Bonaparte (1991) described the first alvarezsaurid,
Alvarezsaurus calvoi, from an incomplete skeleton found in Patagonia, Argentina. Bonaparte also named a
family, Alvarezsauridae, to contain it. He argued that
Alvarezsaurus might be most closely related to the
ornithomimosaurs. In 1993, Perle
et al. described the next alvarezsaur to be discovered, naming it
Mononychus olecranus (meaning "one claw"). A month later they changed the genus name to
Mononykus, because the earlier spelling was already the genus name of an extant beetle. Perle
et al. mistakenly described
Mononykus as a member of
Avialae, one more advanced than
Archaeopteryx. They argued that the family Alvarezsauridae was actually a group of Mesozoic flightless birds on the basis of several features that were supposedly unique to birds. In 1996, Novas described another member of the group called
Patagonykus puertai. Karhu and Rautian (1996) described a Mongolian member of the family;
Parvicursor remotus. Chiappe
et al.(1998) described another Mongolian member,
Shuvuuia deserti, and found it to be a bird as in Perle et al.'s analysis. '' These mistaken assignments of alvarezsaurids to birds were caused primarily by features that are strikingly, or even uniquely, avian. The sternum, for example, is elongated and deeply keeled for an enlarged pectoralis muscle, as it is in
neognathous birds and
volant ratites. One bone in the skull of
Shuvuuia appeared to be an ectethmoid fused to a prefrontal. The ectethmoid is an ossification known only in
Neornithes. Other birdlike characters included the palatine, foramen magnum, cervical and caudal vertebrae, and many others. Several researchers disagreed with Perle
et al. (1993) and Chiappe
et al. (1998), Feduccia (1994), Ostrom (1994), Wellnhofer (1994), Kurochkin (1995), Zhou (1995), and Sereno (1997) considered it unlikely that alvarezsaurids were members of
Avialae. Martin (1997) performed a cladistic analysis but Sereno criticized it strongly, finding it flawed by incorrect codings, use of only select data, and results that did not support his conclusions. Sereno (1999) performed a new analysis, revising the anatomical interpretations and clarifying the characters. He found that alvarezsaurids were more parsimoniously related to the
Ornithomimosauria. As the more primitive members of the Alvarezsauridae were better characterized, the monophyly of the clade was strongly supported, but the more primitive members lacked the most birdlike traits. Some of these traits had been misinterpreted as well. The remaining similarities between birds and alvarezsaurs—like the keeled sterna—are another case of
homoplasy, where the derived alvarezsaurids developed birdlike characters through convergent evolution, rather than inheriting them from a common ancestor with birds. ==Description==