Bonaparte (1991) initially put Alvarezsaurus in Alvarezsauria, but this was largely ignored since his Alvarezsauridae covered known taxa for almost two decades. Similarly, Livezey and Zusi (2007) used Alvarezsauroidea, but that was basically an afterthought in their huge inaccurate Aves analysis and like Alvarezsauria was redundant with Alvarezsauridae as then known. Hu et al. (2009) were the first to propose a phylogenetic definition for either and went with Alvarezsauroidea, defining it as a stem-based clade. Choiniere et al. (2010) then first published a proposed non-alvarezsaurid alvarezsaur (Haplocheirus), and followed Hu et al.'s choice. Later authors have similarly used Alvarezsauroidea, so we have consensus. Hoorah. Except... now Agnolin et al. want to start using Alvarezsauria again "in order to emphasize this morphologically distinctive theropod group", and propose basically the same definition, considering it a senior synonym of Alvarezsauroidea. Yet due to the ICZN, Bonaparte named Alvarezsauroidea the same time he named Alvarezsauridae, so there's no date or author priority, and plenty of big theropod clades end in -oidea (Coelophysoidea, Megalosauroidea, Tyrannosauroidea). As for the subjective level of morphological distinction, might I remind the authors they're writing in the twenty-first century. None of their reasons justify disrupting a taxonomic consensus.
Similarly, once people decided to follow the ICZN and use Parvicursorinae over Mononykinae, the former was given a node-based definition equivalent to the one Mononykinae had (Choiniere et al., 2010). But now the same group of authors (Xu et al., 2012) want to change it to a stem-based definition that ends up including more basal taxa like Linhenykus and Xixianykus too. Why? Because the latter "are morphologically very similar to members of this node-based Parvicursorinae" and "the most informative option is to treat Parvicursorinae as a stem-based taxon." But under their broader Parvicursorinae, the most basal forms will undoubtedly be very similar to Patagonykus, so by their same logic we would then expand the definition again. That's why we should be past these 50's-style typological arguments- evolution is a continuum. Again, even ignoring the faulty logic, phylogenetic taxonomy is only really useful if we leave the names and definitions alone so that when anyone says "Parvicursorinae" everyone will know where it goes in their phylogeny. It's as if people are using phylogenetic taxonomy, but don't really get the point of why it was introduced in the first place.
On a related note, Agnolin et al. propose Patagonykinae for Patagonykus and Bonapartenykus, but never properly define it with a node- or stem-based definition (and external reference taxon for the latter). Not that it's particularly useful, as the clade is united by all of two synapomorphies and has only been found in one analysis of course.
So far, the detailed alvarezsauroid analyses have been based on adding taxa to Longrich and Currie's (2009) little alvarezsaurid analysis, which results in Xixianykus and Linhenykus being basal as noted above. Instead, Agnolin et al. add Ceratonykus, Albinykus, Xixianykus and Linhenykus to Choiniere et al.'s big theropod analysis. Seems smart, until you realize that Choiniere et al. just left tons of known states uncoded. For instance, of all theropods, only Stokesosaurus langhami(!) is coded as lacking the traditional alvarezsaurid character of procoelous caudals. So the analysis is terribly untrustworthy. In any case, Agnolin et al. find a novel topology where Ceratonykus and Albinykus are basal (proposed by their original describers to be sister to the derived Mononykus and Shuvuuia respectively), Albertonykus is sister to Mononykus instead of being basal, and Linhenykus is sister to Shuvuuia instead of being basal. Compare below.
Previously published consensus-
Agnolin et al. (2012)-
While I don't trust the latter matrix as noted above, I am open to alternate alvarezsaurid topologies since Longrich and Currie's matrix is small, and hasn't included Albinykus or Bonapartenykus (I added all other known alvarezsauroids and reinforced Alifanov and Barsbold's suggested placement for Ceratonykus at least). What's irritating though is that Agnolin et al. go on to use their topology to propose and define two new clades. One is Mononykini (Mononykus olecranus <- Parvicursor remotus, Patagonykus puertai, Alvarezsaurus calvoi), based on one character which Parvicursor actually also has. Whoops! The other is Ceratonykini, defined as (Ceratonykus oculatus + Xixianykus zhangi). This is based on two characters, which only Xixianykus and Albinykus have. Whoops again!* Do I even have to say that Agnolin et al. don't test to see how many extra steps the old consensus topology takes to constrain in their analysis? Note that when applied to the standard topology, their definition of Ceratonykini ends up covering a much wider set of taxa, including Mononykini. So not only do we have the awkward nested tribes, but the concept is different than intended by Agnolin et al.. And the whole thing is based on meager character evidence that's incorrect anyway. Surely if you find a new topology from the consensus, you should wait until someone else finds it in a different matrix before applying a name to your novel clade. Unless your new analysis has strong support compared to the old topology, and you should pay attention to see if that support is based on accurate anatomy. And even if you feel the urge to name a new clade with strong and accurate support, please choose a definition that works with the old topology if possible. Agnolin et al. could have just called it Xixianykini and defined it as (Xixianykus zhangi <- Mononykus olecranus). That retains the same position in both trees (sister to Parvicursorinae), being unaffected by the controversial taxa, and Xixianykus actually has the proposed diagnostic characters. It's so easy.
So alvarezsaur authors, please don't redefine clades (especially for typological reasons) and please don't name new clades based on poor, inaccurate support, with definitions that don't work well in other topologies.
* Okay, I was writing this post at my parents' from memory of morphology, and now that I'm back home with the Ceratonykus pdf, it may also have the supposed ceratonykine characters (distal tarsals fused to metatarsals; metatarsals fused). At least the right metatarsus seems to, though the left is more ambiguous. Fusion is never explicitly mentioned, though they are called 'tarsometatarsals' [sic]. I was right about Parvicursor's second cnemial crest though, ha! In any case, I stand by my point that such low character support is problematic to name new clades from.
References- Bonaparte, 1991. Los vertebrados fosiles de la Formacion Ryo Colorado, de la ciudad de Neuquen y cercanyas, Cretacico superior, Argentina. Museo Argentino de Ciencias Naturales "Bernardino Rivadavia", Revista (Seccion Paleontologya). 4, 15-123.
Livezey and Zusi, 2007. Higher-order phylogeny of modern birds (Theropoda, Aves: Neornithes) based on comparative anatomy. II. Analysis and discussion. Zoological Journal of the Linnean Society. 149 (1), 1-95.
Hu, Hou, Zhang and Xu, 2009. A pre-Archaeopteryx troodontid theropod from China with long feathers on the metatarsus. Nature. 461, 640-643.
Longrich and Currie, 2009. Albertonykus borealis, a new alvarezsaur (Dinosauria: Theropoda) from the Early Maastrichtian of Alberta, Canada: Implications for the systematics and ecology of the Alvarezsauridae. Cretaceous Research. 30(1), 239-252.
Choiniere, Xu, Clark, Forster, Guo and Han, 2010. A basal alvarezsauroid theropod from the Early Late Jurassic of Xinjiang, China. Science. 327, 571-574.
Agnolin, Powell, Novas and Kundrat, 2012. New alvarezsaurid (Dinosauria, Theropoda) from uppermost Cretaceous of north-western Patagonia with associated eggs. Cretaceous Research. DOI: 10.1016/j.cretres.2011.11.014
Xu, Upchurch, Ma, Pittman, Choiniere, Sullivan, Hone, Tan, Tan, Xiao and Han, 2012. Osteology of the alvarezsauroid Linhenykus monodactylus from the Upper Cretaceous Wulansuhai Formation of Inner Mongolia, China, and comments on alvarezsauroid biogeography. Acta Palaeontologica Polonica. http://dx.doi.org/10.4202/app.2011.0083