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      A Nomenclature for Vertebral Fossae in Sauropods and Other Saurischian Dinosaurs

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          Abstract

          Background

          The axial skeleton of extinct saurischian dinosaurs (i.e., theropods, sauropodomorphs), like living birds, was pneumatized by epithelial outpocketings of the respiratory system. Pneumatic signatures in the vertebral column of fossil saurischians include complex branching chambers within the bone (internal pneumaticity) and large chambers visible externally that are bounded by neural arch laminae (external pneumaticity). Although general aspects of internal pneumaticity are synapomorphic for saurischian subgroups, the individual internal pneumatic spaces cannot be homologized across species or even along the vertebral column, due to their variability and absence of topographical landmarks. External pneumatic structures, in contrast, are defined by ready topological landmarks (vertebral laminae), but no consistent nomenclatural system exists. This deficiency has fostered confusion and limited their use as character data in phylogenetic analysis.

          Methodology/Principal Findings

          We present a simple system for naming external neural arch fossae that parallels the one developed for the vertebral laminae that bound them. The nomenclatural system identifies fossae by pointing to reference landmarks (e.g., neural spine, centrum, costal articulations, zygapophyses). We standardize the naming process by creating tripartite names from “primary landmarks,” which form the zygodiapophyseal table, “secondary landmarks,” which orient with respect to that table, and “tertiary landmarks,” which further delineate a given fossa.

          Conclusions/Significance

          The proposed nomenclatural system for lamina-bounded fossae adds clarity to descriptions of complex vertebrae and allows these structures to be sourced as character data for phylogenetic analyses. These anatomical terms denote potentially homologous pneumatic structures within Saurischia, but they could be applied to any vertebrate with vertebral laminae that enclose spaces, regardless of their developmental origin or phylogenetic distribution.

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          Most cited references 6

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          Cretaceous sauropods from the sahara and the uneven rate of skeletal evolution among dinosaurs

          Lower Cretaceous fossils from central Niger document the succession of sauropod dinosaurs on Africa as it drifted into geographic isolation. A new broad-toothed genus of Neocomian age ( approximately 135 million years ago) shows few of the specializations of other Cretaceous sauropods. A new small-bodied sauropod of Aptian-Albian age ( approximately 110 million years ago), in contrast, reveals the highly modified cranial form of rebbachisaurid diplodocoids. Rates of skeletal change in sauropods and other major groups of dinosaurs are estimated quantitatively and shown to be highly variable.
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            The last of the dinosaur titans: a new sauropod from Madagascar.

            The Titanosauria, the last surviving group of the giant sauropod dinosaurs, attained a near-global distribution by the close of the Cretaceous period (65 Myr ago). With the exception of a few new discoveries in Argentina, most titanosaurs are known only from fragmentary postcranial skeletons and rare, isolated skull elements. Here we describe the most complete titanosaur yet discovered. Rapetosaurus krausei gen. et sp. nov., from the Maevarano Formation of Madagascar, provides a view of titanosaur anatomy from head to tail. A total-evidence phylogenetic analysis supports a close relationship between brachiosaurids and titanosaurs (Titanosauriformes). The inclusion of cranial data from Rapetosaurus also lays to rest questions concerning the phylogeny of the enigmatic Mongolian genera Nemegtosaurus and Quaesitosaurus. In spite of their elongated, diplodocoid-like skulls, all three taxa are now firmly nested within Titanosauria.
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              Evidence for Avian Intrathoracic Air Sacs in a New Predatory Dinosaur from Argentina

              Background Living birds possess a unique heterogeneous pulmonary system composed of a rigid, dorsally-anchored lung and several compliant air sacs that operate as bellows, driving inspired air through the lung. Evidence from the fossil record for the origin and evolution of this system is extremely limited, because lungs do not fossilize and because the bellow-like air sacs in living birds only rarely penetrate (pneumatize) skeletal bone and thus leave a record of their presence. Methodology/Principal Findings We describe a new predatory dinosaur from Upper Cretaceous rocks in Argentina, Aerosteon riocoloradensis gen. et sp. nov., that exhibits extreme pneumatization of skeletal bone, including pneumatic hollowing of the furcula and ilium. In living birds, these two bones are pneumatized by diverticulae of air sacs (clavicular, abdominal) that are involved in pulmonary ventilation. We also describe several pneumatized gastralia (“stomach ribs”), which suggest that diverticulae of the air sac system were present in surface tissues of the thorax. Conclusions/Significance We present a four-phase model for the evolution of avian air sacs and costosternal-driven lung ventilation based on the known fossil record of theropod dinosaurs and osteological correlates in extant birds: (1) Phase I—Elaboration of paraxial cervical air sacs in basal theropods no later than the earliest Late Triassic. (2) Phase II—Differentiation of avian ventilatory air sacs, including both cranial (clavicular air sac) and caudal (abdominal air sac) divisions, in basal tetanurans during the Jurassic. A heterogeneous respiratory tract with compliant air sacs, in turn, suggests the presence of rigid, dorsally attached lungs with flow-through ventilation. (3) Phase III—Evolution of a primitive costosternal pump in maniraptoriform theropods before the close of the Jurassic. (4) Phase IV—Evolution of an advanced costosternal pump in maniraptoran theropods before the close of the Jurassic. In addition, we conclude: (5) The advent of avian unidirectional lung ventilation is not possible to pinpoint, as osteological correlates have yet to be identified for uni- or bidirectional lung ventilation. (6) The origin and evolution of avian air sacs may have been driven by one or more of the following three factors: flow-through lung ventilation, locomotory balance, and/or thermal regulation.
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                Author and article information

                Contributors
                Role: Editor
                Journal
                PLoS One
                plos
                plosone
                PLoS ONE
                Public Library of Science (San Francisco, USA )
                1932-6203
                2011
                28 February 2011
                : 6
                : 2
                Affiliations
                Museum of Paleontology and Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan, United States of America
                Raymond M. Alf Museum of Paleontology, United States of America
                Author notes

                Conceived and designed the experiments: JAW MDD TI EMM JAW. Performed the experiments: JAW MDD TI EMM JAW. Analyzed the data: JAW MDD TI EMM JAW. Contributed reagents/materials/analysis tools: JAW MDD TI EMM JAW. Wrote the paper: JAW MDD TI EMM JAW.

                Article
                PONE-D-10-03877
                10.1371/journal.pone.0017114
                3046170
                21386963
                bdae4f0b-a291-40a7-a03e-86a728bf5c59
                Wilson et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
                Page count
                Pages: 19
                Categories
                Research Article
                Biology
                Anatomy and Physiology
                Evolutionary Biology
                Paleontology
                Zoology
                Earth Sciences
                Paleontology

                Uncategorized

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