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
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.