Haplocheirus (/ˌhæplʊuˈkəirəs/, meaning "simple hand") is an extinct genus of theropod dinosaur from the Middle Jurassic Shishugou Formation of Xinjiang in China.

[2] 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 maniraptorans[7] as well as the sensory capabilities,[8] diet,[9] and ontogeny[10] of primitive coelurosaurs.

It was discovered in the orange mudstone "Middle Beds" in the upper part of the Shishugou Formation in the Wucaiwan area of the Junggar Basin.

[1] Many of the small theropods 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.

[9] 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.

The similarities between Haplocheirus and other primitive maniraptorans provided evidence that their ancestry was much more conventional and that they were not very closely related to birds, compared to dromaeosaurs and troodontids.

[2][9] 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.

[13] Later authors have noted that the holotype is most-likely a juvenile individual, and Zichuan Qin and colleagues estimated that an adult Haplocheirus could have weighed around 41 kilograms (90 lb).

[9] In 2014, Noah Choiniere, James Clark, Mark Norell, and Xu Xing published a follow-up to the original description of Haplocheirus with a detailed monograph on the anatomy of the holotype's skull.

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 which contacts the squamosal.

The evidence for the alvarezsaurian affinities of Haplocheirus includes the strong inclination of the basisphenoid bone and the long tapering process of the basopterygoids.

[9] 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.

[2] Haplocheirus also demonstrates a trend towards narrowing the second and third metacarpals, which is believed to be a precondition for the full coossification of these bones in animals like Patagonykus.

[15] Another notable feature of Haplocheirus is that it preserves a transitional state in the formation of the semi-lunate carpal, which characterizes most derived maniraptorans.

Prior to their analysis, alvarezsauroids had been recovered either as the sister group of avialae or of ornithomimosauria, but neither of these results were obtained by Choiniere and colleagues.

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.

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.

[10] A study published in 2021 by several authors including Choiniere, James Clark, Xu Xing, and Roger Benson examined the skulls of various theropods in order to infer the possible range of sensory abilities they possessed.

The skull of Haplocheirus is also relatively lightweight due to the large orbits and fenestrae, meaning that it likely could not withstand significant forces.

[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 modern aye-aye.

[19] This region is inland and arid today, but in the Late Jurassic, it formed a coastal basin on the northern shores of the Tethys Ocean.

[20] 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.

[18] Named fossils include the primitive mammal-relative Yuanotherium, the crocodylomorphs Sunosuchus, Nominosuchus, and Junggarsuchus, and the pterosaurs Sericipterus and Kryptodrakon.