| Zuolong | |
|---|---|
 | |
| Skeletal diagram of known material in white and light grey | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Clade: | Coelurosauria (?) |
| Genus: | †Zuolong Choiniereet al.,2010 |
| Type species | |
| †Zuolong salleei Choiniereet al., 2010 | |
Zuolong (/ˌtzwoːlʊŋ/) is an extinctgenus oftetanurantheropod from theLate Jurassic period ofChina. The type and only species isZ. salleei. The generic name ofZuolong is in honor ofGeneral Zuo Zōngtáng (also known as "General Tso") with theChinese word "long" which meansdragon. Thespecific epithet "salleei" is in honor of Hilmar Sallee, who funded the expedition which led to the specimen's discovery.[2]
Zuolong was discovered in the upper part of the Wucaiwan member of theShishugou Formation inXinjiang,China.40Ar/39Ar dating of volcanicfeldspar at this locality places it at the span between theCallovian andOxfordian boundary, andZuolong was discovered in the upper part of this unit, which is interpreted as being Oxfordian in age.[1] The specimen was discovered in 2001 by the Sino-American field expedition, but it was not described until 2010 when Jonah Choiniere, James Clark, Catherine Forester, and Xu Xing published a full analysis of the bones.[2]
Choiniere and colleagues noted that, at the time of its description,Zuolong was one of the oldestcoelurosaurs known to science, but that the implications of its discovery cannot be fully understood until more fossil material is discovered. TheMiddle Jurassic preserves very few coelurosaurs, and the ones which are known are almost all from China, with the exception ofProceratosaurus andKileskus.[2]
The holotype ofZuolong, given the designationIVPP V15912, consists of a partially complete skull and numerous post-cranial elements. The skull preserves amaxilla, apremaxilla, one of thequadrate bones, bothquadratojugals, asquamosal bone, both ectopterygoids, apterygoid bone, alacrimal bone, apostorbital bone, a partialfrontal andparietal, as well as three of theteeth from thelower jaw. Other elements of the skeleton which have been preserved include fivecervical vertebrae, fourdorsal vertebrae, fivesacral vertebrae, eightcaudal vertebrae, ahumerus, theradius andulna from the left arm, one of the hand claws, the leftilium, bothpubic bones, bothfemora, atibia, part of afibula, threemetatarsals from the right foot, three toes, and a single toe claw.[2]
Zuolong was not a large theropod. Choiniere and colleagues used tworegression analyses based on the work of P. Christiansen and R.A. Fariña[3] as well as François Therrien, and Donald M. Henderson[4] to estimate the body mass ofZuolong and they calculated a range of between 16–50 kilograms (35–110 lb) based on the length of the femur and the size of the skull. This would make it about half the estimated size of its contemporary,Guanlong.[2] Later,Gregory S. Paul suggested that the holotype is a juvenile and estimates a total adult length of 3 metres (9.8 ft) and a mass of 50 kilograms (110 lb).[5] Other authors have suggested a larger adult size, giving a total length of 3.35 metres (11.0 ft) meters and a mass of 43 kilograms (95 lb) kilograms.[6] The holotype is also considered byThomas R. Holtz Jr. to almost certainly be from a juvenile theropod.[7]
Choiniere and colleagues provide the following traits as autapomorphies for the skull: a slit-like depression on the surface of thequadrate bone, a square-shapedpremaxillary body, a triangular tapering at the anterior of themaxilla, a relatively shallow antorbital fossa,frontal andjugal processes of thepostorbital bone which contact at a right-angle, apostorbital bone with no anterior process, a ventral anterior process of thelacrimal bone.[2] They also describe several autapomorphies of the post-cranial skeleton including: a centrum of the fifthsacral vertebra with an obliquely angled posterior articulation, a largefovea capitis, a large distal condyle of the thirdmetatarsal, a short post-acetabular wing of theilium, and a lack of apubic tubercle, a straightulna andradius, a ridge on the head of thetibia, a lack of paired lateralforamina on thevertebrae, a lack of lateralfossae on thevertebral centra, a straighthumeral andfemoral shaft, and a high and rounded ilium.[2]
Several parts of the skull of the holotype are preserved completely, albeit with very poor preservation quality. This makes some aspects of the skull anatomy difficult to determine, but enough is known that the authors who described it noted several distinct features. There are very few primitivecoelurosaurs known from complete remains, however the authors are able to draw numerous distinctions betweenZuolong and otherLate Jurassic smalltheropods such asGuanlong,Coelurus, andTanycolagreus. The skull is overall triangular-shaped, with a significant tapering towards the end of the snout. It has very large orbits which face laterally and a pronounced anterior process of the lacrimal, which gives the appearance of a small crest above the eyes, a trait very common among theropods.[2]
The preservedalveoli of the tooth positions are relatively well-preserved, which led Choiniere and colleagues to estimate that in life,Zuolong likely had a total of four premaxillary and twelve maxillary teeth. Of the teeth which are preserved, one is likely apremaxillary tooth, because it is said to be much smaller than the other preserved teeth. It is d-shaped in cross-section, which is the condition seen intyrannosauroids, although they are not quite as convex as they are in those taxa. The other teeth which are preserved were badly damaged by thefossilization process, but they do appear to have some serrations. In most other respects, they resemble the teeth of most other theropods; they are long and recurved with cylindrical roots. This is emblematic of a common trend in the skull anatomy ofZuolong which the authors note. It shares numerous skull characteristics with derived coelurosaurs, but also with more basally-branching theropods likecarcharodontosaurs andmegalosauroids.[2]
Twenty-two vertebrae are preserved, coming from all four vertebral segments. Choiniere and colleagues noted multiple features of thevertebrae ofZuolong which were similar to other theropods from a wide range ofclades. For example, the mid-cervical vertebrae possess a posteriorly-projecting lip on the ventral side, which is a condition seen inMonolophosaurus,Allosaurus, and numerousmaniraptorans. The cervical vertebrae themselves are also highly elongate, and are more than twice as long as they are tall. However, the authors note that this is actually much shorter than the cervical vertebrae in other primitivecoelurosaurs, such asGuanlong andCoelurus, which are more than three times as long as they are tall. Thedorsal vertebrae are not as well-preserved as the cervicals, but they are complete enough to determine that they have twinpleurocoels and lack both fossae and foramina on their lateral surfaces. Thesacrum is similarly incomplete and was damaged duringfossilization, but the authors hypothesized thatZuolong likely had five sacral vertebrae, like most other coelurosaurs. Thecaudal vertebrae, like the dorsals, lack both fossae and formania on their lateral surface, and they also preserve sharply inclined neural spines, though these are not as pronounced as they are inallosauroids.[2]
The lefthumerus andradius are both preserved, and the radius is about 88% the length of the humerus, which is a ratio that is conserved across a wide variety of theropods. The manual claws which are preserved are strongly curved and have lateral grooves down their length.[2]
The hip bones display a supraacetabular crest which extends from the posterior end of theacetabulum. This is a common trait in primitive theropods likeCryolophosaurus, but which disappears in derivedmaniraptorans. Thefemur is also very different than most coelurosaurs because the head of the femur is oriented about 15 degrees toward the anterior of the animal, unlike in other coelurosaurs, where there it only extends medially from the femur at a 90-degree angle. One of the autapomorphies forZuolong, the enlargedfovea capitis, is known to be a pathological condition in maleturkeys, so the authors say they cannot rule this out as a possibility, although they think it is relatively unlikely and cannot be proven until a second specimen is found.[2]
The other leg bones display a patchwork of characteristics seen in other theropods. Thefourth trochanter is much higher up on the femur than it is in more basal theropods, but still not as high as in other, more derived, coelurosaurs. Likewise, the medial side of the distal end of the femur is smooth and does not possess the rugosities seen in that area intyrannosauroids. The tibia is not complete, so it is not possible to know ifZuolong had a tibia which exceeded the femur in length, which is more common in derived coelurosaurs and is commonly viewed as a cursorial adaptation. The metetarsals are generally similar to other basal coelurosaurs such asCoelurus andTanycolagreus, however they do have a unique feature in this part of the body which is a flange on the front-medial edge of the distal condyle of the third metatarsal, which is not seen in similar taxa.[2]
The classification ofZuolong has been uncertain and controversial since its description.[2] Almost every analysis including it in its matrix has recovered this species in a different position.[8][9][10] Different hypotheses of its classification can be seen below.
The most common hypothesis of the taxonomy ofZuolong is that it is a basal member of coelurosauria.[2][8][11][7]
In their description ofZuolong in 2010, Choiniere and colleagues included a wide variety of taxa in their phylogenetic analysis.Zuolong exhibits severalsymplesiomorphies ofcoelurosaurs and othertheropod groups. The species is also from a part of theMesozoic where several theropod groups are known to have originated, so the affinities ofZuolong needed to be tested against a broad range of taxa, includingcoelophysoids andceratosaurs in addition toavetheropods. They also included numerous coelurosaurs of uncertain classification in an attempt to use their analysis to resolve the phylogeny hypotheses at the base of coelurosauria. These includedBagaraatan,Tugulusaurus,Tanycolagreus, andAniksosaurus, although a few of these taxa were eventually excluded from the final analysis.[2]
The resulting analysis recoveredZuolong within amonophyletic coelurosauria, but did not resolve any more specific relationships between basal coelurosaurs outside of well-established groups likemaniraptora andtyrannosauroidea. Their placement ofZuolong as a coelurosaur was based on the followingsynapomorphies: amaxillaryfenestra behind theantorbital fossa, a dorsal ridge of the antorbital fossa formed by thenasal andlacrimal bones, d-shaped premaxillary teeth, maxillary teeth with non-uniform serrations,cervical vertebrae with multiplepleurocoels, afemur shorter than thetibia, acnemial crest level with the posterior proximal condyle of the tibia, a groove on the ascending process of theastragalus, and a lack of a horizontal groove on the astragalar condyles. Another novel result of this phylogeny included a recovery ofProceratosaurus at the base of coelurosauria outside of tyrannosauroidea. A reduced consensus tree compiled from 421 of the most parsimonious trees in their analysis is shown below.[2]
In 2020, a group of several authors led by the Brazilian paleontologist Juliana Manso Sayão described a new genus of coelurosaur from theRomualdo Formation,Aratasaurus.[8] They conducted a phylogenetic analysis using the data set presented in a 2012 paper about the anatomy and relationships ofNqwebasaurus, another enigmatic early coelurosaur, some supplementary data aboutSantanaraptor, and added the recently describedBicentenaria to the dataset.[8][12]
In the resulting analysis the conducted, they recovered this new taxon as thesister-species ofZuolong. This was based on thesynapomorphy that both animals have aginglymoidal joint at the distal end of the thirdmetatarsal. Both taxa were recovered as basalcoelurosaurs, similar to Choiniere and colleagues, based on the following synapomorphies: an antorbital fossa with a dorsal border in lateral view, a medial opening on the ectopterygoid bones of thepalate, a d-shaped cross-section of the premaxillary teeth, a rounded surface on the bottom of thecaudal vertebrae, a shelf-like fossa on theilium, and a lack of an anterior process on thepubic boot.[8]
Other novel results of this analysis included finding aparaphyleticproceratosauridae and recoveringBicentenaria as a relatively derivedstem-maniraptoran and the sister taxon ofOrnitholestes. A consensus tree compiled from the 1,056 most parsimonious trees is shown below.[8]
A recent phylogeny includingZuolong was contained in the paper which described the new taxonMaip, a largemegaraptoran from theLate Cretaceous by Rolando and colleagues in2022. They performed two analyses, one which included fragmentary taxa, and one which did not. Although the discussion of their results focused most heavily on its implications for megaraptoran taxonomy, both of their analyses resolved the same relationships forZuolong and other basal coelurosaurs. The results of their analysis, compiled from a consensus of the 2,560 most parsimonious trees is shown below.[11]
A similar result was recovered byAndrea Cau in his study of theropod phylogeny and ontogeny in 2024.[13]
Zuolong is a part of the analysis conducted byFernando Novas and colleagues in their description of the basal coelurosaurBicentenaria in2012. Their phylogenetic analysis was relatively unresolved as a result of codingSantanaraptor as a wildcard taxon, so it was removed from their final analysis. In this analysis, they observed thatfemur length, which is viewed as a proxy forbody size exhibited a continuous two-step decline at the base ofcoelurosauria and then again at the base ofparaves. This seemingly unbroken trend was used to classifyZuolong as a stem-maniraptoran based on the size estimates published for the holotype. This decrease in size is also explained as the cause of the rapid diversification of coelurosauria in theMiddle orLate Jurassic.[9]
In2020, a group of scientists led by Lida Xing from theChinese Academy of Sciences published their description of acompsognathid,Xunmenglong. Their paper included a phylogenetic analysis based on the data set provided by Choiniere and colleagues in their2014 description ofAorun.[14][15] This analysis was unique in its inclusion of a dual analytical framework; the authors of both papers conducted a conventional phylogenetic analysis as well as one which coded several primary characters to account for theontogeny of the sampled taxa.[15] This was done because theholotype ofAorun was shown byhistology to be a juvenile.[14]
This resulted in very different results depending on the analysis used. In the ontogeny analysis,Zuolong is recovered in the conventional position as a very early-diverging coelurosaur outside oftyrannoraptora. In the conventional analysis,Zuolong is recovered as being withinmaniraptoromorpha as the sister-taxon ofAorun, although they do not list any unambiguoussynapomorphies for this clade. Xing and colleagues use the discrepancy between their two analyses as an example of the need for additional specimens to be described before phylogenetic relationships can be confidently established.[15] The results of both of their analyses can be seen below.
| Full Analysis[15] |
| Ontogeny Analysis[15] |
Andrea Cau conducted a landmark phylogenetic analysis in2018 which included hundreds of taxa and sought to resolve the evolution of theavianbody plan from the base ofarchosauria to the evolution ofcrown birds.[10] This analysis was replicated by Chris Barker and colleagues in 2020 with similar results.[16] Cau's analysis was assembled over the course of a decade using over 1,400 discrete characters and showed support for the controversial "Ornithoscelida hypothesis", which groupstheropods andornithischians as sister taxa to the exclusion ofsauropodomorphs.[10][17] Cau's analysis differs substantially from those conducted by Matthew Baron in 2017[17] and Paul Dieudonné and colleagues in 2020[18] by recovering the enigmaticChilesaurus as a basal member oftetanurae.[10]
Cau recoveredZuolong as being slightly more basal thanChilesaurus, which itself is found to be the sister taxon ofneotetanurae. He suggests that the discrepancy between his analysis and those of others is a result of different out groups being used, and he suggests that his analysis, which uses out groups extensively sampled from throughout thebird-line archosaurs, is the superior analytical method. Synapomorphies recovered for tetanurae in this analysis include: the loss of the lacrimal shelf which overhangs theantorbital fossa, a contact between the lateral ridge and condyle of thequadrate bone, a vertically compressedcervical vertebrae, a reduced shelf over theacetabulum, a perforation of thepubic apron, a medially-facing head of thefemur, and a reduction of the femoral trochlea.[10]Darren Naish and Cau also recovered a basal tetanuran position forZuolong in their2022 re-description ofEotyrannus.[19] The consensus tree from Cau's original analysis, compiled from the 3,072 most parsimonious trees, can be seen below.[10]
The only remains ofZuolong so far described were discovered near the town of Wucaiwan inXinjiang,China.[2] This locality is a part of the upper member of theShishugou Formation,[20] which ranges from 164 to 159 million years ago. This interval spans the transition from theMiddle Jurassic to theLate Jurassic, though most of it has been recently dated to the Late Jurassic.[21] This region is inland and arid today, but in the Late Jurassic, it formed a coastal basin on the northern shores of theTethys Ocean.[22]
The lower (or Wucaiwan) member of the Shishugou consists primarily of redmudstone andsandstone deposits. This is interpreted to have consisted of a woodedalluvial fan environment which experienced periodic flooding, which accounts for the wide variety of small-bodied animal fossils preserved in the area as well as the abundance of fossilized trees. The Wucaiwan member preserves fossils oflungfish,amphibians,crocodilians,tritylodonts, anddinosaurs of various sizes. However, the upper portions of this member, whereZuolong was found, are believed to have consisted of more traditionalfluvial orwetland environments with less-intense flooding than the lower portions of the member.[21] The climate of the area during the Late Jurassic was temperate and seasonally wet and dry.[22] This pattern of rainfall led to the prominence of seasonal mires, possibly exacerbated by substrate liquefaction by the footfalls of massivesauropods which created "death pits" that trapped and buried small animals.[21][23]
There have also been significantvolcanic ash deposits found in the Wucaiwan member, indicating that volcanic activity in the western part of China was increasing at this time.[21]
A variety of small animals have been uncovered from theShishugou Formation. Various remains of small animals have been referred to various groups but have yet to be givenbinomial names. These include remains oflungfish,brachyopoid amphibians,docodont andtritylodontmammaliamorphs,lizards, andturtles. Some of these are preserved almost completely and in articulation.[21] There is also a small crocodylomorph which may be related toJunggarsuchus that has yet to receive a formal description or name.[24][25] Various dinosaur remains that have not yet been named have also been recovered from the area. These includestegosaurs,ankylosaurs,ornithopods,tetanurans, and a putativeornithomimosaur.[20][26]
Named fossils include the primitive mammal-relativeYuanotherium, thecrocodylomorphsSunosuchus andJunggarsuchus, and the pterosaursSericipterus andKryptodrakon.[21] Dinosaurs are the most common and diverse terrestrial fauna found in the Shishugou.[22] They are represented by smallornithischians such asYinlong,Hualianceratops, and "Eugongbusaurus" as well as by thesauropodsKlamelisaurus,Bellusaurus, andMamenchisaurus sinocanadorum. All large terrestrial predators weretheropods. These ranged from smallcoelurosaurs likeHaplocheirus,Aorun, andGuanlong to largecarnosaurs likeSinraptor. Also notable in the area was the smallceratosaurLimusaurus, which was preserved in one of the muddy "death pits".[21]
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