Editing Alvarezsauridae (section)

==History of study==
Bonaparte (1991) described the first alvarezsaurid, ''[[Alvarezsaurus calvoi]]'', from an incomplete skeleton found in Patagonia, Argentina. Bonaparte also named a [[Family (biology)|family]], Alvarezsauridae, to contain it. He argued that ''Alvarezsaurus'' might be most closely related to the [[Ornithomimosauria|ornithomimosaurs]].<ref name="Bonaparte1991">{{cite journal |last1=Bonaparte |first1=José F. |title=Los vertebrados fósiles de la formación Río Colorado, de la ciudad de Neuquén y cercanías, Cretácico Superior, Argentina |journal=Revista del Museo Argentino de Ciencias Naturales "Bernardino Rivadavia". Paleontología |date=1991 |volume=4 |issue=3 |pages=15–123 |oclc=29480292 }}</ref>

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.<ref name="Perleetal1993b">{{cite journal |last1=Altangerel |first1=Perle |last2=Norell |first2=Mark A. |last3=Chiappe |first3=Luis M. |last4=Clark |first4=James M. |title=Flightless bird from the Cretaceous of Mongolia |journal=Nature |date=April 1993 |volume=362 |issue=6421 |pages=623–626 |doi=10.1038/362623a0 |bibcode=1993Natur.362..623A |s2cid=4252852 }}</ref> 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.<ref name="Perleetal1993a">{{cite journal |last1=Altangerel |first1=Perle |last2=Norell |first2=Mark A. |last3=Chiappe |first3=Luis M. |last4=Clark |first4=James M. |title=Flightless bird from the Cretaceous of Mongolia |journal=Nature |date=15 April 1993 |volume=362 |issue=6421 |pages=623–626 |doi=10.1038/362623a0 |bibcode=1993Natur.362..623A |s2cid=4252852 }}</ref>

In 1996, Novas described another member of the group called ''[[Patagonykus puertai]]''.<ref name="Novas1996">{{cite journal |id={{BHL page|40091128}} |last1=Novas |first1=Fernando E. |title=Alvarezsauridae, Cretaceous basal birds from Patagonia and Mongolia |journal=Memoirs of the Queensland Museum |date=1996 |volume=39 |pages=675–702 }}</ref> Karhu and Rautian (1996) described a Mongolian member of the family; ''[[Parvicursor]] remotus''.<ref name="Karhu&Rautian1996">{{cite journal |last1=Karhu |first1=A. A. |last2=Rautian |first2=A. S. |title=A new family of Maniraptora (Dinosauria: Saurischia) from the Late Cretaceous of Mongolia |journal=Paleontological Journal |volume=30 |issue=5 |year=1996 |pages=583–592 }}</ref> Chiappe ''et al.''(1998) described another Mongolian member, ''[[Shuvuuia]] deserti'', and found it to be a bird as in Perle et al.'s analysis.<ref name="Chiappeetal1998">{{cite journal |last1=Chiappe |first1=Luis M. |last2=Norell |first2=Mark A. |last3=Clark |first3=James M. |title=The skull of a relative of the stem-group bird Mononykus |journal=Nature |date=March 1998 |volume=392 |issue=6673 |pages=275–278 |doi=10.1038/32642 |bibcode=1998Natur.392..275C |s2cid=4426807 }}</ref>

[[Image:Patagonykus.jpg|thumb|left|Reconstructed skeleton of ''[[Patagonykus puertai]]'']] 
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 [[Flying and gliding animals|volant]] [[ratite]]s. 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.<ref name="Sereno2001">{{cite book |last1=Sereno |first1=Paul |year=2001 |chapter=Alvarezsaurids: Birds or ornithomimosaurs? |editor1-last=Gauthier |editor1-first=Jacques |editor2-last=Ostrom |editor2-first=John H. |title=New Perspectives on the Origin and Early Evolution of Birds |publisher=Peabody Museum of Natural History Yale University |isbn=978-0-912532-57-8 }}</ref>

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]].<ref name="Sereno2001"/>

As the more primitive members of the Alvarezsauridae were better characterized, the monophyly of the clade was strongly supported,<ref name="Novas1996"/> but the more primitive members lacked the most birdlike traits. Some of these traits had been misinterpreted, also. 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.<ref name="Sereno2001"/>