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Alvarezsauroids
Temporal range:
Late Jurassic-Late Cretaceous,
160–66 Ma
Skeletal mount of Patagonykus
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Saurischia
Clade: Theropoda
Clade: Maniraptora
Clade: †Alvarezsauria
Bonaparte, 1991
Superfamily: †Alvarezsauroidea
Bonaparte, 1991
Type species
†Alvarezsaurus calvoi
Bonaparte, 1991
Subgroups
List of genera
Synonyms
For Alvarezsauridae

Alvarezsauroidea is a group of small maniraptoran dinosaurs. Alvarezsauroidea, Alvarezsauridae, and Alvarezsauria are named for the historian Gregorio Álvarez. The group was first formally proposed by Choiniere and colleagues in 2010, to contain the family Alvarezsauridae, which had been named some years prior, and non-alvarezsaurid alvarezsauroids, such as Haplocheirus.[1] 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.

Alvarezsauridae is a family within Alvarezsauroidea whose members represent the most derived and latest-surviving alvarezsauroids. When they were first discovered, they originally thought to represent the earliest known flightless birds. However, 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.

History of study edit

 
A skeletal mount of Alvarezsaurus on display in Copenhagen

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]

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 Alvarezsauroidea 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]

Anatomy edit

 
A skeletal diagram of Shuvuuia, an archetypal alvarezsaurid

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. Some non-alvarezsaurid alvarezsauroids grew to greater sizes, such as Haplocheirus, which was comparable in size to the largest alvarezsaurids. 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]

Hands edit

The differences in the morphology of the hand of basic Alvarezsauroidea and the derived members are characterized by digit reduction. In the evolution of theropod dinosaurs, modifications of the hand were typical. The digital reduction, for instance, is a striking evolutionary phenomenon that is clearly exemplified in theropod dinosaurs.[1] The enlargement of the manual digit II in alvarezsauroids and the concurrent reduction of the lateral digits, created one functional medial digit and two very small, and presumably vestigial, lateral digits. These morphological changes have been interpreted as adaptations for digging.

One possible interpretation suggests that alvarezsauroids fed on insects, using their hands to search beyond the tree bark. This interpretation is consistent with their long, elongate snout and small teeth. Another interpretation suggests 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.[10] In contrast to the digit reduction of the hand of derived alvarezsauroid to a claw used for digging, Haplocheirus was still able to grab things. However, Haplocheirus already shows the enlargement of the second manual digit. Important data on the evolution of the alvarezsauroid hand is also provided by the basal parvicursorine Linhenykus.[11]

Integument edit

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.[12]

Classification edit

Evolution edit

Sister of ornithomimosaurs
Basal maniraptorans

The phylogenetic placement of Alvarezsauroidea is still unclear. At first, they were interpreted as a sister group of Avialae (birds) or nested within the group Avialae[1] and considered to be flightless birds,[11] because they share many morphological characteristics with them, such as a loosely sutured skull, a keeled sternum, fused wrist elements, and a posteriorly directed pubis.[1] But this association was reevaluated after the discovery of the primitive forms like Haplocheirus, Patagonykus and Alvarezsaurus, which do not show all bird-like features as the first discovered species Mononykus and Shuvuuia.[10] This shows that bird-like characteristics were developed multiple times within the Maniraptora. Furthermore, the Alvarezsauroidea had simplified homogenous dentition, convergent with that of some extant insectivorous mammals. More recently, they have been placed within the Coelurosauria basal to the Maniraptora or as a sister taxa of Ornithomimosauria within the Ornithomimiformes.

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.[13]

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.[14]

Taxonomy edit

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.[1]

 
Haplocheirus, an early alvarezsauroid
 
Shuvuuia, a derived alvarezsaurid

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.[1] 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.[15][16][17][18][19]

The cladogram presented here follows a 2011 phylogenetic analysis by Xu and colleagues.[20]

Alvarezsauroidea

Haplocheirus

Alvarezsauridae

Achillesaurus

Alvarezsaurus

Patagonykus

Parvicursorinae

Linhenykus

Albertonykus

"Ornithomimus" minutus

Xixianykus

Parvicursor

Tugriken Shireh alvarezsaur

Species edit

 
A size diagram of several alvarezsaurids
 
A life reconstruction of Linhenykus by Julius T. Csotonyi

Paleoecology edit

Behavior and diet edit

 
A speculative restoration of an alvarezsauroid feeding on termites

The lifestyle of the derived 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.[21] 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.[22]

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.[21] 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.[23]

Another difference between Alvarezsauridae and stem-alvarezsauroids like Haplocheirus is their dentition. While derived alvarezsauroids show a simplified homogenous dentition, Haplocheirus on the other side possesses recurved serrated teeth. The dentition of Haplocheirus and their basal phylogenetic position, suggest that carnivory was the primitive condition for the clade. Furthermore, Haplocheirus possessed more teeth on the maxilla than other alvarezsauroids.[1]

Distribution edit

 
The spatial and temporal locations of the alvarezsaurs discovered in Mongolia

At first, alvarezsauroids were thought to have been originated in South America. However, the discovery of Haplocheirus, and its basal phylogenetic position, as well as its early temporal position, suggests they derived in Asia rather than South America. Xu et al. (2011) suggested that at least three dispersal events of alvarezsauroids took place; one from Asia to Gondwana, one from Gondwana to Asia, and one from Asia to North America. This hypothesis is consistent with faunal interchanges.[11]

On the other hand, some theropod groups are inconsistent with this hypothesis. Xu et al. (2013) used event−based tree−fitting to perform a quantitative analysis of alvarezsauroid biogeography.[24] Their results showed an absence of statistical support for previous biogeographic hypotheses that favour pure vicariance or pure dispersal scenarios as explanations for the distributions of alvarezsauroids across South America, North America and Asia. They instead found that statistically significant biogeographic reconstructions suggest a dominant role for sympatric events (“within area” ones), combined with a mix of vicariance, dispersal and regional extinction. The Asian origin of alvarezsauroids is also bolstered by the discovery of alvarezsaurid specimens from the Turonian-age Bissekty Formation (some of which were named Dzharaonyx in 2022[19]) of Uzbekistan and Bannykus, Tugulusaurus, and Xiyunykus from the Early Cretaceous of China.[25][26]

See also edit

References edit

  1. ^ a b c d e f g Choiniere, J.N., Xu, X., Clark, J.M., Forster, C.A., Guo, Y. and Han, F. (2010). "A basal alvarezsauroid theropod from the early Late Jurassic of Xinjiang, China." Science, 327: 571-574. doi:10.1126/science.1182143 PMID 20110503 Cite error: The named reference "haplocheirus" was defined multiple times with different content (see the help page).
  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.
  4. ^ Altangerel, Perle; Norell, Mark A.; Chiappe, Luis M.; Clark, James M. (15 April 1993). "Flightless bird from the Cretaceous of Mongolia". Nature. 362 (6421): 623–626. Bibcode:1993Natur.362..623A. doi:10.1038/362623a0. S2CID 4252852.
  5. ^ a b 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. ^ a b c 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. ^ a b Holtz, R.T. (2007). "Ornithomimosaurs and Alvarezsaurs". Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages. ISBN 978-0-375-82419-7.
  11. ^ a b c Xu, X., Sullivan, C., Pittman, M., Choniere, J.N., Hone, D., Upchurch, P., Tan, Q., Xiao, D., Tan, L. and Han, F. (2011). "A monodactyl nonavian dinosaur and the complex evolution of the alvarezsauroid hand." PNAS, 108: no.6. doi:10.1073/pnas.1011052108
  12. ^ 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.
  13. ^ 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.
  14. ^ Choiniere, J. (2010). Guest Post: Haplocheirus, the Skillful One Dave Hone's Archosaur Musings, April 23, 2011.
  15. ^ 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.
  16. ^ 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.
  17. ^ 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.
  18. ^ 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.
  19. ^ a b 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.
  20. ^ 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.
  21. ^ a b 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.
  22. ^ 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.
  23. ^ 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.
  24. ^ Xu, X., Upchurch, P., Ma, Q., Pittman, M., Choiniere, J., Sullivan, C., Hone, D.W.E., Tan, Q., Tan, L., Xiao, D., and Han, F., 2013. Osteology of the Late Cretaceous alvarezsauroid Linhenykus monodactylus from China and comments on alvarezsauroid biogeography. Acta Palaeontologica Polonica 58 (1): 25–46.
  25. ^ Averianov A, Sues H-D (2017) The oldest record of Alvarezsauridae (Dinosauria: Theropoda) in the Northern Hemisphere. PLoS ONE12(10): e0186254. https://doi.org/10.1371/journal.pone.0186254
  26. ^ Xing Xu; Jonah Choiniere; Qingwei Tan; Roger B.J. Benson; James Clark; Corwin Sullivan; Qi Zhao; Fenglu Han; Qingyu Ma; Yiming He; Shuo Wang; Hai Xing; Lin Tan (2018). "Two Early Cretaceous fossils document transitional stages in alvarezsaurian dinosaur evolution". Current Biology. Online edition. doi:10.1016/j.cub.2018.07.057.
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