Abstract
BackgroundThe 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 FindingsWe 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/SignificanceThe 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.
Highlights
Living archosaurs are characterized by the presence of pneumatic outpocketings of the respiratory epithelium that invade certain bones
Among non-volant ornithodirans, axial pneumaticity is perhaps best developed in sauropod dinosaurs, in which pneumatic diverticulae leave their traces in postatlantal vertebrae and ribs, but apparently not chevrons (Fig. 1)
The nomenclatural system for vertebral fossae we propose here is based on our combined collections research at the institutions listed below
Summary
Living archosaurs (i.e., birds and crocodylians) are characterized by the presence of pneumatic outpocketings of the respiratory epithelium that invade certain bones. Axial pneumaticity can take the form of deep and sometimes complex invasion of internal bone, or in the form of spaces enclosed by bony laminae connecting the processes projecting from the neural arch The former, which we refer to as ‘‘internal pneumaticity,’’ displays variation that appears to characterize sauropod subgroups [7] and has important implications for sauropod paleobiology [8]. On the other hand, are typically bounded by vertebral laminae and homologized within and between taxa These structures display important phylogenetic variation that has not been extensively sampled far. 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
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