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Two Early Cretaceous Fossils Document Transitional Stages in Alvarezsaurian Dinosaur Evolution

2018; Elsevier BV; Volume: 28; Issue: 17 Linguagem: Inglês

10.1016/j.cub.2018.07.057

1879-0445

Xing Xu, Jonah N. Choiniere, Qingwei Tan, Roger Benson, James M. Clark, Corwin Sullivan, Qi Zhao, Fenglu Han, Qingyu Ma, Yiming He, Shuo Wang, Xing Hai, Lin Tan,

Ichthyology and Marine Biology

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 [1Chiappe L.M. Norell M.A. Clark J.M. The Cretaceous, short-armed Alvarezsauridae: Mononykus and its kin.in: Chiappe L.M. Witmer L.M. Mesozoic Birds: Above the Heads of Dinosaurs. University of California Press, 2002: 87-120Google Scholar, 2Bonaparte 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: 17-123Google Scholar, 3Perle A. Norell M.A. Chiappe L. Clark J.M. Flightless bird from the Cretaceous of Mongolia.Nature. 1993; 362: 623-626Crossref Scopus (108) Google Scholar, 4Naish 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: 385-401Google Scholar, 5Chiappe L.M. Norell M.A. Clark J.M. The skull of a relative of the stem-group bird Mononykus.Nature. 1998; 392: 275-278Crossref Scopus (102) Google Scholar, 6Sereno P.C. Dinosaurian biogeography; vicariance, dispersal and regional extinction.National Science Museum Monographs. 1999; 15: 249-257Google Scholar, 7Rauhut O.W.M. The interrelationships and evolution of basal theropod dinosaurs.Palaeontology. 2003; 69: 1-215Google Scholar, 8Norell M.A. Clark J.M. Makovicky P.J. Phylogenetic relationships among coelurosaurian dinosaurs.in: Gauthier J. Gall L.F. New Perspectives on the Origin and Evolution of Birds. Yale University Press, 2001: 49-67Google Scholar, 9Turner 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: 1378-1381Crossref PubMed Scopus (247) Google Scholar, 10Senter P. A new look at the phylogeny of coelurosauria (Dlnosauria: Theropoda).J. Syst. Palaeontology. 2007; 5: 429-463Crossref Scopus (98) Google Scholar, 11Holtz T. Jr R. A new phylogeny of the carnivorous dinosaurs.Gaia. 1998; 15: 5-61Google Scholar]. A 70-million-year ghost lineage exists between them and the Late Jurassic putative alvarezsaurian Haplocheirus [12Choiniere 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: 571-574Crossref PubMed Scopus (110) Google Scholar], which preserves so few derived features that its membership in Alvarezsauria has recently been questioned [13Lee M.S.Y. Worthy T.H. Likelihood reinstates Archaeopteryx as a primitive bird.Biol. Lett. 2012; 8: 299-303Crossref PubMed Scopus (58) 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 [14Weishampel 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.in: Weishampel D.B. Dodson P. Osmolska H. The Dinosauria. Second Edition. University of California Press, 2004: 517-606Crossref 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 [1Chiappe L.M. Norell M.A. Clark J.M. The Cretaceous, short-armed Alvarezsauridae: Mononykus and its kin.in: Chiappe L.M. Witmer L.M. Mesozoic Birds: Above the Heads of Dinosaurs. University of California Press, 2002: 87-120Google Scholar, 15Xu 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: 2338-2342Crossref PubMed Scopus (44) 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.