Late Jurassic-Late Cretaceous,
|Reconstructed skeleton of a Mononykus olecranus|
Alvarezsauroidea is a group of small maniraptoran dinosaurs. Alvarezsauroidea, Alvarezsauridae, and Alvarezsauria are named for the historian Don Gregorio Alvarez, not the more familiar physicist Luis Alvarez, who 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 (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.
The first fossil Alvarezsauroids were recognized in the 1990s. Since then, the number of specimen found has dramatically increased. Most of the recent Alvarezsauroids are found in China. But they are also known from North- and South-America, as well as Europe. They existed from the Late Jurassic to the Late Cretaceous. The basalmost and oldest alvarezsauroid of the Chinese alvarezsaurs is from the Shishugou Formation in Xinjiang (earliest Late Jurassic). Additionally, two derived members of the derived alvarezsaurs group Parvicursorinae are known from the Inner Mongolia and Henan (Late Cretaceous). The size of the derived members of Alvarezsauroidea range between 0,5 m and 2 m (20 – 80 inches), but some members may have been larger. 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. Minaturizations are very rare in dinosaurs, but convergently evolved in Paraves.
The phylogenetic placement of Alvarezsauroid is still unclear. At first, they were interpreted as a sister group of Avialae (birds) or nested within the group Avialae  and considered to be flightless birds, 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. But this association was relativize 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. The systematical position of the Alvarezsauroidea is still controversial. They are placed within the Coelurosauria to the Maniraptora or as a sister taxa of Ornithomimosauria within the Ornithomimiformes. 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.
The cladogram below is based on Choniere et al. 2010.
In the beginning, 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. On the other side, some theropod groups are inconsistent with this hypothesis and therefore further investigations are needed.
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. 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 the Alvarezsauroids feed on insects, using their hand to search beyond the tree bark. This interpretation is consistent with their long snout, as well as with their elongate snout and small teeth. Another interpretation suggests that they used their claws to break into ant and termite colonies. But the arm anatomy of an Alvarezsaurid would require the animal to lie on its chest against a termite nest. 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 modified hand of Alvarezsauroid is also provided by the basal Parvicursorinae Linhenykus. Another difference between Alvarezsauridae and Haplocheirus is the dentition. While 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 possesses more teeth on the maxilla than other Alvarezsauroids.
- 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. "Bernadino Rivadavia", Paleontol. 4:16-123.
- Choiniere, J. (2010). Guest Post: Haplocheirus, the Skillful One Dave Hone's Archosaur Musings, April 23, 2011
- 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
- Hutchinson, Chiappe (1998). "The first known alvarezsaurid (Theropoda: Aves) from North America". "Journal of Vertebrate Paleontology". 18(3): 447-450.
- 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/-/DCSupplemental
- 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.