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Haplocheirus (, from Ancient Greek ἁπλός ( haplós), meaning "simple", and χείρ ( kheír), meaning "hand") is an extinct genus of theropod dinosaur from the Middle Jurassic Shishugou Formation of Xinjiang in China. It is generally considered to be an alvarezsauroid, although some researchers have questioned this assignment. The genus contains a monotypic taxon, H. sollers, which is known from a mostly complete skeleton including the skull.
The quality of the fossilization in the only known specimen of Haplocheirus is near-perfect and preserves the animal in three dimensions. This makes Haplocheirus one of the most well-known Jurassic from anywhere in the world. The specimen has been relatively well-studied in comparison with other comparable taxa like Zuolong or Guanlong, which has allowed researchers to gain insights into the evolution of as well as the sensory capabilities, diet, and ontogeny of primitive coelurosaurs.
Many of the small known from the Shishugou Formation, such as Guanlong and Limusaurus are known from mired specimens which are believed to have suffocated in mud, Haplocheirus was discovered in a more traditional fine-grained red or brown mudstone. 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 distal missing from the specimen. Subsequent publications have noted that the holotype was also discovered with the articulated remains of a crocodyliform.
The formal description of Haplocheirus was published in 2010 in the journal Science by Jonah Choiniere, Xu Xing, James M. Clark, Catherine A. Forster, Yu Guo, and Fenglu Han. Their explanation of the etymology is that the genus name comes from the Greek words " haplo", meaning "simple", and " cheirus", meaning "hand". The specific epithet, H. sollers is derived from the Latin word for "skillful". Their description was brief, only comprising three pages, but they described several apomorphy features of the skeleton as well as a phylogeny analysis of the taxon. An amended diagnosis was published by many of the same authors in 2014 along with a detailed osteological monograph about the specimen's skull.
Prior to the discovery of Haplocheirus, the phylogenetic placement of was uncertain. Some authors believed them to be the sister group to avialae, while others believed the anatomical similarity to were the result of convergent evolution. The similarities between Haplocheirus and other primitive provided evidence that their ancestry was much more conventional and that they were not very closely related to birds, compared to and . The scientists who described Haplocheirus in 2010 were vocal in the literature that the discovery of the genus resolved a wide variety of so-called "paradoxes" that related to the origin of birds. It also narrowed the significant ghost lineage in the evolution of alvarezsauroids which would necessarily exist if they were basal maniraptorans.
A detailed description of its skull anatomy was published by several of the same authors who described it in 2014. Several subsequent publications have contained detailed anatomical information, but the genus has not received a comprehensive osteological description.
The general anatomy of Haplocheirus seems to preserve the plesiomorphic condition of in the possession of a long snout and relatively long arms with three claws on each hand. This has led to some confusion in its classification as subsequent researchers have suggested that it belongs to an earlier-diverging coelurosaur clade. 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 the pterygoid bone, a vertically inclined sphenoid bone, a large tuberosity within the humerus, a large ectepicondyle on the humerus, a furrow on the surface of the first phalanges of the second finger, and a conical shape to the lateral condyle of the femur.
In their description of the cranial osteology of Haplocheirus in 2014, Choiniere and colleagues provided a revised diagnosis for the genus. It possesses two unambiguous autapomorphies: a twisted ventral edge of the paroccipital process and a 1:2 ratio in the lengths of the third metacarpal to the second metacarpal. It also differs from all over alvarezsauroids by the following apomorphic features: a dorsal expansion of the jugal bone process of the maxilla, heterodonty, an enlarged fourth dentary 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 been taphonomy in origin.
Haplocheirus is different from derived in the presence of heterodonty dentition. The teeth are circular in cross-section and lack serrations whereas the teeth are mediolaterally flattened and have serrations along some of their length. The maxillary teeth also decrease in size substantially posteriorly. Haplocheirus possessed four premaxillarly teeth, like most ; and at least 30 maxillary teeth and between 30-40 dentary teeth, although some of the tooth alveolus are obscured by the matrix. Very few theropods have tooth counts this high, with the only taxa approaching this total being Pelecanimimus, Shuvuuia, Falcarius, and Byronosaurus.
The skull of Haplocheirus differs considerably from derived alvarezsaurs like Shuvuuia and Parvicursor. It is much more similar to basal members of theropod clades like Ornitholestes, Pelecanimimus, and Nqwebasaurus in the presence of a triradiate jugal bone, a complete postorbital bar, and an ascending ramus of the quadrate bone which contacts the squamosal. These similarities are possibly the result of Haplocheirus preserving the plesiomorphic maniraptoran skull morphology. The evidence for the alvarezsaurian affinities of Haplocheirus 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 only Gallimimus and the unnamed troodontid taxon represented by the specimen IGM 100/1128. However, some authors believe that these traits indicate closer affinities with ornithomimosaurs or at least that the placement of Haplocheirus as an alvarezsauroid is not very robust.
One of the most unique aspects of the postcranial anatomy Haplocheirus is the robust first finger in comparison to the other two fingers. This is intermediate between the generally equally-robust fingers on taxa like Allosaurus and the greatly reduced second and third fingers in Mononykus and its relatives. Haplocheirus also demonstrates a trend towards narrowing the second and third , which is believed to be a precondition for the full coossification of these bones in animals like Patagonykus.
Another notable feature of Haplocheirus is that it preserves a transitional state in the formation of the lunate bone, 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.
Similar phylogenies have been recovered by Steve Brusatte and colleagues in 2014 and by Choiniere and colleagues in a different publication when they described the related genus Aorun in 2013. Xu Xing and colleagues provided further corroboration to this position in 2018 in the description of Bannykus and Xiyunykus, and Kubo and colleagues in 2023 with the description of Jaculinykus, using the same dataset.
However, this classification is not universally accepted. At least two other hypotheses regarding its relationships have been put forward. In their description of the troodontid genus Hesperornithoides, 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 recovered Haplocheirus as a compsognathid, 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.
Federico Agnolín and colleagues published a re-analysis of the skull material from Haplocheirus in 2022 and they conducted a phylogenetic analysis as well. They found that the support for the placement of Haplocheirus within alvarezsaurs and within compsognathids was roughly equivalent to the support values of the trees which placed it in . They do not suggest that this implies Haplocheirus was an unambiguous ornithomimosaur, but rather that a robust classification must await the discovery of new taxa or new character information.
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 modern Aye-Aye. However, other authors have suggested that it was unremarkable in its ecology and likely had a diet very similar to most similarly sized theropods.
The lower (or Wucaiwan) member of the Shishugou consists primarily of red mudstone and sandstone deposits. This is interpreted to have consisted of a wooded alluvial 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 of lungfish, amphibians, crocodilians, tritylodonts, and dinosaurs of various sizes. However, the upper portions of this member, where Haplocheirus was found, are believed to have consisted of more traditional fluvial or wetland environments with less-intense flooding than the lower portions of the member. The climate of the area during the Late Jurassic was temperate and seasonally wet and dry. This pattern of rainfall led to the prominence of seasonal mires, possibly exacerbated by substrate liquefaction by the footfalls of massive sauropods which created "death pits" that trapped and buried small animals. There have also been significant volcanic ash deposits found in the Wucaiwan member, indicating that volcanic activity in the western part of China was increasing at this time.
Named fossils include the primitive mammal-relative Yuanotherium, the crocodylomorphs Sunosuchus, Nominosuchus, and Junggarsuchus, and the pterosaurs Sericipterus and Kryptodrakon. Dinosaurs are the most common and diverse part of the terrestrial fauna found in the Shishugou. They are represented by small ornithischians such as Yinlong, Hualianceratops, and "Eugongbusaurus" as well as by the sauropods Klamelisaurus, Bellusaurus, and Mamenchisaurus sinocanadorum. All large terrestrial predators in the ecosystem were theropods. These ranged from small coelurosaurs like Zuolong and Guanlong to large carnosaurs like Sinraptor. Also notable in the area was the small ceratosaur Limusaurus, which was preserved in one of the muddy "death pits".
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