Alvarezsauridae
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 Restoration of Mononykus
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The Alvarezsauridae are an enigmatic family of small, long-legged running dinosaurs. Although originally thought to represent primitive birds, most recent work suggests that they are more primitive members of the Maniraptora. Alvarezsaurs are highly specialized, with forelimbs adapted for digging or tearing, elongate jaws, and minute teeth. They may have been adapted to prey on colonial insects such as termites.
Alvarezsaurids range from 0.5–2 m (20–80 inches) in length (although some possible members may have been substantially larger, including the European Heptasteornis (also called Elopteryx) that may have reached 2.5 m (8 ft), and the Australian Rapator, based on a single finger bone that, although obviously not nearly enough material from which to make a good extrapolation, would have been a few meters long.
At least one species of Alvarezsaurid, Shuvuuia deserti, is known to have had down-like feathery integuments. The family is generally recognized by their smoothly shaped slender heads, and short powerful arms with one big claw.
Before the discovery of Shuvuuia, Mononykus was the most complete species, and based on that, the Alvarezsaurid hand was believed to have only one big finger. Now it is known that Alvarezsaurids probably never lost the two other fingers, but they had shrunk to almost nothing.
Alvarezsaurus, and thus Alvarezsaurinae, Alvaresauridae, and Alvarezsauria are named for the historian Don Gregorio Alvarez, not the more familiar physicist Luis Alvarez, who proposed that the Cretaceous-Tertiary extinction event was caused by an impact event.
[edit] Classification history
Alvarezsaurus was the first discovered, and was used in 1991 by José Bonaparte as the anchor taxon of the family and its containing taxon (clade), Alvarezsauria, which was loosely classified as a ceratosaurian. In 1993, the bird-like Mononykus was named by Perle, Clark and Norell, and placed in a separate family, Mononykidae. It was the 1996 description of Patagonykus, which was clearly a link between the more primitive (basal) Alvarezsaurus and the more advanced (derived) Mononykus that started to clarify their relationship.
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.
While its exact position is uncertain, Alvarezsauridae is placed just a step or two away from Archaeopteryx. Alvarezsaurid material has also been reassigned to Ornithomimus, and there may be a relationship between the alvarezsaurids and the Ornithomimosauria. Classification is difficult because the known specimens are all very derived forms from the late Cretaceous, which provides little information on what early forms they evolved from.
[edit] Taxonomy
- ?Arctometatarsalia
- FAMILY ALVAREZSAURIDAE
- ?Rapator
- Alvarezsaurus
- Patagonykus
- Subfamily Parvicursorinae
[edit] External links
- RE: Alvarezsauridae splitting, by Thomas R. Holtz, Jr., from the Dinosaur Mailing List.
- Vertebrates 350.200 Aves: Metornithes, from Palæos.
- The Holy of Holies... Dinosauria II, by Thomas R. Holtz, Jr., from the Dinosaur Mailing List (mentions in passing the Rapator = ?alvaresaurid hypothesis seeing print for the first time; it had been rumored on the list for several years prior)
