Alvarezsauroidea

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Alvarezsauroids
Temporal range:
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

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,[1] which is the basalmost of the Alvarezsauroidea (from the Late Jurassic, Asia). 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.

Introduction

The first fossil alvarezsauroids were recognized in the 1990s. Since then, the number of specimens found has dramatically increased. Most of the recent alvarezsauroids are found in China, but they are also known from the Americas and Europe.

Parvicursorinae
are known from the Inner Mongolia and Henan (Late Cretaceous).

The size[specify] of the derived members of Alvarezsauroidea range between 0.5 and 2 m (1 ft 8 in and 6 ft 7 in), but some members may have been larger.[2] Haplocheirus, for example, was the largest member of the Alvarezsauroidea. Because of the size of Haplocheirus and its basal phylogenetic position, a pattern of miniaturization for the Alvarezsauroidea is suggested. Miniaturizations are very rare in dinosaurs, but convergently evolved in Paraves.[1]

Classification

The phylogenetic placement of Alvarezsauroidea is still unclear. At first, they were interpreted as a sister group of

Ornithomimiformes.

Shuvuuia skeletal by Headden

The cladogram below is based on Choniere et al. (2010).[1]

Alvarezsauroidea

Geographical distribution

At first, alvarezsauroids were thought to have been originated in

theropod groups are inconsistent with this hypothesis. Xu et al. (2013) used event−based tree−fitting to perform a quantitative analysis of alvarezsauroid biogeography.[5] 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[6]) of Uzbekistan and Bannykus, Tugulusaurus, and Xiyunykus from the Early Cretaceous of China.[7][8]

Hand morphology

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.[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.[4] In contrast to the digit reduction of the hand of derived alvarezsauroid to a claw used for digging,
parvicursorine Linhenykus.[3]
Another difference between
carnivory was the primitive condition for the clade.[1] Furthermore, Haplocheirus possesses more teeth on the maxilla
than other alvarezsauroids.

Haplocheirus restoration

References

  1. ^
    PMID 20110503
  2. ^ Hutchinson, Chiappe (1998). "The first known alvarezsaurid (Theropoda: Aves) from North America". "Journal of Vertebrate Paleontology". 18(3): 447-450.
  3. ^
    doi:10.1073/pnas.1011052108
  4. ^
    ISBN 978-0-375-82419-7
    .
  5. ^ 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.
  6. S2CID 247391327
    .
  7. ^ 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
  8. doi:10.1016/j.cub.2018.07.057
    .

Sources

  • Nesbitt, S.J., Clarke, J.A., Turner, A.H., Norell, M.A. (2011): "A small alvarezsauroid from eastern Gobi Desert offers insight into evolutionary patterns in the Alvarezsauroidea". Journal of Vertebrate Paleontology. 31:1. 144–153.
  • Turner, A.H., Nesbit, S.J., Norell, M.A. (2009): "A Large Alvarezsaurid from the Late Cretaceous of Mongolia". American Museum Novitates. Number: 3648.
  • Bonaparte, J.F. (1991). "Los vertebrados fosiles de la formacion Rio Colorado, de la ciudad de Neuquen y Cercanias, Creatcio Superior, Argentina" Rev. Mus. Agent. Cienc. "Bernardino Rivadavia", Paleontol. 4:16-123.
  • Choiniere, J. (2010). Guest Post: Haplocheirus, the Skillful One Dave Hone's Archosaur Musings, April 23, 2011
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