| Haplocheirus | |
|---|---|
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| Holotype skull | |
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| Holotype postcania | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Superfamily: | †Alvarezsauroidea (?) |
| Genus: | †Haplocheirus Choiniereet al., 2010 |
| Type species | |
| †Haplocheirus sollers Choiniereet al., 2010 | |
Haplocheirus (/ˌhæplʊuˈkəirəs/, meaning "simple hand") is anextinctgenus oftheropoddinosaur from theMiddle JurassicShishugou Formation ofXinjiang inChina. It is generally considered to be analvarezsauroid,[2][3][4][5] although some researchers have questioned this assignment.[6][7] The genus contains asingle species,H. sollers, which is known from a mostly complete skeleton including the skull.[2]
The quality of thepreservation in the only known specimen ofHaplocheirus is near-perfect and preserves the animal in three dimensions. This makesHaplocheirus one of the most well-knownJurassiccoelurosaurs from anywhere in the world.[2] The specimen has been relatively well-studied in comparison with other comparable taxa likeZuolong orGuanlong, which has allowed researchers to gain insights into the evolution ofmaniraptorans[7] as well as the sensory capabilities,[8] diet,[9] and ontogeny[10] of primitive coelurosaurs.
Thetype and only specimen ofHaplocheirus was given the designationIVPP V15988. It was discovered in the orange mudstone "Middle Beds" in the upper part of theShishugou Formation in the Wucaiwan area of theJunggar Basin.[2]40Ar/39Ar dating of volcanicfeldspar at this locality places it at the span between theCallovian andOxfordian boundary, andHaplocheirus was discovered in the upper part of this unit, which is interpreted as being Oxfordian in age.[1]
Many of the smalltheropods known from the Shishugou Formation, such asGuanlong andLimusaurus are known from mired specimens which are believed to have suffocated in mud,Haplocheirus was discovered in a more traditional fine-grained red or brownmudstone.[9] The exceptional preservation quality was noted by the team who described it, and the skeleton is almost perfectly preserved in three-dimensions, with only a few of the distaltail vertebrae missing from the specimen.[2] Subsequent publications have noted that the holotype was also discovered with the articulated remains of acrocodyliform.[9]
The formal description ofHaplocheirus was published in 2010 in the journalScience by Jonah Choiniere,Xu Xing, James M. Clark,Catherine A. Forster, Yu Guo, and Fenglu Han. Their explanation of theetymology is that thegenus name comes from theGreek words "haplo", meaning "simple", and "cheirus", meaning "hand". The specific epithet,H. sollers is derived from theLatin word for "skillful". Their description was brief, only comprising three pages, but they described severalapomorphic features of the skeleton as well as aphylogenetic analysis of the taxon.[2] An amended diagnosis was published by many of the same authors in 2014 along with a detailed osteological monograph about the specimen's skull.[9]
Prior to the discovery ofHaplocheirus, the phylogenetic placement ofalvarezsauroids was uncertain. Some authors believed them to be the sister group toavialae, while others believed the anatomical similarity tobirds were the result ofconvergent evolution. The similarities betweenHaplocheirus and other primitivemaniraptorans provided evidence that their ancestry was much more conventional and that they were not very closely related to birds, compared todromaeosaurs andtroodontids.[2][9] The scientists who describedHaplocheirus in 2010 were vocal in the literature that the discovery of the genus resolved a wide variety of so-called "paradoxes" that related to theorigin of birds. It also narrowed the significantghost lineage in the evolution of alvarezsauroids which would necessarily exist if they were basal maniraptorans.[11]
Haplocheirus was a relatively smallcoelurosaur, but it was one of the largestalvarezsaurs.[10]Gregory S. Paul estimated that it was about 2 metres (6.6 ft) long and weighed about 21 kilograms (46 lb).[12] Rubén Molina-Pérez and Asier Larramendi gave a similar size estimate to Paul and additionally estimate that it would have been about 60 centimetres (2.0 ft) tall at the hip.[13] Later authors have noted that theholotype is most-likely a juvenile individual, and Zichuan Qin and colleagues estimated that an adultHaplocheirus could have weighed around 41 kilograms (90 lb).[10]
A detailed description of itscranial anatomy was published by several of the same authors who described it in2014.[9] Several subsequent publications have contained detailed anatomical information,[5][14][15] but the genus has not received a comprehensive osteological description.
The general anatomy ofHaplocheirus seems to preserve theplesiomorphic condition ofmaniraptorans in the possession of a long snout and relatively long arms with three claws on each hand.[9] This has led to some confusion in its classification as subsequent researchers have suggested that it belongs to an earlier-diverging coelurosaur clade.[6][7] It was identified as an alvarezsaur in its original description due to the presence of several unambiguous alvarezsauroid synapomorphies. These included: a long process on thepterygoid bone, a vertically-inclinedbasisphenoid, a large tuberosity within thehumerus, a large ectepicondyle on the humerus, a furrow on the surface of the firstphalanx of thesecond finger, and a conical shape to the lateralcondyle of thefemur.[2]
In their description of the cranial osteology ofHaplocheirus in 2014, Choiniere and colleagues provided a revised diagnosis for the genus. It possesses two unambiguousautapomorphies: a twisted ventral edge of the paroccipital process and a 1:2 ratio in the lengths of thethird metacarpal to thesecond metacarpal. It also differs from all over alvarezsauroids by the following apomorphic features: a dorsal expansion of thejugal process of themaxilla,heterodonty, an enlarged fourthdentary tooth, a convex dorsal alveolar margin of the dentary, and serrations on the distal part of the carinae. One of the suggested autapomorphies — a second mandibular fenestra — is believed to have beentaphonomic in origin.[9]
In 2014, Noah Choiniere, James Clark,Mark Norell, andXu Xing published a follow-up to the original description ofHaplocheirus with a detailed monograph on the anatomy of the holotype'sskull. The skull itself is almost fully complete and is missing only the rightpostorbital, half of theparietal, most of the leftsquamosal, and small pieces of thevomer, leftlacrimal,nasals, and leftjugal. Thebraincase andmandible are also almost fully-preserved and are described in detail in the publication.[9]
Haplocheirus is different from derivedalvarezsaurids in the presence ofheterodont dentition. Thepremaxillary teeth are circular in cross-section and lack serrations whereas themaxillary teeth are mediolaterally flattened and have serrations along some of their length.[9] The maxillary teeth also decrease in size substantially posteriorly.[2]Haplocheirus possessed four premaxillarly teeth, like mosttheropods; and at least 30 maxillary teeth and between 30-40dentary teeth, although some of thealveoli are obscured by the matrix. Very few theropods have tooth counts this high, with the only taxa approaching this total beingPelecanimimus,Shuvuuia,Falcarius, andByronosaurus.[9]
The skull ofHaplocheirus differs considerably from derived alvarezsaurs likeShuvuuia andParvicursor. It is much more similar to basal members of theropod clades likeOrnitholestes,Pelecanimimus, andNqwebasaurus in the presence of a triradiate jugal bone, a complete postorbital bar, and an ascending ramus of thequadrate which contacts the squamosal. These similarities are possibly the result ofHaplocheirus preserving theplesiomorphic maniraptoran skull morphology. The evidence for the alvarezsaurian affinities ofHaplocheirus includes the strong inclination of the basisphenoid bone and the long tapering process of the basopterygoids. These traits are very rare outside of alvarezsauroidea, being present in onlyGallimimus and the unnamedtroodontid taxon represented by the specimen IGM 100/1128.[9] However, some authors believe that these traits indicate closer affinities with ornithomimosaurs or at least that the placement ofHaplocheirus as an alvarezsauroid is not very robust.[7]
The type specimen ofHaplocheirus is almost fully-complete. It has not received a full osteological description, but its initial description stated that the entire skeleton was present, except for the distal-mostcaudal vertebrae. It also preserves the ancestralsaurischianpubic condition, unlike derived alvarezsaurs, which have theornithischian pubic condition.[2]
One of the most unique aspects of the postcranial anatomyHaplocheirus is the robust first finger in comparison to the other two fingers. This is intermediate between the generally equally-robust fingers on taxa likeAllosaurus and the greatly reduced second and third fingers inMononykus and its relatives.[2]Haplocheirus also demonstrates a trend towards narrowing the second and thirdmetacarpals, which is believed to be a precondition for the full coossification of these bones in animals likePatagonykus.[15]
Another notable feature ofHaplocheirus is that it preserves a transitional state in the formation of thesemi-lunate carpal, which characterizes most derived maniraptorans. While several of the carpals are coossified, the resulting structure is asymmetrical due to the presence of a long mediodorsal process. The fused carpals are also not equal in size, with the third distal carpal being significantly smaller than the second.[14]
In their description of the genus, Choiniere and colleagues conducted aphylogenetic analysis using 98 othertheropod taxa. They recoveredHaplocheirus as the mostbasal member ofalvarezsauroidea. Their also analysis supported a monophyletic alvarezsauroidea at the base ofmaniraptora. Prior to their analysis, alvarezsauroids had been recovered either as the sister group ofavialae or ofornithomimosauria, but neither of these results were obtained by Choiniere and colleagues. However, they do remark that an affinity with ornithomimosaurs is much more strongly supported than the former hypothesis. Choiniere and colleagues are circumspect about this assignment of alvarezsauroidea and remark that robust results would have to wait for the description of new material. An abbreviated version of the phylogenetic tree displayed in the paper is shown below.[2]
Similar phylogenies have been recovered bySteve Brusatte and colleagues in 2014[3] and by Choiniere and colleagues in a different publication when they described the related genusAorun in 2013.[4] Xu Xing and colleagues provided further corroboration to this position in2018 in the description ofBannykus andXiyunykus,[5] and Kubo and colleagues in 2023 with the description ofJaculinykus, using the same dataset.[16]
However, this classification is not universally accepted. At least two other hypotheses regarding its relationships have been put forward. In their description of thetroodontid genusHesperornithoides, Scott Hartman and colleagues conducted a phylogenetic analysis using 380 taxa and 700 characters which was based on a heavily modified version of the data set used by Brusatte and colleagues in 2014. In this analysis, they recoveredHaplocheirus as acompsognathid, however they only published an abbreviated topology in their final publication which did not include detailed trees of the various stem-maniraptoran groups like compsognathidae.[6]
Federico Agnolín and colleagues published a re-analysis of the skull material fromHaplocheirus in 2022 and they conducted a phylogenetic analysis as well. They found that the support for the placement ofHaplocheirus within alvarezsaurs and within compsognathids was roughly equivalent to the support values of the trees which placed it inornithomimosaurs. They do not suggest that this impliesHaplocheirus was an unambiguous ornithomimosaur, but rather that a robust classification must await the discovery of new taxa or new character information.[7]
Haplocheirus seems to have been able to use its hands as proficiently as othertheropods with similarly-proportioned arms and hands. This is only notable in comparison to the much more aberrant morphology of the forelimbs of mostalvarezsaurs.[15]
The holotype and only specimen ofHaplocheirus is believed to belong to a juvenile individual. A histological sample from the holotype was examined by Qin and colleagues in2021. They identified four distinct lines of arrested growth (or LAGs), which strongly implies that the individual was four-years-old when it died. The holotypes of the closely-related taxaShishugounykus andXiyunykus are both adults, and they were also sampled histologically and determined to be nine-years-old when they died. From their estimated adult sizes, Qin and colleagues constructed a hypothetical growth curve for basalalvarezsaurs and used it to estimate the adult mass ofHaplocheirus. They give a range of between 38 kilograms (84 lb) and 43 kilograms (95 lb) for an adult individual. They also estimate that the ancestral mass of alvarezsaurs to be roughly 23 kilograms (51 lb).[10]
A study published in2021 by several authors including Choiniere, James Clark, Xu Xing, and Roger Benson examined the skulls of varioustheropods in order to infer the possible range of sensory abilities they possessed. In their findings, they suggested that the largeorbits and widescleral aperture suggest thatHaplocheirus and its relativeShuvuuia were adapted fornocturnality. They also remarked that thecochlear canals ofHaplocheirus and its relatives were proportionally longer than those of other theropods, which is an adaptation believed to correlate with improved hearing capabilities, and one which is very uncommon in non-avian theropods.[8]
The teeth ofHaplocheirus are relatively slender compared with the contemporaneous generaZuolong andMonolophosaurus, and the narrow morphology of thedentary and rostrum implies that there is a relatively low power-to-velocity ratio. The skull ofHaplocheirus is also relatively lightweight due to the largeorbits andfenestrae, meaning that it likely could not withstand significant forces. These traits suggest thatHaplocheirus primarily fed on small vertebrates.[9] Choiniere and colleagues have also suggested thatcarnivory is the plesiomorphic condition forManiraptora andAlvarezsauroidea as a whole.[2]
Some authors have also suggested that the increase in the robustness of the first finger may have been an adaptation for digging in tree trunks for insects, similar to the modernaye-aye.[15] However, other authors have suggested that it was unremarkable in its ecology and likely had a diet very similar to most similarly-sized theropods.[8]
The only remains ofHaplocheirus so far described were discovered near the town of Wucaiwan inXinjiang,China.[17] This locality is a part of the lower member of theShishugou Formation,[18] 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.[19] This region is inland and arid today, but in the Late Jurassic, it formed a coastal basin on the northern shores of theTethys Ocean.[20]
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, whereHaplocheirus was found, are believed to have consisted of more traditionalfluvial orwetland environments with less-intense flooding than the lower portions of the member.[19] The climate of the area during the Late Jurassic was temperate and seasonally wet and dry.[20] 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.[19][21]
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.[19]
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.[19] Various dinosaur remains that have not yet been named have also been recovered from the area. These includestegosaurs,ankylosaurs,ornithopods,tetanurans, and a putativeornithomimosaur.[18]
Named fossils include the primitive mammal-relativeYuanotherium, thecrocodylomorphsSunosuchus,Nominosuchus, andJunggarsuchus, and the pterosaursSericipterus andKryptodrakon.[19] Dinosaurs are the most common and diverse part of the terrestrial fauna found in the Shishugou.[20] They are represented by smallornithischians such asYinlong,Hualianceratops, and "Eugongbusaurus" as well as by thesauropodsKlamelisaurus,Bellusaurus, andMamenchisaurus sinocanadorum. All large terrestrial predators in the ecosystem weretheropods. These ranged from smallcoelurosaurs likeZuolong andGuanlong to largecarnosaurs likeSinraptor. Also notable in the area was the smallceratosaurLimusaurus, which was preserved in one of the muddy "death pits".[19]
Also known as Haplocheirus type locality
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