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      Postcranial skeletal anatomy of the holotype and referred specimens of Buitreraptor gonzalezorum Makovicky, Apesteguía and Agnolín 2005 (Theropoda, Dromaeosauridae), from the Late Cretaceous of Patagonia

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          Here we provide a detailed description of the postcranial skeleton of the holotype and referred specimens of Buitreraptor gonzalezorum. This taxon was recovered as an unenlagiine dromaeosaurid in several recent phylogenetic studies and is the best represented Gondwanan dromaeosaurid discovered to date. It was preliminarily described in a brief article, but a detailed account of its osteology is emerging in recent works. The holotype is the most complete specimen yet found, so an exhaustive description of it provides much valuable anatomical information. The holotype and referred specimens preserve the axial skeleton, pectoral and pelvic girdles, and both fore- and hindlimbs. Diagnostic postcranial characters of this taxon include: anterior cervical centra exceeding the posterior limit of neural arch; eighth and ninth cervical vertebral centra with lateroventral tubercles; pneumatic foramina only in anteriormost dorsals; middle and posterior caudal centra with a complex of shallow ridges on lateral surfaces; pneumatic furcula with two pneumatic foramina on the ventral surface; scapular blade transversely expanded at mid-length; well-projected flexor process on distal end of the humerus; dorsal rim of the ilium laterally everted; and concave dorsal rim of the postacetabular iliac blade. A paleohistological study of limb bones shows that the holotype represents an earlier ontogenetic stage than one of the referred specimens (MPCA 238), which correlates with the fusion of the last sacral vertebra to the rest of the sacrum in MPCA 238. A revised phylogenetic analysis recovered Buitreraptor as an unenlagiine dromaeosaurid, in agreement with previous works. The phylogenetic implications of the unenlagiine synapomorphies and other characters, such as the specialized pedal digit II and the distal ginglymus on metatarsal II, are discussed within the evolutionary framework of Paraves.

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          A pre-Archaeopteryx troodontid theropod from China with long feathers on the metatarsus.

          The early evolution of the major groups of derived non-avialan theropods is still not well understood, mainly because of their poor fossil record in the Jurassic. A well-known result of this problem is the 'temporal paradox' argument that is sometimes made against the theropod hypothesis of avian origins. Here we report on an exceptionally well-preserved small theropod specimen collected from the earliest Late Jurassic Tiaojishan Formation of western Liaoning, China. The specimen is referable to the Troodontidae, which are among the theropods most closely related to birds. This new find refutes the 'temporal paradox'1 and provides significant information on the temporal framework of theropod divergence. Furthermore, the extensive feathering of this specimen, particularly the attachment of long pennaceous feathers to the pes, sheds new light on the early evolution of feathers and demonstrates the complex distribution of skeletal and integumentary features close to the dinosaur-bird transition.
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            Four-winged dinosaurs from China.

            Although the dinosaurian hypothesis of bird origins is widely accepted, debate remains about how the ancestor of birds first learned to fly. Here we provide new evidence suggesting that basal dromaeosaurid dinosaurs were four-winged animals and probably could glide, representing an intermediate stage towards the active, flapping-flight stage. The new discovery conforms to the predictions of early hypotheses that proavians passed through a tetrapteryx stage.
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              The theropod ancestry of birds: new evidence from the late cretaceous of madagascar

              A partial skeleton of a primitive bird, Rahona ostromi, gen. et sp. nov., has been discovered from the Late Cretaceous of Madagascar. This specimen, although exhibiting avian features such as a reversed hallux and ulnar papillae, retains characteristics that indicate a theropod ancestry, including a pubic foot and hyposphene-hypantra vertebral articulations. Rahona has a robust, hyperextendible second digit on the hind foot that terminates in a sicklelike claw, a unique characteristic of the theropod groups Troodontidae and Dromaeosauridae. A phylogenetic analysis places Rahona with Archaeopteryx, making Rahona one of the most primitive birds yet discovered.

                Author and article information

                PeerJ Inc. (San Francisco, USA )
                26 March 2018
                : 6
                [1 ]Instituto Multidisciplinario de Investigaciones Biológicas (IMIBIO-SL), CONICET—Universidad Nacional de San Luis , San Luis, Argentina
                [2 ]Section of Earth Sciences, Integrative Research Center, Field Museum of Natural History , Chicago, IL, USA
                [3 ]CONICET, Fundación de Historia Natural ‘Félix de Azara’, CEBBAD, Universidad Maimónides , Buenos Aires, Argentina
                [4 ]CONICET, Instituto de Investigación en Paleobiología y Geología, Universidad Nacional de Río Negro , General Roca, Río Negro, Argentina
                © 2018 Gianechini et al.

                This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, reproduction and adaptation in any medium and for any purpose provided that it is properly attributed. For attribution, the original author(s), title, publication source (PeerJ) and either DOI or URL of the article must be cited.

                Funded by: NASA astrobiology grant to The Field Museum
                Funded by: Agencia Nacional de Promoción Científica y Tecnológica
                Award ID: PICT 2014-1449
                Funded by: US National Science Foundation
                Award ID: EAR 0228693; ANT 1341475
                Fieldwork was supported by a NASA astrobiology grant to The Field Museum, and research was supported by funds of the Agencia Nacional de Promoción Científica y Tecnológica (PICT 2014-1449), The Jurassic Foundation and the American Museum of Natural History Collection Study Grant Program to Federico Gianechini, awards from the US National Science Foundation (EAR 0228693; ANT 1341475) to Peter J. Makovicky, and CONICET to Federico Gianechini, Sebastián Apesteguía and Ignacio Cerda. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.


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