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Paper Code  
Title   Two Early Cretaceous Fossils Document Transitional Stages in Alvarezsaurian Dinosaur Evolution
Authors   Xing Xu
Corresponding Author   Xing Xu
Year   2018
Title of Journal   Current Biology
Volume   28
Number   17
Page   2853-2860

Highly specialized animals are often difficult to place phylogenetically. The Late Cretaceous members of Alvarezsauria represent such an example, having been posited as members of various theropod lineages, including birds [ 1 Chiappe L.M. Norell M.A. Clark J.M. The Cretaceous, short-armed Alvarezsauridae: Mononykus and its kin.: 87-120University of California Press, ; 2002 Google Scholar, 2 Bonaparte J.F. Los vertebrados fósiles de la Formación Río Colorado, de la ciudad de Neuquén y cercanías, Cretácico superior, Argentina. Rev. del Museo argent. de Ciencias Naturales” Bernardino Rivadavia. Paleontologia.1991; 4: 17-123 Google Scholar, 3 Perle A. Norell M.A. Chiappe L. Clark J.M. Flightless bird from the Cretaceous of Mongolia. Nature.1993; 362: 623-626 Crossref Scopus (83) Google Scholar, 4 Naish D. Dyke G.J. Heptasteornis was no ornithomimid, troodontid, dromaeosaurid or owl: the first alvarezsaurid (Dinosauria: Theropoda) from Europe. Neues Jahrb. Geol. Palaontol., Monatsh.2004; 7: 385-401 Google Scholar, 5 Chiappe L.M. Norell M.A. Clark J.M. The skull of a relative of the stem-group bird Mononykus. Nature.1998; 392: 275-278 Crossref Scopus (78) Google Scholar, 6 Sereno P.C. Dinosaurian biogeography; vicariance, dispersal and regional extinction. National Science Museum Monographs.1999; 15: 249-257 Google Scholar, 7 Rauhut O.W.M. The interrelationships and evolution of basal theropod dinosaurs. Palaeontology.2003; 69: 1-215 Google Scholar, 8 Norell M.A. Clark J.M. Makovicky P.J. Phylogenetic relationships among coelurosaurian dinosaurs.: 49-67Yale University Press, ; 2001 Google Scholar, 9 Turner A.H. Pol D. Clarke J.A. Erickson G.M. Norell M.A. A basal dromaeosaurid and size evolution preceding avian flight. Science.2007; 317: 1378-1381 Crossref PubMed Scopus (189) Google Scholar, 10 Senter P. A new look at the phylogeny of coelurosauria (Dlnosauria: Theropoda). J. Syst. Palaeontology.2007; 5: 429-463 Crossref Scopus (0) Google Scholar, 11 Holtz T. Jr R. A new phylogeny of the carnivorous dinosaurs. Gaia.1998; 15: 5-61 Google Scholar]. A 70-million-year ghost lineage exists between them and the Late Jurassic putative alvarezsaurian Haplocheirus [ 12 Choiniere J.N. Xu X. Clark J.M. Forster C.A. Guo Y. Han F. A basal alvarezsauroid theropod from the early Late Jurassic of Xinjiang, China. Science.2010; 327: 571-574 Crossref PubMed Scopus (0) Google Scholar], which preserves so few derived features that its membership in Alvarezsauria has recently been questioned [ 13 Lee M.S.Y. Worthy T.H. Likelihood reinstates Archaeopteryx as a primitive bird. Biol. Lett.2012; 8: 299-303 Crossref PubMed Scopus (0) Google Scholar]. If Haplocheirus is indeed an alvarezsaurian, then the 70-million-year gap between Haplocheirus and other alvarezsaurians represents the longest temporal hiatus within the fossil record of any theropod subgroup [ 14 Weishampel D.B. Barrett P.M. Coria R.A. Loeuff J.L. Xu X. Zhao X.J. Sahni A. Gomani E. Noto C.R. Dinosaur distribution.: 517-606University of California Press, ; 2004 Crossref Google Scholar]. Here we report two new alvarezsaurians from the Early Cretaceous of Western China that document successive, transitional stages in alvarezsaurian evolution. They provide further support for Haplocheirus as an alvarezsaurian and for alvarezsaurians as basal maniraptorans. Furthermore, they suggest that the early biogeographic history of the Alvarezsauria involved dispersals from Asia to other continents. The new specimens are temporally, morphologically, and functionally intermediate between Haplocheirus and other known alvarezsaurians and provide a striking example of the evolutionary transition from a typical theropod forelimb configuration (i.e., the relatively long arm and three-digit grasping hand of typical tetanuran form in early-branching alvarezsaurians) to a highly specialized one (i.e., the highly modified and shortened arm and one-digit digging hand of Late Cretaceous parvicursorines such as Linhenykus [ 1 Chiappe L.M. Norell M.A. Clark J.M. The Cretaceous, short-armed Alvarezsauridae: Mononykus and its kin.: 87-120University of California Press, ; 2002 Google Scholar, 15 Xu X. Sullivan C. Pittman M. Choiniere J.N. Hone D. Upchurch P. Tan Q. Xiao D. Tan L. Han F. A monodactyl nonavian dinosaur and the complex evolution of the alvarezsauroid hand. Proc. Natl. Acad. Sci. USA.2011; 108: 2338-2342 Crossref PubMed Scopus (22) Google Scholar]). Comprehensive analyses incorporating data from these new finds show that the specialized alvarezsaurian forelimb morphology evolved slowly and in a mosaic fashion during the Cretaceous.

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