Haplocheirus

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Haplocheirus
Temporal range: Late Jurassic[1]
~161.2 to 158.7 Ma - Oxfordian
Life reconstruction of Haplocheirus covered in a dense coat of feathers
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Saurischia
Clade: Theropoda
Superfamily: †Alvarezsauroidea (?)
Genus: †Haplocheirus
Choiniere et al., 2010
Type species
†Haplocheirus sollers
Choiniere et al., 2010

Haplocheirus (

alvarezsauroid,[2][3][4][5][6] although some researchers have questioned this assignment.[7][8] The genus contains a single species, H. sollers, which is known from a mostly complete skeleton including the skull.[2]

The quality of the

coelurosaurs from anywhere in the world.[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[8] as well as the sensory capabilities,[9] diet,[10] and ontogeny[11]
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.[2] 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.[1]

Many of the small

crocodyliform.[10]

The formal description of Haplocheirus was published in 2010 in the journal

phylogenetic 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.[10]

Prior to the discovery of Haplocheirus, the phylogenetic placement of

troodontids.[2][10] 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.[12]

Description

Size of Haplocheirus relative to a human

Haplocheirus was a relatively small

alvarezsaurs.[11] Gregory S. Paul estimated that it was about 2 metres (6.6 ft) long and weighed about 18 kilograms (40 lb).[13] 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.[14] 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).[11]

A detailed description of its cranial anatomy was published by several of the same authors who described it in 2014.[10] Several subsequent publications have contained detailed anatomical information,[6][15][16] but the genus has not received a comprehensive osteological description.

The general anatomy of Haplocheirus seems to preserve the

second finger, and a conical shape to the lateral condyle of the femur.[2]

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

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.[10]

Skull

The skull of Haplocheirus after being removed from the matrix

In 2014, Noah Choiniere, James Clark,

braincase and mandible are also almost fully-preserved and are described in detail in the publication.[10]

Haplocheirus is different from derived

alveoli 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.[10]

The skull of Haplocheirus differs considerably from derived alvarezsaurs like Shuvuuia and

troodontid taxon represented by the specimen IGM 100/1128.[10] 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.[8]

Postcranial skeleton

The holotype of Haplocheirus still obscured by the matrix

The type specimen of Haplocheirus 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-most

ornithischian pubic condition.[2]

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

metacarpals, which is believed to be a precondition for the full coossification of these bones in animals like Patagonykus.[16]

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. 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.[15]

Classification

In their description of the genus, Choiniere and colleagues conducted a

theropod taxa. They recovered Haplocheirus as the most basal member of alvarezsauroidea. Their also analysis supported a monophyletic alvarezsauroidea at the base of maniraptora. 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. 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]

Maniraptora

Similar phylogenies have been recovered by

Steve Brusatte and colleagues in 2014[3] and by Choiniere and colleagues in a different publication when they described the related genus Aorun in 2013.[4]

However, this classification is not universally accepted. At least two other hypotheses regarding its relationships have been put forward. In their description of the

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.[7]

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

ornithomimosaurs. 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.[8] Xu Xing and colleagues responded to these suggestions in 2018 when they published the description of Bannykus and Xiyunykus, conducted an analysis which corroborated Haplocheirus as an alvarezsaur.[6]

The most recent phylogenetic analysis to include Haplocheirus was the analysis conducted by Kohta Kubo and colleagues with their description of the genus Jaculinykus in 2023. They obtained similar results to Choiniere and colleagues in their original description, finding it to be one of the most basal alvarezsaurs, being slightly more derived than the contemporary genus Aorun. An abbreviated version of the tree they give can be seen below.[5]

Maniraptora

Paleobiology

Arm function

Haplocheirus seems to have been able to use its hands as proficiently as other

alvarezsaurs.[16]

Growth and histology

The holotype and only specimen of Haplocheirus is believed to belong to a juvenile individual. A histological sample from the holotype was examined by Qin and colleagues in

alvarezsaurs and used it to estimate the adult mass of Haplocheirus. 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).[11]

Senses

A study published in

nocturnality. They also remarked that the cochlear canals of Haplocheirus 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.[9]

Paleoecology

Diet

The teeth of Haplocheirus are relatively slender compared with the contemporaneous genera

carnivory is the plesiomorphic condition for maniraptora and alvarezsauroidea 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 modern

Aye-Aye.[16] However, other authors have suggested that it was unremarkable in its ecology and likely had a diet very similar to most similarly-sized theropods.[9]

Paleoenvironment

A climatological map of the world during the middle to late Jurassic, with the Shishugou Formation labeled "C1"

The only remains of Haplocheirus so far described were discovered near the town of Wucaiwan in Xinjiang, China.[17] This locality is a part of the lower member of the Shishugou Formation,[18] which ranges from 164 to 159 million years ago. This interval spans the transition from the Middle Jurassic to the Late 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 the Tethys Ocean.[20]

The lower (or Wucaiwan) member of the Shishugou consists primarily of red

sauropods which created "death pits" that trapped and buried small animals.[19][21]

A depiction of some of the animals of the Shishugou Formation in their environment

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.[19]

Contemporary Fauna

A variety of small animals have been uncovered from the

ornithopods, tetanurans, and a putative ornithomimosaur.[18]

Named fossils include the primitive mammal-relative

ceratosaur Limusaurus, which was preserved in one of the muddy "death pits".[19]

See also

References

  1. ^ a b Clark, J. M.; Xu, X.; Eberth, D. E.; Forster, C. A.; Machlus, M.; Hemming, S.; Yuan, W.; Hernandez, R. (2006). "The Middle-to-Late Jurassic terrestrial transition: new discoveries from the Shishugou Formation, Xinjiang, China". 9th International Symposium, Mesozoic Terrestrial Ecosystems and Biota. University of Manchester: 26–28.
  2. ^
    S2CID 36904501
    .
  3. ^
    S2CID 8879023
    .
  4. ^
    S2CID 53538348. Archived from the original
    (PDF) on 11 January 2019.
  5. ^
    PMID 37967055
    .
  6. ^
    PMID 30146153
    .
  7. ^
    PMID 31333906
    .
  8. ^
    S2CID 236221732
    .
  9. ^
    S2CID 233872840
    .
  10. ^
    S2CID 4667472
    .
  11. ^
    S2CID 235752037
    .
  12. PMID 20110471
    .
  13. ISBN 978-0-691-13720-9
    .
  14. ISBN 978-0691180311
    .
  15. ^
    PMID 25116378
    .
  16. ^
    S2CID 251254776
    .
  17. ^ Benson, Roger (2012). "Wucaiwan area Sino-American expedition - orange mudstone beds (Jurassic of China)". The Paleobiology Database. Also known as Haplocheirus type locality
  18. ^
    ISBN 0-520-24209-2
    .
  19. ^
    S2CID 251934171
    .
  20. ^
    PMID 20838442
    .
  21. S2CID 131520314
    .

External links
Retrieved from "https://en.wikipedia.org/w/index.php?title=Haplocheirus&oldid=1219168111"