| Aorun | |
|---|---|
 | |
| Life reconstruction ofAorun as a coelurosaur | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Clade: | Avetheropoda |
| Genus: | †Aorun Choiniereet al.,2013 |
| Type species | |
| †Aorun zhaoi Choiniereet al. 2013 | |
Aorun (pronunciationⓘ) (敖闰pinyin Áo rùn) is agenus ofcarnivoroustheropoddinosaur first discovered in 2006, with its scientificdescription published in 2013.[1] It is generally considered one of the oldest knowncoelurosaurian dinosaurs and is estimated to have lived ~161.6 million years ago during theLate JurassicPeriod, though some researchers consider it to be acarnosaurian instead.[2]
Thefossil which included theskull with numerous teeth, somevertebrae and leg bones were discovered by James Clark, the Ronald B. Weintraub Professor of Biology, in the Department of Biological Sciences of GW'sColumbian College of Arts and Sciences with his then doctoral student Jonah Choiniere, along with a team of international researchers in a remote region of Xinjiang in China in 2006. They originally spotted a portion of a leg bone exposed on the surface, and when they dug it up, they found the skull underneath.[3]
Thetype speciesAorun zhaoi was named and described in 2013 by Jonah Choiniere, James Clark,Catherine Forster,Mark Norell,David Eberth,Gregory Erickson,Chu Hongjun andXu Xing. Thegeneric name is derived fromMandarin Chinese and is actually ashortenedmasculine name ofAo Run, who is the Chinese mythicaldeity, theDragon King of the West Sea in theepicJourney to the West.[1][4] Thespecific name honours ProfessorZhao Xijin, who led several important vertebrate paleontological expeditionsto the Junggar Basin.Aorun zhaoi is the only species under the genus, which thus ismonotypic.
Theholotype,IVPP V15709, consists of a skull, the lower jaws, a neck vertebra, a dorsal vertebra, three tail vertebrae, the leftulna and hand, the lower ends of both pubes and both lower legs. In the skull, the rightorbit contains a nearly completesclerotic ring which is composed of overlappingossicles. The gracile hand of this specimen, which has particularly thin metacarpals III and IV more closely approximates the hands of derived non-avian coelurosaurs than the hands of more basal theropods.[5]
It is estimated that the number of teeth in thepremaxilla is four, in themaxilla twelve, and in thedentary between twenty-five and thirty. Its twelve maxillary tooth positions is also suggestive of a juvenile condition, with adult coelurosaurs typically having 15 or more. The teeth are distinctive, because they have no serrations (in the premaxillary and in some dentary teeth), or bear very fine serrations (~10/mm) only on the distal carinae (maxillary teeth and some dentary teeth). The authors noted, however, that the variability in the dentition may represent the juvenile condition.[1]
The specimen, as can be judged from the availableskeletal structure, represents a small bipedal predator. Thetype specimen is a juvenile individual and measures about 1 m (3.3 ft) long, weighing about 1.5 kilograms (3.3 lbs).[1] Choiniere et al. (2013) noted that based on a histological analysis of itsfemur andtibia and other characteristics of the holotype, this specimen ofAorun is at most one year old, and is clearly not aperinate. The relatively large orbit ofAorun is not indicative of what would be expected of an adult morphology.
The describing authors established some diagnostic traits. In the depression for a skull opening, thefenestra antorbitalis, to the front a second opening is present, afenestra maxillaris, that occupies much of the front part of this depression. The teeth in themaxilla only on their rear edges havedenticles which are very small and directed towards the point of the teeth. The neck vertebrae are mildlyopisthocoelous: with centra that are convex in front and concave at the rear. The claws of the hand are different from each other: the thumb claw is large and curved, but the other two claws are smaller and feature a straight underside. The shinbone has on its front outer side only a high narrow groove functioning as contact with the upper part of theastragalus. Thisprocessus ascendens however, though indeed positioned on the outer side, is low.
Aorun is different from other theropods discovered from the same region, such asGuanlong,Haplocheirus,Limusaurus,Monolophosaurus,Sinraptor andZuolong. It is different fromGuanlong in lacking a medial crest on thepremaxilla,nasals andfrontal bones. It also lacks high externalnostrils, and a short anterior maxillary process. It has a shorter premaxillary body, but with a larger maxillary fenestra, a rodlikejugal, closely spaced fine serrations on the distal tooth carinae of the maxilla and dentary. The cervicalvertebral centra are elongated with two pneumaticforamina, theneural spines are short and elongated towards the posterior in the dorsal vertebrae, and thepubic shaft is curved at the end. The limb bones are also substantially different.[1]
The authors who describedAorun placed it in a basal position within Coelurosauria and a member ofCoeluridae.[1] The assignment ofAorun to this position is due to its lack ofsynapomorphies that would provide evidence of its affinity with more derived coelurosaurian taxa. Tykoski (2005) and others have demonstrated that when immature taxa, as is the case with this specimen, are coded as adults in a phylogenetic analysis, the immature taxa were recovered in artificially basal positions relative to adults of the same taxon.[6][7][8] In their cladistic analysis ofBannykus andXiyunykus, Xu et al. (2018) recoveredAorun as a basal alvarezsaur.[9] This assignment has been questioned.[10] Analysis conducted by Kohta Kubo and colleagues with their description of the genusJaculinykus in2023 found it to be one the basalmost alvarezsaurs, being slightly more basal than the contemporary genusHaplocheirus. An abbreviated version of the tree they give can be seen below.[11]
Cau (2024) recoveredAorun as a basal member of the superfamilyAllosauroidea.[2] A simplified version of the cladogram is shown below.
The remains of the type specimen ofAorun zhaoiIVPP V15709 was recovered in theWucaiwan locality, in the lower half of theShishugou Formation, in the Junggar Basin inXinjiang,China. The specimen was collected by IVPP-GWU Field Expedition in 2006, in terrestrial red-brown siltstone that was shown byradiometric dating to have been deposited approximately 161.6 million years ago,[12] at the boundary of theOxfordian/Callovian stages of theJurassic period. This find is significant temporally, because coelurosaur fossils dating to the Jurassic are rare. This specimen is housed in the collection of theInstitute of Vertebrate Paleontology and Paleoanthropology, inBeijing, China.[1]
Thepaleoenvironment of theShishugou Formation was warm but had a basin-wide seasonal dry climate. TheWucaiwan locality of theShishugou Formation has produced the remains of other theropods, such as the basalceratosaurLimusaurus inextricabilis,[13] the basalcoelurosaurZuolong salleei,[14] the basaltyrannosauroidGuanlong wucaii[15] and the basalalvarezsauroidsHaplocheirus sollers andShishugounykus.[16] These other theropods, however, were recovered in sediments that wereOxfordian in age. This rich paleofauna also includedpterosaurs likeSericipterus,ornithischians likeJiangjunosaurus andYinlong and thesauropodsBellusaurus,Klamelisaurus,Tienshanosaurus andMamenchisaurus.Aorun was probably apredator of smalllizards andmammals.[17]Aorun is the seventh theropod, and oldest coelurosaur known from the Shishugou Formation, which is considered one of the most phylogenetically and trophically diverse middle to late Jurassic theropod faunas.[1]
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