Alvarezsaurus calvoi  was a small, lightly-built, bird-like, bipedal theropod.  It was discovered in the Rio Colorado Formation, Coniancian - Santonian, in Argentina. It is known from vertebrae, scapula , partial pelvis and partial hindlimbs and teeth.  No skull has been found.
   Although the skeleton is incomplete, it does show that it had very long legs, long feet, short arms, a long s-shaped neck and an extremely long, thin and flat tail.  The tail was over half of its length. The long legs indicate that it was a fast runner. It differed from other theropods as it did not have ridges on its back and had a unique large hooked forelimbs. It had small unserrated teeth in the front of its snout. It may have been an insectivore.
    Alvarezsaurus calvoi  was very different to other carnivorous dinosaurs in America at the same time period. Because of this it was originally classified as a dinosaur in its own special family. 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. It was originally classified as a dinosaurs, but subsequent study re-classified it as a flightless bird.  Later discoveries of related species have led to the all the Alvarezsauridae being classified as dinosaurs.