Jianianhualong
Jianianhualong | |
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Type specimen of J. tengi | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Clade: | Dinosauria |
Clade: | Saurischia |
Clade: | Theropoda |
Family: | †Troodontidae |
Genus: | †Jianianhualong Xu et al., 2017 |
Type species | |
Jianianhualong tengi Xu et al., 2017
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Jianianhualong (meaning "Jianianhua dragon") is a
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[1] rocks of the Yixian Formation in the locality of Baicai Gou (Chinese: 白菜溝), located in the Yixian County of Liaoning.[2] The Dakangpu Bed (equivalent to the Dawangzhangzi, Daxinfangzi, Hejiaxin, Sichakou, and Senjiru Beds),[3] which Baicai Gou is a part of,[4] 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.[5][6][7]
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.[8] The genus name combines the name of a company that sponsored the research of this taxon, Jianianhua (Chinese: 嘉年華), and the suffix long (Chinese: 龍), meaning "dragon". The specific name, tengi, honours Ms. Teng Fangfang who acquired the specimen and made it available for study.[2]
Description
The type specimen of Jianianhualong measures approximately 1 metre (3 ft 3 in) as preserved; since the back of the tail is missing, the animal would probably have been around 1.12 metres (3 ft 8 in) long in life. Scaling from a femoral length of 11.7 cm (4.6 in), Xu et al. found that it weighed 2.4 kilograms (5.3 lb).[2][9] Despite its small size, the specimen was mature, as indicated by the fusion of the sutures in the vertebrae.[2][10][11]
Skull
The skull of Jianianhualong is small and sub-triangular, with a short snout and a wide skull roof like
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.[12][13][14] The bar of bone separating the fenestrae is narrow, like in Sinovenator.[2]
The
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
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.[2][15]
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 16 cm (6.3 in) long, the torso (i.e. the dorsal vertebrae) is 17 cm (6.7 in) long, and the tail (i.e. the caudal vertebrae) is 54 cm (21 in) long. Overall, there may have been 31 caudal vertebrae in total.[2]
Uniquely among troodontids, the
Similar to other
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.[2]
The hand bears three digits, typical of the Maniraptora, which respectively bear 2, 3, and 4
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.[2]
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.[2]
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 30 mm (1.2 in). Those along the back and over the hip are longer, with a length of about 75 mm (3.0 in). Behind the tibia, there are also some feathers measuring 70 mm (2.8 in) in length. Additionally, some poorly preserved feathers are associated with the humerus and ulna, having an unknown length.[2]
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 120 mm (4.7 in) 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.[2]
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.[2][16]
Within the troodontids, Jianianhualong displays a unique combination of characteristics from both
For their 2019 description of Hesperornithoides, 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, a group that was named in 2017 by Caizhi Shen and colleagues in their description of Daliansaurus. 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 required a phylogenetic tree that was only one step longer, and thus marginally less likely according to the principle of maximum parsimony.[17]
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 includes Troodon, Saurornithoides, and Zanabazar).[2][17]
Topology A: Xu et al. (2017)[2]
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Topology B: Hartman et al. (2019)[17]
Paraves |
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Implications for troodontid evolution
The conjunction of basal and derived features as distinct regions of the body in Jianianhualong and Sinosonasus represent a possible case of mosaic evolution, in which natural selection acts upon distinct "modules" of the body.[18] Indeed, the pattern in which basal and derived features are present seems to follow the domains of the body that are regulated by Hox genes, which have previously been discussed in the context of pterosaurs such as Darwinopterus.[19] 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.[2]
An
Paleobiology
Jianianhualong preserves the first evidence of feathers in an unquestionable troodontid, with the troodontid affinities of taxa such as Anchiornis, Eosinopteryx, Jinfengopteryx, and Xiaotingia having been questioned by various studies.[20][21][22][23] 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 microraptorians Microraptor, Changyuraptor, and possibly Zhenyuanlong.[24][25] Asymmetrical feathers are correlated with flight capabilities among paravians,[26] and their emergence marked a major step in the evolution of modern avian feathers.[27][28][29] This trait may have been selected for due to its augmentation of locomotion on the ground or in the air.[2][30]
However, the presence of asymmetrical feathers in Jianianhualong does not guarantee that it was capable of either gliding or powered flight.
Paleoecology
This section needs additional citations for verification. (January 2021) |
After Mei, Sinovenator, Sinusonasus, and Jinfengopteryx, Jianianhualong is the fifth named troodontid that has been discovered in the Jehol Biota 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.[2] Asides from troodontids, the Yixian Formation also preserves a diverse assemblage of other animals, both dinosaurian and non-dinosaurian. The hadrosauroid Jinzhousaurus 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 Hyphalosaurus.
From elsewhere in the Dakangpu/Dawangzhangzi Beds, specimens have been discovered belonging to the birds Confuciusornis, Hongshanornis, Grabauornis, Liaoxiornis, Longicrusavis, Shanweiniao, Shenqiornis, and Zhongornis; the non-avialan theropod Sinosauropteryx; the ceratopsian Psittacosaurus; the pterosaur Cathayopterus; the lizards Dalinghosaurus and Yabeinosaurus; the choristoderes Hyphalosaurus and Monjurosuchus, of which the former is highly abundant; the turtle Manchurochelys; the mammals Acristatherium, Akidolestes, Chaoyangodens, Eomaia, Sinobaatar, and Sinodelphys; the frog Liaobatrachus; and fish, including Lycoptera (of which L. davidi is numerous), Peipiaosteus, and Protopsephurus.
Environmentally, Jianianhualong lived in a lacustrine area, as evinced by the tuffaceous sandstone 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 10 ± 4 °C (50 ± 7 °F). The local flora consists of a mixed assemblage of gymnosperms, such as Czekanowskia, Schizolepis, and the ephedroids Amphiephedra, Chengia, Ephedrites, and Liaoxia; and also early angiosperms, including Archaefructus, Hyrcantha (formerly Sinocarpus), Leefructus, and Potamogeton. Leefructus in particular is a eudicot, being among the earliest records of the group.
See also
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