Mononykini
| Alvarezsaurids Temporal range: Late Cretaceous, 86–66 Ma |
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| Reconstructed skeleton of Mononykus olecranus | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Order: | Saurischia |
| Suborder: | Theropoda |
| Superfamily: | †Alvarezsauroidea |
| Family: |
†Alvarezsauridae Bonaparte, 1991 |
| Type species | |
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†Alvarezsaurus calvoi Bonaparte, 1991 |
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| Subgroups | |
| Synonyms | |
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Parvicursoridae Karhu & Rautian, 1996 |
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Parvicursoridae Karhu & Rautian, 1996
Mononykidae Chiappe et al., 1998
Alvarezsauridae was a group of small, long-legged dinosaurs. Although originally thought to represent the earliest known flightless birds, a consensus of recent work suggests that they evolved from an early branch of maniraptoran theropods. Alvarezsaurids were highly specialized. They had tiny but stout forelimbs, with compact, bird-like hands. Their skeletons suggest that they had massive breast and arm muscles, possibly adapted for digging or tearing. They had long, tube-shaped snouts filled with tiny teeth. They may have been adapted to prey on colonial insects such as termites.
Alvarezsaurus, the namesake member of the group, was named for the historian Gregorio Álvarez, not the more familiar physicist Luis Alvarez, who proposed that the Cretaceous–Paleogene extinction event was caused by an impact event.
Alvarezsaurids ranged from 50 centimetres (20 in) to 2 metres (6.6 ft) in length, although some possible members may have been larger, including the European Heptasteornis that may have reached 2.5 metres (8.2 ft) long. Fossils attributed to alvarezsaurids have also been found in North and South America and Asia, and range in age from about 86 to 66 million years ago.
At least one specimen of alvarezsaurid, from the species Shuvuuia deserti, preserved down-like, feathery, integumental structures covering the fossil. Schweitzer et al. (1999) subjected these filaments to microscopic, morphological, mass spectrometric, and immunohistochemical studies and found that they consisted of beta-keratin, which is the primary protein in feathers.
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