaps between the chevrons of ''Jianianhualong''.


Limbs

The bottom of the large, four-sided
coracoid A coracoid is a paired bone which is part of the shoulder assembly in all vertebrates except therian mammals (marsupials and placentals). In therian mammals (including humans), a coracoid process is present as part of the scapula, but this is n ...
extends backwards with a hook-like projection, the postglenoid process, which is similar in length to that of ''Sinornithoides'' but longer than that of ''Sinovenator''. Unlike ''Sinornithoides'' but like ''Mei'', the
furcula The (Latin for "little fork"; : furculae) or wishbone is a forked bone found in most birds and some species of non-avian dinosaurs, and is either an interclavicle or formed by the fusion of the two clavicles. In birds, its primary function is ...
in ''Jianianhualong'' is robust, flattened, and U-shaped like that of ''Mei''. Like derived troodontids, the slender
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 shorter than the femur. 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 ...
exhibits a slight backward bow; ''Mei'' has a much stronger bow, while the bow is absent in most other troodontids altogether. Contrary to most other theropods, the top portion of the radius bears a backwards bow similar to that of the ulna. The hand bears three digits, typical of the Maniraptora, which respectively bear 2, 3, and 4
phalanges The phalanges (: phalanx ) are digit (anatomy), digital bones in the hands and foot, feet of most vertebrates. In primates, the Thumb, thumbs and Hallux, big toes have two phalanges while the other Digit (anatomy), digits have three phalanges. ...
. The overall length of the hand is . Several distinguishing characteristics of ''Jianianhualong'' are found among the phalanges. The first phalanx of the first digit is unusually long; it bears a "heel" at its top end, as well as a large groove on the interior surface that extends for more than half the bone's length. Additionally, the second phalanx of the second digit is also very long, being longer than the
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 ...
of the corresponding digit. The claws are highly robust, with their bases being at least half as wide as their overall length; the claw of the first digit is the largest. The pelvis of ''Jianianhualong'' is overall similar to that of ''Sinovenator''. Like ''
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" ...
'' and basal dromaeosaurids, the ilium is small. A distinguishing trait is that the top margin of the ilium is slightly concave. Below the ilium, the left and right pubes are conjoined into the unusually broad "pubic apron" that is characteristic of troodontids, with a ridge on the rear surface as in ''Sinovenator''. Behind the pubes, the forward-projecting
obturator process The obturator process is an anatomical feature on the pelvis of archosaurs. It is a raised area of the ischium bone of the pelvis. It is the origin of muscles that attach to the femur and aid in running. These muscles are called M. pubo-ischio-fem ...
of the
ischium The ischium (; : is ...
is located near the bottom of the bone. There is a small recess, or lamina, on the margin of the process, another distinguishing trait of ''Jianianhualong''. Among the four
metatarsal bones 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 ...
, the second is the shortest and most slender after the first (corresponding to the
hallux Toes are the digits of the foot of a tetrapod. Animal species such as cats that walk on their toes are described as being ''digitigrade''. Humans, and other animals that walk on the soles of their feet, are described as being ''plantigrade''; ...
). The third metatarsal is shorter relative to the femur than that of ''Sinovenator''. Unlike other troodontids, the bottom end of the second metatarsal lacks a
hinge joint A hinge joint (ginglymus or ginglymoid) is a bone joint where the articular surfaces are molded to each other in such a manner as to permit motion only in one plane. According to one classification system they are said to be uniaxial (having o ...
, and a flange on the bottom of metatarsal IV is also missing. The latter is a distinguishing characteristic. Further below, like derived troodontids and dromaeosaurids, the "sickle claw" of the second digit is strongly curved and bears a prominent flexor tubercle (the portion which articulates with the phalanx). It is also much larger than the other phalanges in the digit; it is longer than the first phalanx, whereas they have the same length in ''Sinovenator''.


Feathering

Feathers are preserved alongside the entire vertebral column of ''Jianianhualong''. On the bottom of the neck, feathers are preserved with a length of at least . Those along the back and over the hip are longer, with a length of about . Behind the tibia, there are also some feathers measuring in length. Additionally, some poorly preserved feathers are associated with the humerus and ulna, having an unknown length. Like '' Jinfengopteryx'', ''
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 ...
'', and ''Archaeopteryx'', the tail of ''Jianianhualong'' supports a frond-like fan of
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 along its entire length. Unlike ''Archaeopteryx'' and potentially ''Jinfengopteryx'', the feathers are curved. Feathers from the middle of the tail are long. At least some of the long pennaceous feathers lining the tail are asymmetric; one feather has a trailing (hind) vane that is about twice as wide as the leading (front) vane, with the barbs being respectively at angles of 10° and 15° for the two vanes.


Classification

A number of characteristics allow ''Jianianhualong'' to be identified as a member of the Troodontidae. These include the long forward-projecting branch and flange of the lacrimal bone; the foramina on the nasal bone; the smooth transition between the eye socket and the backward-projecting branch of the frontal bone; the ridge on the forward-projecting branch of the jugal bone; the triangular dentary bearing a widening groove; the robust forward-projecting branch of the surangular bone; the relatively large number of unevenly-distributed teeth; the flattened chevrons with blunt forward projections and bifurcated backward projections; and the broad and flat "pubic apron" formed by the pubic bones. Within the troodontids, ''Jianianhualong'' displays a unique combination of characteristics from both basal ("unspecialized") and derived ("specialized") troodontids, organized in distinct regions of its body. The forelimbs and pelvis largely resemble basal troodontids such as ''Sinovenator'': the short deltopectoral crest on the humerus; the long hand, metacarpal II, and phalanx III-2 on the hand; the small ilium; the ridge on the pubic apron; and the short ischium with two projections and an obturator process near the bottom of the bone. At the same time, the skull and hindlimbs resemble derived troodontids: the deep jugal branch of the maxilla bearing a prominent groove, the sub-triangular antorbital fossa; the relatively long maxillary fenestra; the short jugal branch of the postorbital bone; the long hallux; the slender and short metatarsal II; the "heel" on phalanx II-2; and the large, curved sickle claw. ''Sinusonasus'' exhibits a similar condition: the skull is like those of basal troodontids, and the pelvis and hindlimb are like those of derived troodontids. In a
phylogenetic analysis In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical data ...
, Xu and colleagues found ''Jianianhualong'' in a
polytomy An internal node of a phylogenetic tree is described as a polytomy or multifurcation if (i) it is in a rooted tree and is linked to three or more child subtrees or (ii) it is in an unrooted tree and is attached to four or more branches. A tree ...
with ''Sinusonasus'' and several other intermediate troodontids. For their 2019 description of ''
Hesperornithoides ''Hesperornithoides'' (meaning "western bird form"; nicknamed "Lori") is a genus of troodontid theropod dinosaur that lived in North America during the Late Jurassic period. Discovery Although several Troodontid teeth were found, with the trood ...
'', Scott Hartman and colleagues added ''Jianianhualong'' to an expanded and updated version of the phylogenetic dataset that was used for the analysis of Xu and colleagues. They found it as a member of the
Sinovenatorinae Troodontidae is a clade of bird-like theropod dinosaurs from the Late Jurassic to Late Cretaceous. During most of the 20th century, troodontid fossils were few and incomplete and they have therefore been allied, at various times, with many dinos ...
, a group that was named in 2017 by Caizhi Shen and colleagues in their description of ''
Daliansaurus ''Daliansaurus'' (meaning "Dalian reptile") is a genus of small troodontid theropod dinosaur, measuring approximately long, from the Early Cretaceous of China. It contains a single species, ''D. liaoningensis'', named in 2017 by Shen and colleag ...
''. However, Hartman and colleagues noted that a position similar to that found by Xu and colleagues was also plausible: forcing ''Jianianhualong'' to be closer to the more derived
Troodontinae Troodontidae is a clade of bird-like theropod dinosaurs from the Late Jurassic to Late Cretaceous. During most of the 20th century, troodontid fossils were few and incomplete and they have therefore been allied, at various times, with many dinos ...
required a
phylogenetic tree A phylogenetic tree or phylogeny is a graphical representation which shows the evolutionary history between a set of species or taxa during a specific time.Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA. In ...
that was only one step longer, and thus marginally less likely according to the principle of
maximum parsimony In phylogenetics and computational phylogenetics, maximum parsimony is an optimality criterion under which the phylogenetic tree that minimizes the total number of character-state changes (or minimizes the cost of differentially weighted charact ...
. The strict consensus phylogenetic tree recovered by Xu and colleagues is reproduced below at left. The partially expanded strict consensus tree recovered by Hartman and colleagues is reproduced below at right (note that
Troodontinae Troodontidae is a clade of bird-like theropod dinosaurs from the Late Jurassic to Late Cretaceous. During most of the 20th century, troodontid fossils were few and incomplete and they have therefore been allied, at various times, with many dinos ...
includes ''
Troodon ''Troodon'' ( ; ''Troödon'' in older sources) is a controversial genus of relatively small, bird-like theropod dinosaurs definitively known from the Campanian age of the Late Cretaceous period (about 77 million years ago). It includes at l ...
'', ''
Saurornithoides ''Saurornithoides'' ( ) is a genus of troodontid maniraptoran dinosaur, which lived during the Late Cretaceous period. These creatures were predators, which could run fast on their hind legs and had excellent sight and hearing. The name is deri ...
'', and ''
Zanabazar Öndör Gegeen Zanabazar (born Eshidorji) was the first '' Jebtsundamba Khutuktu'' and the first ''Bogd Gegeen'' or supreme spiritual authority, of the Gelugpa (Yellow Hat) lineage of Tibetan Mahayana Buddhism in Mongolia. The son of a Mongol ...
''). Topology A: Xu ''et al.'' (2017) Topology B: Hartman ''et al.'' (2019)


Implications for troodontid evolution

The conjunction of basal and derived features as distinct regions of the body in ''Jianianhualong'' and ''Sinusonasus'' represent a possible case of
mosaic evolution Mosaic evolution (or modular evolution) is the concept, mainly from palaeontology, that evolutionary change takes place in some body parts or systems without simultaneous changes in other parts. Another definition is the "evolution of characters ...
, in which
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the Heredity, heritable traits characteristic of a population over generation ...
acts upon distinct "modules" of the body. Indeed, the pattern in which basal and derived features are present seems to follow the domains of the body that are regulated by
Hox gene Hox genes, a subset of homeobox, homeobox genes, are a gene cluster, group of related genes that Evolutionary developmental biology, specify regions of the body plan of an embryo along the craniocaudal axis, head-tail axis of animals. Hox protein ...
s, which have previously been discussed in the context of
pterosaur Pterosaurs are an extinct clade of flying reptiles in the order Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 million to 66 million years ago). Pterosaurs are the earli ...
s such as ''
Darwinopterus ''Darwinopterus'' (meaning "Darwin's wing") is a genus of pterosaur, discovered in China and named after biologist Charles Darwin. Between 30 and 40 fossil specimens have been identified, all collected from the Tiaojishan Formation, which dates t ...
''. However, this hypothesis is weakened by the presence of characteristics in both troodontids that do not follow the pattern. In ''Jianianhualong'', for instance, the large antorbital fenestra, long tooth row of the maxilla, and finely-serrated teeth represent basal characteristics in an otherwise derived skull. Similarly, digit II of the foot in ''Sinusonasus'' is not as specialized as those of derived troodontids, despite the hindlimb being overall derived. Conversely, the forelimb of ''Jianianhualong'' is short overall as in derived troodontids, despite the presence of basal traits. An
ancestral state reconstruction Ancestral reconstruction (also known as ''Character Mapping'' or ''Character Optimization'') is the extrapolation back in time from measured characteristics of individuals, populations, or species to their common ancestors. It is an important appli ...
conducted along with the phylogenetic analysis (results labelled in the above phylogenetic tree) suggests that wing feather asymmetry evolved only once, at the
last common ancestor A most recent common ancestor (MRCA), also known as a last common ancestor (LCA), is the most recent individual from which all organisms of a set are inferred to have descended. The most recent common ancestor of a higher taxon is generally assu ...
of the Paraves. Asymmetrical tail feathers would then also have evolved once, at the common ancestor of all paravians excluding scansoriopterygids and avialans. However, given the uncertainty of whether dromaeosaurids possessed asymmetrical wing feathers, the support for a single emergence of asymmetrical wing feathers in the common ancestor of the Paraves is equivocal, the alternative scenario that it evolved twice being plausible. The distinct possibility thus remains that troodontids had asymmetrical tail feathers but symmetrical arm feathers, suggesting that asymmetry in the tail appeared first, but this is not supported by the ancestral state reconstruction.


Paleobiology

''Jianianhualong'' preserves the first evidence of feathers in an unquestionable troodontid, with the troodontid affinities of taxa such as ''Anchiornis'', ''
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 ...
'', ''Jinfengopteryx'', and ''Xiaotingia'' having been questioned by various studies. Additionally, ''Jianianhualong'' represents the first record of asymmetrical feathers among troodontids, thus making troodontids the second group among non-avialan theropods to possess asymmetrical feathers; asymmetrical arm feathers are found in the
microraptoria Microraptoria (Greek, μίκρος, ''mīkros'': "small"; Latin, ''raptor'': "one who seizes") is a clade of basal Dromaeosauridae, dromaeosaurid theropod dinosaurs. Definitive microraptorians lived during the Barremian to Aptian stages of the Ear ...
ns ''Microraptor'', ''
Changyuraptor ''Changyuraptor'' is a genus of "four-winged", predatory dinosaurs. It is known from a single fossil specimen representing the species ''Changyuraptor yangi'', which was discovered from Early Cretaceous (125 million year old) deposits in Liaonin ...
'', and possibly ''
Zhenyuanlong ''Zhenyuanlong'' (meaning ''"Zhenyuan's dragon"'', from Chinese language, Chinese Pinyin :wikt:龙, 龙/:wikt:龍, 龍 lóng ''"dragon"'') is a genus of Dromaeosauridae, dromaeosaurid dinosaur from the Yixian Formation of Liaoning, China. It liv ...
''. Asymmetrical feathers are correlated with flight capabilities among paravians, and their emergence marked a major step in the evolution of modern avian feathers. This trait may have been selected for due to its augmentation of locomotion on the ground or in the air. However, the presence of asymmetrical feathers in ''Jianianhualong'' does not guarantee that it was capable of either gliding or powered flight. The feathers in the tail of ''Jianianhualong'' could certainly have functioned in either maintaining feather stability (the barbs of the leading edge, having a low angle of 10°, augmented the vane's rigidity) or controlling airflow (through the variation of the feathers'
blade pitch Blade pitch or simply pitch refers to the angle of a blade in a fluid. The term has applications in aeronautics, shipping, and other fields. Aeronautics In aeronautics, blade pitch refers to the angle of the blades of an aircraft propeller or ...
). However, these feathers differ from those of modern birds in that the barbs of the trailing edge also have a relatively low angle of approximately 15°. Thus, aerodynamically speaking, the feathers in the tail of ''Jianianhualong'' likely did not have the same function as those of modern birds. Regardless, the subtriangular outline of the tail frond, as well as the slots separating the tips of individual feathers, would have provided
drag Drag or The Drag may refer to: Places * Drag, Norway, a village in Tysfjord municipality, Nordland, Norway * ''Drág'', the Hungarian name for Dragu Commune in Sălaj County, Romania * Drag (Austin, Texas), the portion of Guadalupe Street a ...
reduction when the tail was in use.


Paleoecology

After ''Mei'', ''Sinovenator'', ''Sinusonasus'', and ''Jinfengopteryx'', ''Jianianhualong'' is the fifth named troodontid that has been discovered in the
Jehol Biota 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 d ...
as a whole, with the first three other troodontids also originating from the Yixian Formation. With ''Jianianhualong'' and ''Sinusonasus'' representing "transitional" and more derived forms of troodontids compared to ''Mei'' and ''Sinovenator'', the diversity of troodontids within the Jehol was appreciable. Asides from troodontids, the Yixian Formation also preserves a diverse assemblage of other animals, both dinosaurian and non-dinosaurian. The
hadrosauroid Hadrosauroidea is a clade or superfamily of ornithischian dinosaurs that includes the "duck-billed" dinosaurs, or Hadrosauridae, and all dinosaurs more closely related to them than to ''Iguanodon''. Their remains have been recovered in Asia, Euro ...
''
Jinzhousaurus ''Jinzhousaurus'' is a genus of hadrosauroid dinosaur of the Early Cretaceous of China. The type species is ''Jinzhousaurus yangi''. The generic name refers to the town Jinzhou. The specific name honours Yang Zhongjian is the founder of Chinese p ...
'' is known from Baicai Gou, the same locality as that of ''Jianianhualong'', as is the maniraptoran ''Yixianosaurus'' as well as hundreds of specimens from the
choristodere Choristodera (from the Greek χωριστός ''chōristos'' + δέρη ''dérē'', 'separated neck') is an extinct order of semiaquatic diapsid reptiles that ranged from the Middle Jurassic, or possibly Triassic, to the Miocene (168 to 20 or p ...
''
Hyphalosaurus ''Hyphalosaurus'' (meaning "submerged lizard") is a genus of freshwater aquatic reptiles, belonging to the extinct order Choristodera. They lived during the early Cretaceous period (late Barremian to early Aptian age), about 123-120 million year ...
''. From elsewhere in the Dakangpu/Dawangzhangzi Beds, specimens have been discovered belonging to the birds ''
Confuciusornis ''Confuciusornis'' is a genus of basal crow-sized avialan 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, bu ...
'', ''
Hongshanornis ''Hongshanornis'' is a genus of ornithuromorph birds known from early Cretaceous lake deposits of the Yixian Formation, Inner Mongolia, China. The holotype specimen, recovered in 2005, is currently held by the Institute of Vertebrate Paleontology ...
'', ''
Grabauornis ''Grabauornis'' is an extinct genus of enantiornitheans from the Early Cretaceous of China. The type species, ''Grabauornis lingyuanensis'', was named and described in 2014 by Johan Dalsätt, Per Ericson and Zhou Zhonghe. The genus name combin ...
'', ''
Liaoxiornis ''Liaoxiornis'' is a dubious genus of enantiornithean bird. The only named species is ''Liaoxiornis delicatus'', described by Hou and Chen in 1999.Hou, L., Chen, P.-J. (1999) "Liaoxiornis delicatus gen. et sp. nov., the smallest Mesozoic bird. C ...
'', ''
Longicrusavis ''Longicrusavis'' (meaning "long shin bird" in Latin) is an extinct genus of basal ornithuromorph bird found only at Dawangzhangzi village in Liaoning Province, China. ''Longicrusavis'' was a ground dwelling carnivore, a wader, and part of bio ...
'', ''
Shanweiniao ''Shanweiniao'' is a genus of long-snouted enantiornithean birds from Early Cretaceous China. One species is known, ''Shanweiniao cooperorum''. There is one known fossil, a slab and counterslab. The fossil is in the collection of the Dalian Nat ...
'', '' Shenqiornis'', and ''
Zhongornis ''Zhongornis'' (meaning "intermediate bird"Li, D.; Sulliven, C.; Zhou, Z; Zhang, Z. (2010). "Basal birds from China: a brief review." ''Chinese Birds'', 1(2): 83-96 ) is a genus of primitive maniraptoran dinosaurs that lived during the Early Cret ...
''; the non-avialan theropod ''
Sinosauropteryx ''Sinosauropteryx'' (meaning "Chinese reptilian wing") is an extinct genus of coelurosaurian theropod dinosaurs. Described in 1996, it was the first dinosaur taxon outside of Avialae (birds and their immediate relatives) to be found with eviden ...
''; the
ceratopsia Ceratopsia or Ceratopia ( or ; Ancient Greek, Greek: "horned faces") is a group of herbivore, herbivorous, beaked dinosaurs that thrived in what are now North America, Asia and Europe, during the Cretaceous Period (geology), Period, although ance ...
n ''
Psittacosaurus ''Psittacosaurus'' ( ; "parrot lizard") is a genus of extinct ceratopsian dinosaur from the Early Cretaceous of what is now Asia, existing between 125 and 105 million years ago. It is notable for being the most species-rich non-avian dinosaur ...
''; the pterosaur ''
Cathayopterus ''Cathayopterus'' is an extinct genus of ctenochasmatid pterosaur from the Early Cretaceous-age Yixian Formation of Liaoning, China. The name means "China wing", using the word "Cathay" as an old alternative name for China. The type species is '' ...
''; the lizards '' Dalinghosaurus'' and ''
Yabeinosaurus ''Yabeinosaurus'' is an extinct genus of lizard from the Early Cretaceous Jehol Group of northeastern China. The type species ''Yabeinosaurus tenuis'' is known from many well-preserved skeletons belonging to both juvenile and adult individuals. F ...
''; the
choristodere Choristodera (from the Greek χωριστός ''chōristos'' + δέρη ''dérē'', 'separated neck') is an extinct order of semiaquatic diapsid reptiles that ranged from the Middle Jurassic, or possibly Triassic, to the Miocene (168 to 20 or p ...
s ''Hyphalosaurus'' and ''
Monjurosuchus ''Monjurosuchus'' is a genus of choristoderan reptile that lived in what is now China and Japan during the Early Cretaceous. It has large eyes, a rounded skull, robust legs with short claws, and a long, thin tail. Fossils have been found that pr ...
'', of which the former is highly abundant; the turtle ''
Manchurochelys ''Manchurochelys'' is an extinct genus of turtle. It existed during the early Cretaceous of what is now northeast China. It has been found in the Jianshangou Bed of West Liaoning's Yixian Formation. However, it is a rarely found fossil. ''Manchu ...
''; the mammals ''
Acristatherium ''Acristatherium yanensis'' is an extinct basal eutherian from the Early Cretaceous (early Aptian, about ) Lujiatun Bed of the Yixian Formation. It was described on the basis of a single specimen (holotype) from Beipiao, Liaoning, China, by Yaom ...
'', ''
Akidolestes ''Akidolestes'' is an extinct genus of mammals of the family Spalacotheriidae, a group of mammals related to therians (the subclass containing marsupials and placentals). The genus name, ''Akidolestes'', is derived from ''akido'', Greek for p ...
'', '' Chaoyangodens'', ''
Eomaia ''Eomaia'' ("dawn mother") is a genus of extinct fossil mammals containing the single species ''Eomaia scansoria'', discovered in rocks that were found in the Yixian Formation, Liaoning Province, China, and dated to the Barremian Age of the Lower ...
'', ''
Sinobaatar ''Sinobaatar'' is a genus of extinct mammal from the Lower Cretaceous of China. It is categorized within the also extinct order Multituberculata and among these it belongs to the plagiaulacid lineage (a possible infraorder). ''Sinobaatar'' was a ...
'', and ''
Sinodelphys ''Sinodelphys'' is an extinct mammal from the Early Cretaceous, estimated to be 125 million years old. It was discovered and described in 2003 in rocks of the Yixian Formation in Liaoning Province, China, by a team of scientists including Zhe-X ...
''; the frog ''
Liaobatrachus ''Liaobatrachus'' (meaning "Liaoning frog") is a genus of prehistoric frog, the first fossil specimen of which was recovered from the Yixian Formation of Liaoning Province, China. It was the first Mesozoic The Mesozoic Era is the Era (geology) ...
''; and fish, including ''
Lycoptera ''Lycoptera'' is an extinct genus of fish that lived from Early Cretaceous, Lower Cretaceous, Barremian to Aptian in present-day China, North Korea, Mongolia and Siberia. Although there is record from Jurassic Formation in Siberia, its age remain ...
'' (of which ''L. davidi'' is numerous), ''
Peipiaosteus ''Peipiaosteus'' is an extinct genus of prehistoric Chondrostei, chondrostean Actinopterygii, ray-finned fish. Its fossils are found in the Early Cretaceous Jiufotang Formation, Pani Lake, Liaoning Province, China.Liu, H. T. and Zhou, J . J. 1 ...
'', and ''
Protopsephurus ''Protopsephurus'' is an extinct genus of paddlefish containing the single species ''Protopsephurus liui,'' known from the Yixian, Jiufotang and Huajiying formations in Liaoning, northern China from the Barremian to Aptian ages of the Early Cr ...
''. Environmentally, ''Jianianhualong'' lived in a
lacustrine A lake is often a naturally occurring, relatively large and fixed body of water on or near the Earth's surface. It is localized in a basin or interconnected basins surrounded by dry land. Lakes lie completely on land and are separate from t ...
area, as evinced by the
tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock co ...
aceous
sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...
present throughout the Dawangzhangzi Beds. Like the rest of the Yixian Formation, the environment represented by these assemblages was seasonally semi-arid, with a low mean air temperature of . The local
flora Flora (: floras or florae) is all the plant life present in a particular region or time, generally the naturally occurring (indigenous (ecology), indigenous) native plant, native plants. The corresponding term for animals is ''fauna'', and for f ...
consists of a mixed assemblage of
gymnosperm The gymnosperms ( ; ) are a group of woody, perennial Seed plant, seed-producing plants, typically lacking the protective outer covering which surrounds the seeds in flowering plants, that include Pinophyta, conifers, cycads, Ginkgo, and gnetoph ...
s, such as '' Czekanowskia'', '' Schizolepis'', and the ephedroids '' Amphiephedra'', '' Chengia'', '' Ephedrites'', and '' Liaoxia''; and also early
angiosperms Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (). The term angiosperm is derived from the Greek words (; 'container, vessel') and (; 'seed'), meaning that the seeds are enclosed within a fruit. T ...
, including ''
Archaefructus ''Archaefructus'' is an extinct genus of herbaceous aquatic seed plants with three known species. Fossil material assigned to this genus originates from the Yixian Formation in northeastern China, originally dated as late Jurassic but now unde ...
'', '' Hyrcantha'' (formerly '' Sinocarpus''), ''
Leefructus ''Leefructus'' is an extinct genus of eudicot plants which existed in Yixian Formation, China during the early Cretaceous period. It was first named by Ge Sun, David L. Dilcher, Hongshan Wang and Zhiduan Chen in 2011 and the type species I ...
'', and ''
Potamogeton ''Potamogeton'' is a genus of aquatic, mostly freshwater, plants of the family Potamogetonaceae. Most are known by the common name pondweed, although many unrelated plants may be called pondweed, such as Canadian pondweed (''Elodea canadensis' ...
''. ''Leefructus'' in particular is a
eudicot The eudicots or eudicotyledons are flowering plants that have two seed leaves (cotyledons) upon germination. The term derives from ''dicotyledon'' (etymologically, ''eu'' = true; ''di'' = two; ''cotyledon'' = seed leaf). Historically, authors h ...
, being among the earliest records of the group.


See also

*
Timeline of troodontid research This timeline of troodontid research is a chronological listing of events in the History of paleontology, history of paleontology focused on the troodontids, a group of bird-like theropod dinosaurs including animals like ''Troodon''. Troodontid r ...
*
2017 in archosaur paleontology The year 2017 in archosaur paleontology was eventful. Archosaurs include the only living dinosaur group — birds — and the reptile crocodilians, plus all extinct dinosaurs, extinct crocodilian relatives, and pterosaurs. Archosaur palaeontolo ...


References

{{Taxonbar , from=Q29715232 Troodontidae Dinosaur genera Barremian dinosaurs Yixian Formation Fossil taxa described in 2017 Taxa named by Xu Xing Taxa named by Philip J. Currie Taxa named by Lü Junchang Dinosaurs of China Feathered dinosaurs