Jianianhualong (meaning "Jianianhua dragon") is a genus of troodontid theropod dinosaur from the Early Cretaceous of China. 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 differences from the flight feathers of modern birds, the feathers in the tail vane of Jianianhualong could have functioned in drag 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.
Jianianhualong possesses a combination of traits seen in basal as well as traits seen in derived troodontids. This is consistent with its phylogenetically 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, where natural selection acts upon the form of the body in a modular way. Similar transitional patterns of traits are seen in the troodontid Sinusonasus, 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.
Discovery and naming
Jianianhualong is known from the type 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 specimen is stored under the collection number DLXH 1218 in the Dalian Xinghai Museum, Liaoning Province, China. It was excavated using mechanical tools (geologist's hammers and chisels) from Early Cretaceous rocks of the Yixian Formation in the locality of Baicai Gou (), 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.
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, Hu Dongyu, and Yu Congyu, in a research paper published in Nature Communications. In order to investigate the preserved plumage of the specimen, Xu and colleagues made use of laser-stimulated fluorescence using a 405 nm laser. The genus name combines the name of a company that sponsored the research of this taxon, Jianianhua (), and the suffix long (), meaning "dragon". The specific name, 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, the main body of which is taller than long. This contrasts Jianianhualong from all other members of the Troodontidae, 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.
Behind the maxilla, the indentation known as the antorbital fossa bears two large openings (the antorbital fenestra and the elongated, enlarged maxillary fenestra) that stretch from the top margin to the bottom margin of the fossa; Sinovenator, Sinornithoides, and Sinusonasus have one additional opening known as the promaxillary opening. The bar of bone separating the fenestrae is narrow, like in Sinovenator.
The lacrimal bone, 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 is short and broad like Mei. The inner surface of the frontal suggests details about the braincase, including the widened olfactory tract and back of the cerebrum. Compared to Sinovenator, the forward branch of the postorbital bone 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 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 projects upwards behind the dentary, like Sinovenator and various other members of the Deinonychosauria.
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) is long, the torso (i.e. the dorsal vertebrae) 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 of the axis, 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 ribs lack uncinate process, and the sternum is also unossified, like other troodontids. Like other theropods, the first pair of the gastralia near the front of the torso is noticeably more robust, particularly in the central portion.
Similar to other paravians, 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 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 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 in Jianianhualong is robust, flattened, and U-shaped like that of Mei. Like derived troodontids, the slender humerus is shorter than the femur. The ulna 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 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 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 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 of the ischium 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 second is the shortest and most slender after the first (corresponding to the hallux). 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, 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, and Archaeopteryx, the tail of Jianianhualong supports a frond-like fan of pennaceous feathers 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