Alvarezsauroidea is a recently discovered, highly specialized group of maniraptoran theropods. Haplocheirus is the oldest known form, from the early part of the Late Jurassic of China; otherwise, the remaining alvarezsauroids (collectively the Alvarezsauridae) are from the Late Cretaceous. Alvarezsaurids are known from South and North America, Europe, and Asia. They have numerous bird-like features, and were once thought to have been specialized flightless birds. Alvarezsaurids range in the chicken-to-rhea sizes.
Alvarezsaurids have small beaky skulls with tiny teeth and hands in which the thumb is much more powerful than the other fingers.
In the Cretaceous Alvarezsauridae, the foprelimbs are further transformed into bizarrely poweful arms with a huge thumb claw and exceedingly small digits II and III. The alvarezsaurids have a backwards pointing pubis. Unlike the therizinosauroid and ornithischian situation, this backwards position of the pubis is more likely associated with changes in the locomotory muscles towards knee-driven power from the ancestral tail-and-femur driven power. Only a little is known of Alvarezsaurus itself (the basalmost form); somewhat more is known for the more derived Patagonykus and Achillesaurus (all from South America).
The highly derived Parvicursorinae (also called "Mononykinae"), in contrast, are known from many excellent specimens. The best studied are the Asian taxa Mononykus, Parvicursor, Xixianykus, and Shuvuuia. (However, North American forms such as Albertonykus are known). Parvicursorines have an extreme version of the arctometatarsus, in which the upper portion of metatarsal III is entirely missing. The parvicursorines show numerous cursorial adaptations, but these were almost certainly defensive. They seem to have been insectivores, and their forelimbs may have been used to batter into ant and termite nests. They have been found from deserts to well-watered environments.
The remaining maniraptorans are the oviraptorosaurs and the eumaniraptorans. These two groups are united by several important characteristics:
- Another increase of brain size
- Laterally directed shoulder joint
- Honest-to-goodness true feathers on at least the arms and tail
- Brooding on nests of eggs (may have been present in more basal coelurosaurs)