Alvarezsauridae

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Alvarezsaurids
Temporal range: Late Cretaceous,
97–66  Ma
Mononykus.jpg
Mounted holotype of Mononykus
Alvarezsaurus.jpg
Skeletal mount of Alvarezsaurus
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Saurischia
Clade: Theropoda
Superfamily:† Alvarezsauroidea
Family:† Alvarezsauridae
Bonaparte, 1991
Type species
† Alvarezsaurus calvoi
Bonaparte, 1991
Subgroups
Synonyms

Alvarezsauridae is a family of small, long-legged dinosaurs. Although originally thought to represent the earliest known flightless birds, they are now thought to be an early diverging branch of maniraptoran theropods. Alvarezsaurids were highly specialized. They had tiny but stout forelimbs, with compact, bird-like hands. Their skeletons suggest that they had massive breast and arm muscles, possibly adapted for digging or tearing. They had long, tube-shaped snouts filled with tiny teeth. They have been interpreted as myrmecophagous, adapted to prey on colonial insects such as termites, with the short arms acting as effective digging instruments to break into nests.

Contents

Alvarezsaurus , the type genus of the family, was named for the historian Gregorio Álvarez. [1]

History of study

Bonaparte (1991) described the first alvarezsaurid, Alvarezsaurus calvoi , from an incomplete skeleton found in Patagonia, Argentina. Bonaparte also named a family, Alvarezsauridae, to contain it. He argued that Alvarezsaurus might be most closely related to the ornithomimosaurs. [2]

In 1993, Perle et al. described the next alvarezsaur to be discovered, naming it Mononychus olecranus (meaning "one claw"). A month later they changed the genus name to Mononykus , because the earlier spelling was already the genus name of an extant beetle. [3] Perle et al. mistakenly described Mononykus as a member of Avialae, one more advanced than Archaeopteryx . They argued that the family Alvarezsauridae was actually a group of Mesozoic flightless birds on the basis of several features that were supposedly unique to birds. [4]

In 1996, Novas described another member of the group called Patagonykus puertai . [5] Karhu and Rautian (1996) described a Mongolian member of the family; Parvicursor remotus. [6] Chiappe et al.(1998) described another Mongolian member, Shuvuuia deserti, and found it to be a bird as in Perle et al.'s analysis. [7]

Reconstructed skeleton of Patagonykus puertai Patagonykus.jpg
Reconstructed skeleton of Patagonykus puertai

These mistaken assignments of alvarezsaurids to birds were caused primarily by features that are strikingly, or even uniquely, avian. The sternum, for example, is elongated and deeply keeled for an enlarged pectoralis muscle, as it is in neognathous birds and volant ratites. One bone in the skull of Shuvuuia appeared to be an ectethmoid fused to a prefrontal. The ectethmoid is an ossification known only in Neornithes. Other birdlike characters included the palatine, foramen magnum, cervical and caudal vertebrae, and many others. [8]

Several researchers disagreed with Perle et al. (1993) and Chiappe et al. (1998), Feduccia (1994), Ostrom (1994), Wellnhofer (1994), Kurochkin (1995), Zhou (1995), and Sereno (1997) considered it unlikely that alvarezsaurids were members of Avialae. Martin (1997) performed a cladistic analysis but Sereno criticized it strongly, finding it flawed by incorrect codings, use of only select data, and results that did not support his conclusions. Sereno (1999) performed a new analysis, revising the anatomical interpretations and clarifying the characters. He found that alvarezsaurids were more parsimoniously related to the Ornithomimosauria. [8]

As the more primitive members of the Alvarezsauridae were better characterized, the monophyly of the clade was strongly supported, [5] but the more primitive members lacked the most birdlike traits. Some of these traits had been misinterpreted, also. The remaining similarities between birds and alvarezsaurs, like the keeled sterna, are another case of homoplasy; where the derived alvarezsaurids developed birdlike characters through convergent evolution, rather than inheriting them from a common ancestor with birds. [8]

Description

Size comparison of several parvicursorines: Parvicursor remotus (green), Ceratonykus oculatus (red), Shuvuuia deserti (blue), and Mononykus olecranus (violet). Alvarezsaurid scale martyniuk.png
Size comparison of several parvicursorines: Parvicursor remotus (green), Ceratonykus oculatus (red), Shuvuuia deserti (blue), and Mononykus olecranus (violet).

Alvarezsaurids ranged from 50 centimetres (20 in) to 2 metres (6.6 ft) in length, although some possible members may have been larger, including the European Heptasteornis that may have reached 2.5 metres (8.2 ft) long. Fossils attributed to alvarezsaurids have also been found in North and South America and Asia, and range in age from about 86 to 66 million years ago. [9]

Feathers

At least one specimen of alvarezsaurid, from the species Shuvuuia deserti , preserved down-like, feathery, integumental structures covering the fossil. Schweitzer et al. (1999) subjected these filaments to microscopic, morphological, mass spectrometric, and immunohistochemical studies and found that they consisted of beta-keratin, which is the primary protein in feathers. [10]

Lifestyle

The lifestyle of alvarezsaurids has been debated since the nature of these dinosaurs was established. It has been suggested by numerous palaeontologists that they used their claws to break into ant and termite colonies, though the arm anatomy of an alvarezsaurid would require the animal to lie on its chest against a termite nest. It is also possible that the alvarezsaurids filled some ecological niche that has not yet been considered. [11] Studies of the tails in various alvarezsaur genera also suggest they were possessed of an incredible ability to change their rotational inertia, and combined with their forelimbs, this suggests their ecological niches were similar to those of aardvarks, pangolins, and anteaters. [12]

Additionally, it is known that alvarezsaurids, with their long legs, appear to be built for speed. What implications this has on possible lifestyle is unknown. [11] The discovery of Qiupanykus in association with oviraptorid eggs, indicates that the advanced alvarezsaurids may also have been specialists in nest raiding, using their robust thumb claws to crack open eggshells. [13]

Classification

Life restoration of Shuvuuia deserti Shuvuuia.jpg
Life restoration of Shuvuuia deserti

Turner et al. (2007) place the alvarezsaurs as the most basal group in the Maniraptora, one step more derived than Ornitholestes and two more derived than the Ornithomimosauria. The alvarezsaurs are more primitive than the Oviraptorosauria. [14]

Novas' 1996 description of Patagonykus , demonstrated that it was a link between the more primitive (basal) Alvarezsaurus and the more advanced (derived) Mononykus, and reinforced their monophyly. Parvicursor was discovered shortly after, and placed in its own family Parvicursoridae, and then Shuvuuia in 1998. Everything has since been lumped into Alvarezsauridae, with Mononykinae surviving as a subfamily.

There may be a relationship between the alvarezsaurids and the Ornithomimosauria as sister clades within either Thomas Holtz's Arctometatarsalia or Paul Sereno's Ornithomimiformes. The discovery of Haplocheirus , which exhibits transitional features between the more derived alvarezsaurs and other maniraptorans, particularly in relation to the skull structure and development of the hand, has provided further support for that relationship. [15]

The taxonomy of the alvarezsaurs has been somewhat confused, due to different authors using different names for groups with the same definition. The family Alvarezsauridae was first coined by Jose Bonaparte in 1991, but given no specific phylogenetic definition. Novas later defined the group as the most recent common ancestor of Alvarezsaurus and Mononykus plus all its descendants, though others, such as Paul Sereno, used a more inclusive definition, such as all dinosaurs closer to Shuvuuia than to modern birds. In 2009, Livezey and Zusi used the name Alvarezsauroidea for the total group of all alvarezsaurs, restricting the name Alvarezsauridae to the clade defined by Alvarezsaurus + Mononykus. This was followed by Choiniere and colleagues in 2010, who described the first non-alvarezsaurid alvarezsauroid, Haplocheirus . [16] Some authors have used the name Mononykinae for the sub-group of alvarezsaurs including the advanced Mongolian species. However, Choiniere and colleagues argued that Parvicursorinae has priority, since its coordinate name under the ICZN Code, Parvicursoridae, was named earlier. [16] Another subfamily, Patagonykinae, has been named to include the South American Patagonykus and Bonapartenykus, but a few recent studies have placed them just outside Alvarezsauridae, some of which do not even recover them in a single clade, making Patagonykinae turn out to be paraphyletic. [17] [18] [19] [20] [21] [22]


The cladogram presented here follows a 2012 phylogenetic analysis by Agnolin and colleagues. [17]

Alvarezsauridae
Patagonykinae

Patagonykus

Bonapartenykus

unnamed

Alvarezsaurus

Parvicursorinae
cladeCeratonykini

Ceratonykus

Xixianykus

Albinykus

Parvicursor

Mononykini

Albertonykus

Mononykus

Linhenykus

Shuvuuia

Cladogram after Xu et al., 2011: [23]

Alvarezsauroidea

Haplocheirus

Alvarezsauridae

Achillesaurus

Alvarezsaurus

Patagonykus

Parvicursorinae

Linhenykus

Albertonykus

"Ornithomimus" minutus

Xixianykus

Parvicursor

Tugriken Shireh alvarezsaur

Shuvuuia

Mononykus

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<i>Shuvuuia</i> Extinct family of bird-like dinosaurs

Shuvuuia is a genus of bird-like theropod dinosaur from the late Cretaceous period of Mongolia. It is a member of the family Alvarezsauridae, small coelurosaurian dinosaurs which are characterized by short but powerful forelimbs specialized for digging. The type species is Shuvuuia deserti, or "desert bird". The name Shuvuuia is derived from the Mongolian word shuvuu (шувуу) meaning "bird".

<i>Parvicursor</i> Extinct genus of dinosaurs

Parvicursor is a genus of tiny maniraptoran dinosaur with long slender legs for fast running.

<span class="mw-page-title-main">Maniraptora</span> Clade of dinosaurs

Maniraptora is a clade of coelurosaurian dinosaurs which includes the birds and the non-avian dinosaurs that were more closely related to them than to Ornithomimus velox. It contains the major subgroups Avialae, Dromaeosauridae, Troodontidae, Oviraptorosauria, and Therizinosauria. Ornitholestes and the Alvarezsauroidea are also often included. Together with the next closest sister group, the Ornithomimosauria, Maniraptora comprises the more inclusive clade Maniraptoriformes. Maniraptorans first appear in the fossil record during the Jurassic Period, and survive today as living birds.

<span class="mw-page-title-main">Ornithomimosauria</span> Extinct clade of theropod dinosaurs

Ornithomimosauria are theropod dinosaurs which bore a superficial resemblance to the modern-day ostrich. They were fast, omnivorous or herbivorous dinosaurs from the Cretaceous Period of Laurasia, as well as Africa and possibly Australia. The group first appeared in the Early Cretaceous and persisted until the Late Cretaceous. Primitive members of the group include Nqwebasaurus, Pelecanimimus, Shenzhousaurus, Hexing and Deinocheirus, the arms of which reached 2.4 m (8 feet) in length. More advanced species, members of the family Ornithomimidae, include Gallimimus, Struthiomimus, and Ornithomimus. Some paleontologists, like Paul Sereno, consider the enigmatic alvarezsaurids to be close relatives of the ornithomimosaurs and place them together in the superfamily Ornithomimoidea.

<i>Alvarezsaurus</i> Extinct genus of dinosaurs

Alvarezsaurus is a genus of alvarezsaurid dinosaur from the Late Cretaceous, living in Argentina approximately 86 - 83 million years ago. It was a small dinosaur, measuring 1–1.4 m (3.3–4.6 ft) long and weighing approximately 3 kg (6.6 lb). It was found in the Bajo de la Carpa Formation and was named by paleontologist José Bonaparte in 1991 after the historian Don Gregorio Alvarez. The type species is A. calvoi.

<i>Mononykus</i> Extinct genus of dinosaurs

Mononykus is a genus of alvarezsaurid dinosaur that lived during the Late Cretaceous in what is now Asia on the Nemegt Formation, about 70 million years ago.

<i>Patagonykus</i> Extinct genus of dinosaurs

Patagonykus is a genus of theropod dinosaur from the Upper Cretaceous of Argentina. This alvarezsauroid was discovered in exposures of the Portezuelo Formation (Turonian-Coniacian) of the Rio Neuquén Subgroup in the Neuquén Basin, Neuquen Province of Patagonia, Argentina. The holotype consists of an incomplete but well-preserved skeleton, lacking a skull, but including many vertebrae, the coracoids, a partial forelimb, pelvic girdle, and hindlimbs. Patagonykus has been classed with the Alvarezsauridae, a family which includes such taxa as the Mongolian Mononykus and the Argentinian Alvarezsaurus. In 2010 Gregory S. Paul estimated its length at 1 meter and its weight at 3.5 kg.

<span class="mw-page-title-main">Maniraptoriformes</span> Clade of dinosaurs

Maniraptoriformes is a clade of dinosaurs with pennaceous feathers and wings that contains ornithomimosaurs and maniraptorans. This group was named by Thomas Holtz, who defined it as "the most recent common ancestor of Ornithomimus and birds, and all descendants of that common ancestor."

<i>Ceratonykus</i> Extinct genus of dinosaurs

Ceratonykus is a monospecific genus of alvarezsaurid dinosaur from Mongolia that lived during the Late Cretaceous in what is now the Barun Goyot Formation. The type and only species, Ceratonykus oculatus, is known from a fragmentary skeleton, including an incomplete skull, of an adult individual. It was named and described in 2009 by Vladimir Alifanov and Rinchen Barsbold. Its describers questioned the traditional placement of alvarezsaurs in Theropoda, instead suggesting they were ornithischians, but this has not been accepted since. Ceratonykus has an estimated length of 75 centimetres and weight of 760 grams. It has been considered as a possible junior synonym of Parvicursor.

<i>Kol ghuva</i> Extinct species of dinosaur

Kol is an extinct genus of coelurosaurian theropod dinosaur from the Late Cretaceous of Mongolia. The type and only species is Kol ghuva. The type specimen was excavated from the Ukhaa Tolgod locality of the Djadochta Formation, dating to about 75 million years ago. It is believed to have been about twice the size of the contemporaneous Shuvuuia. However, unlike Shuvuuia, which is known from many well preserved specimens, and although Ukhaa Tolgod has been thoroughly explored, Kol is known only from one complete foot, suggesting that it must have been relatively rare in that ecosystem.

<i>Haplocheirus</i> Extinct genus of dinosaurs

Haplocheirus 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.

<span class="mw-page-title-main">Alvarezsauroidea</span> Extinct superfamily of dinosaurs

Alvarezsauroidea is a group of small maniraptoran dinosaurs. Alvarezsauroidea, Alvarezsauridae, and Alvarezsauria are named for the historian Gregorio Álvarez, not the more familiar physicist Luis Alvarez, or his son geologist Walter Alvarez who jointly proposed that the Cretaceous–Paleogene extinction event was caused by an impact event. The group was first formally proposed by Choiniere and colleagues in 2010, to contain the family Alvarezsauridae and non-alvarezsaurid alvarezsauroids, such as Haplocheirus, which is the basalmost of the Alvarezsauroidea. The discovery of Haplocheirus extended the stratigraphic evidence for the group Alvarezsauroidea about 63 million years further in the past. The division of Alvarezsauroidea into the Alvarezsauridae and the non-alvarezsaurid alvarezsauroids is based on differences in their morphology, especially in their hand morphology.

<i>Linhenykus</i> Extinct genus of dinosaurs

Linhenykus is an extinct genus of alvarezsaurid theropod dinosaur from the Late Cretaceous of Inner Mongolia, China. It is the most basal known member of the Parvicursorinae. The genus gets its name from Linhe, a city near the site where the fossil was first found and Greek nykus, "claw". The specific name is derived from Greek monos, "single", and daktylos, "finger", a reference to the fact that it is the only known non-avian dinosaur to have had but a single digit.

Bonapartenykus is a monospecific genus of alvarezsauroid dinosaur from Argentina that lived during the Late Cretaceous (Campanian-Maastrichtian) in what is now the upper Allen Formation of the Río Negro Province. The type and only species, Bonapartenykus ultimus, is known from a nearly articulated but partial skeleton that was found in close association to two incomplete eggs and several clusters of eggshells belonging to the oogenus Arriagadoolithus. Bonapartenykus was named in 2012 by Federico L. Agnolin, Jaime E. Powell, Fernando E. Novas and Martin Kundrát. Bonapartenykus has an estimated length of 2.5 m (8.2 ft) and weight of 72 kg (159 lb), making it the largest member of the clade Alvarezsauroidea.

<i>Aorun</i> Extinct genus of dinosaurs

Aorun (pron.:"AW-roon") is an extinct genus of carnivorous theropod dinosaur first discovered in 2006, with its scientific description published in 2013. It is one of the oldest known coelurosaurian dinosaurs and is estimated to have lived ~161.6 million years ago during the Late Jurassic Period. It is the fifth theropod discovered from Wucaiwan, China.

Qiupanykus is a genus of alvarezsaurid coelurosaur theropod from the Late Cretaceous Qiupa Formation of southern China.

<i>Jaculinykus</i> Genus of alvarezsaurid dinosaurs

Jaculinykus is an extinct genus of alvarezsaurid theropod dinosaur from the Late Cretaceous Baruungoyot Formation of Mongolia. The genus contains a single species, J. yaruui, known from a nearly complete articulated skeleton including bones of the skull. Jaculinykus is notable for its unique hand, which has a hypertrophied first digit and greatly reduced second digit, which is intermediate between the tridactyl hand of Shuvuuia and monodactyl hand of Linhenykus.

References

  1. ↑ i.e. not the more familiar physicist Luis Alvarez, who proposed that the Cretaceous–Paleogene extinction event was caused by an impact event
  2. ↑ Bonaparte, José F. (1991). "Los vertebrados fósiles de la formación Río Colorado, de la ciudad de Neuquén y cercanías, Cretácico Superior, Argentina". Revista del Museo Argentino de Ciencias Naturales "Bernardino Rivadavia". Paleontología. 4 (3): 15–123. OCLC   29480292.
  3. ↑ Altangerel, Perle; Norell, Mark A.; Chiappe, Luis M.; Clark, James M. (April 1993). "Flightless bird from the Cretaceous of Mongolia". Nature. 362 (6421): 623–626. Bibcode:1993Natur.362..623A. doi:10.1038/362623a0. S2CID   4252852.
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  5. 1 2 Novas, Fernando E. (1996). "Alvarezsauridae, Cretaceous basal birds from Patagonia and Mongolia". Memoirs of the Queensland Museum. 39: 675–702. BHL page 40091128.
  6. ↑ Karhu, A. A.; Rautian, A. S. (1996). "A new family of Maniraptora (Dinosauria: Saurischia) from the Late Cretaceous of Mongolia". Paleontological Journal. 30 (5): 583–592.
  7. ↑ Chiappe, Luis M.; Norell, Mark A.; Clark, James M. (March 1998). "The skull of a relative of the stem-group bird Mononykus". Nature. 392 (6673): 275–278. Bibcode:1998Natur.392..275C. doi:10.1038/32642. S2CID   4426807.
  8. 1 2 3 Sereno, Paul (2001). "Alvarezsaurids: Birds or ornithomimosaurs?". In Gauthier, Jacques; Ostrom, John H. (eds.). New Perspectives on the Origin and Early Evolution of Birds. Peabody Museum of Natural History Yale University. ISBN   978-0-912532-57-8.
  9. ↑ Hutchinson; Chiappe (1998). "The first known alvarezsaurid (Theropoda: Aves) from North America". Journal of Vertebrate Paleontology. 18 (3): 447–450. doi:10.1080/02724634.1998.10011073.
  10. ↑ Schweitzer, M. H.; Watt, J. A.; Avci, R.; Knapp, L.; Chiappe, L.; Norell, M.; Marshall, M. (1999). "Beta-keratin specific immunological reactivity in feather-like structures of the Cretaceous Alvarezsaurid, Shuvuuia deserti". Journal of Experimental Zoology. 285 (2): 146–157. doi:10.1002/(SICI)1097-010X(19990815)285:2<146::AID-JEZ7>3.0.CO;2-A. PMID   10440726.
  11. 1 2 Holtz, Thomas R. Jr. (2007). "Ornithomimosaurs and Alvarezsaurs" . Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages. Random House Children's Books. ISBN   978-0-375-82419-7.
  12. ↑ Meso, J.G.; Qin, Z.; Pittman, M.; Canale, J.I.; Salgado, L.; Díez Díaz, V. (March 2021). "Tail anatomy of the Alvarezsauria (Theropoda, Coelurosauria), and its functional and behavioural implications". Cretaceous Research. 124: 104830. doi:10.1016/j.cretres.2021.104830. S2CID   233858300.
  13. ↑ Lü, Jun-chang; Xu, Li; Chang, Hua-li; Jia, Song-hai; Zhang, Ji-ming; Gao, Dian-song; Zhang, Yi-yang; Zhang, Cheng-jun; Ding, Fang (2018). "A new alvarezsaurid dinosaur from the Late Cretaceous Qiupa Formation of Luanchuan, Henan Province, central China". China Geology. 1 (1): 28–35. doi: 10.31035/cg2018005 .
  14. ↑ Turner, A. H.; Pol, D.; Clarke, J. A.; Erickson, G. M.; Norell, M. A. (7 September 2007). "A Basal Dromaeosaurid and Size Evolution Preceding Avian Flight". Science. 317 (5843): 1378–1381. Bibcode:2007Sci...317.1378T. doi: 10.1126/science.1144066 . PMID   17823350.
  15. ↑ Choiniere, J. (2010). Guest Post: Haplocheirus, the Skillful One Dave Hone's Archosaur Musings, April 23, 2011.
  16. 1 2 Choiniere, J. N.; Xu, X.; Clark, J. M.; Forster, C. A.; Guo, Y.; Han, F. (29 January 2010). "A Basal Alvarezsauroid Theropod from the Early Late Jurassic of Xinjiang, China". Science. 327 (5965): 571–574. Bibcode:2010Sci...327..571C. doi:10.1126/science.1182143. PMID   20110503. S2CID   36904501.
  17. 1 2 Agnolin, Federico L.; Powell, Jaime E.; Novas, Fernando E.; Kundrát, Martin (June 2012). "New alvarezsaurid (Dinosauria, Theropoda) from uppermost Cretaceous of north-western Patagonia with associated eggs". Cretaceous Research. 35: 33–56. doi:10.1016/j.cretres.2011.11.014.
  18. ↑ Xu, Xing; Choiniere, Jonah; Tan, Qingwei; Benson, Roger B.J; Clark, James; Sullivan, Corwin; Zhao, Qi; Han, Fenglu; Ma, Qingyu; He, Yiming; Wang, Shuo; Xing, Hai; Tan, Lin (2018). "Two Early Cretaceous Fossils Document Transitional Stages in Alvarezsaurian Dinosaur Evolution". Current Biology. 28 (17): 2853–2860.e3. doi: 10.1016/j.cub.2018.07.057 . PMID   30146153.
  19. ↑ Qin, Zichuan; Clark, James; Choiniere, Jonah; Xu, Xing (2019). "A new alvarezsaurian theropod from the Upper Jurassic Shishugou Formation of western China". Scientific Reports. 9 (1): 11727. Bibcode:2019NatSR...911727Q. doi:10.1038/s41598-019-48148-7. PMC   6692367 . PMID   31409823.
  20. ↑ Denver W. Fowler; John P. Wilson; Elizabeth A. Freedman Fowler; Christopher R. Noto; Daniel Anduza; John R. Horner (2020). "Trierarchuncus prairiensis gen. et sp. nov., the last alvarezsaurid: Hell Creek Formation (uppermost Maastrichtian), Montana". Cretaceous Research. 116: Article 104560. doi: 10.1016/j.cretres.2020.104560 . S2CID   225630913.
  21. ↑ Averianov AO, Lopatin AV (2022). "A re-appraisal of Parvicursor remotus from the Late Cretaceous of Mongolia: implications for the phylogeny and taxonomy of alvarezsaurid theropod dinosaurs". Journal of Systematic Palaeontology. 19 (16): 1097–1128. doi:10.1080/14772019.2021.2013965. S2CID   247222017.
  22. ↑ Averianov AO, Sues HD (2022). "New material and diagnosis of a new taxon of alvarezsaurid (Dinosauria, Theropoda) from the Upper Cretaceous Bissekty Formation of Uzbekistan". Journal of Vertebrate Paleontology. 41 (5): e2036174. doi:10.1080/02724634.2021.2036174. S2CID   247391327.
  23. ↑ Xu, Xing; Sullivan, Corwin; Pittman, Michael; Choiniere, Jonah N.; Hone, David; Upchurch, Paul; Tan, Qingwei; Xiao, Dong; Tan, Lin; Han, Fenglu (8 February 2011). "A monodactyl nonavian dinosaur and the complex evolution of the alvarezsauroid hand". Proceedings of the National Academy of Sciences of the United States of America. 108 (6): 2338–2342. Bibcode:2011PNAS..108.2338X. doi: 10.1073/pnas.1011052108 . PMC   3038769 . PMID   21262806.
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