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 single species, H. sollers, which is known from a mostly complete skeleton including the skull. The quality of the preservation 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 coelurosaurs 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 maniraptorans as well as the sensory capabilities, diet, and ontogeny of primitive coelurosaurs.
Discovery
The type and only specimen of Haplocheirus was given the designation IVPP V15988. 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. 40Ar/39Ar dating of volcanic feldspar at this locality places it at the span between the Callovian and Oxfordian boundary, and Haplocheirus was discovered in the upper part of this unit, which is interpreted as being Oxfordian in age. 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. 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 tail vertebrae 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 apomorphic features of the skeleton as well as a phylogenetic 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 alvarezsauroids was uncertain. Some authors believed them to be the sister group to avialae, while others believed the anatomical similarity to birds were the result of convergent evolution. 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. 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.
Description
Haplocheirus was a relatively small coelurosaur, but it was one of the largest alvarezsaurs. Gregory S. Paul estimated that it was about 2 metres (6.6 ft) long and weighed about 21 kilograms (46 lb). 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. 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). A detailed description of its cranial 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 maniraptorans 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 basisphenoid, a large tuberosity within the humerus, a large ectepicondyle on the humerus, a furrow on the surface of the first phalanx 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 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 taphonomic in origin.
Skull
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