John A. Whitlock

A Nomenclature for Vertebral Fossae in Sauropods and Other Saurischian Dinosaurs

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.

Inferences of Diplodocoid (Sauropoda: Dinosauria) Feeding Behavior from Snout Shape and Microwear Analyses

Background As gigantic herbivores, sauropod dinosaurs were among the most important members of Mesozoic communities. Understanding their ecology is fundamental to developing a complete picture of Jurassic and Cretaceous food webs. One group of sauropods in particular, Diplodocoidea, has long been a source of debate with regard to what and how they ate. Because of their long lineage duration (Late Jurassic-Late Cretaceous) and cosmopolitan distribution, diplodocoids formed important parts of multiple ecosystems. Additionally, fortuitous preservation of a large proportion of cranial elements makes them an ideal clade in which to examine feeding behavior. Methodology/Principal Findings Hypotheses of various browsing behaviors (selective and nonselective browsing at ground-height, mid-height, or in the upper canopy) were examined using snout shape (square vs. round) and dental microwear. The square snouts, large proportion of pits, and fine subparallel scratches in Apatosaurus, Diplodocus, Nigersaurus, and Rebbachisaurus suggest ground-height nonselective browsing; the narrow snouts of Dicraeosaurus, Suuwassea, and Tornieria and the coarse scratches and gouges on the teeth of Dicraeosaurus suggest mid-height selective browsing in those taxa. Comparison with outgroups (Camarasaurus and Brachiosaurus) reinforces the inferences of ground- and mid-height browsing and the existence of both non-selective and selective browsing behaviors in diplodocoids. Conclusions/Significance These results reaffirm previous work suggesting the presence of diverse feeding strategies in sauropods and provide solid evidence for two different feeding behaviors in Diplodocoidea. These feeding behaviors can subsequently be tied to paleoecology, such that non-selective, ground-height behaviors are restricted to open, savanna-type environments. Selective browsing behaviors are known from multiple sauropod clades and were practiced in multiple environments.

Description of a Nearly Complete Juvenile Skull of Diplodocus (Sauropoda: Diplodocoidea) from the Late Jurassic of North America

ABSTRACT More than any other sauropod dinosaur group, the long-necked herbivores belonging to Diplodocoidea have been defined by their skulls. Their unique skull shape, which is extremely elongate antorbitally, with a transversely broad, square snout packed at its anterior extreme with narrow-crowned, pencil-like teeth, has served as a touchstone for describing the biology of these animals ever since the discovery of the first skull in the late 19th century. In particular, the unusual diplodocoid skull has been discussed frequently in the context of examining feeding behavior, spawning hypotheses ranging from branch stripping, propalinal shearing, and aquatic plant ‘grazing.’ Here, we describe a juvenile skull of Diplodocus (Carnegie Museum 11255) that does not share the unusually blunted snout and anteriorly sequestered teeth seen in adult specimens, suggesting that adults and juveniles may have differed greatly in their feeding behavior, an ontogenetic distinction that may be unique among sauropodomorphs.

Evolution of high tooth replacement rates in sauropod dinosaurs.

Background Tooth replacement rate can be calculated in extinct animals by counting incremental lines of deposition in tooth dentin. Calculating this rate in several taxa allows for the study of the evolution of tooth replacement rate. Sauropod dinosaurs, the largest terrestrial animals that ever evolved, exhibited a diversity of tooth sizes and shapes, but little is known about their tooth replacement rates. Methodology/Principal Findings We present tooth replacement rate, formation time, crown volume, total dentition volume, and enamel thickness for two coexisting but distantly related and morphologically disparate sauropod dinosaurs Camarasaurus and Diplodocus. Individual tooth formation time was determined by counting daily incremental lines in dentin. Tooth replacement rate is calculated as the difference between the number of days recorded in successive replacement teeth. Each tooth family in Camarasaurus has a maximum of three replacement teeth, whereas each Diplodocus tooth family has up to five. Tooth formation times are about 1.7 times longer in Camarasaurus than in Diplodocus (315 vs. 185 days). Average tooth replacement rate in Camarasaurus is about one tooth every 62 days versus about one tooth every 35 days in Diplodocus. Despite slower tooth replacement rates in Camarasaurus, the volumetric rate of Camarasaurus tooth replacement is 10 times faster than in Diplodocus because of its substantially greater tooth volumes. A novel method to estimate replacement rate was developed and applied to several other sauropodomorphs that we were not able to thin section. Conclusions/Significance Differences in tooth replacement rate among sauropodomorphs likely reflect disparate feeding strategies and/or food choices, which would have facilitated the coexistence of these gigantic herbivores in one ecosystem. Early neosauropods are characterized by high tooth replacement rates (despite their large tooth size), and derived titanosaurs and diplodocoids independently evolved the highest known tooth replacement rates among archosaurs.

Biology of tooth replacement in amniotes

Tooth replacement is a common trait to most vertebrates, including mammals. Mammals, however, have lost the capacity for continuous tooth renewal seen in most other vertebrates, and typically have only 1–2 generations of teeth. Here, we review the mechanisms of tooth replacement in reptiles and mammals, and discuss in detail the current and historical theories on control of timing and pattern of tooth replacement and development.

The dentary of Suuwassea emilieae (Sauropoda: Diplodocoidea)

Sauropod cranial elements, despite their rarity, contain a significant proportion of the known phylogenetically important character data (see Wilson, 2002; Upchurch et al., 2004). In particular, cranial characters are important in distinguishing between the two main neosauropod lineages, Diplodocoidea and Macronaria, as well as between the various lineages of diplodocoid sauropods. One recently described diplodocoid, Suuwassea emilieae, has proven difficult to place phylogenetically despite a relative wealth of cranial data (Harris, 2006a). As a putatively basal member of the Late Jurassic radiation of diplodocoids, Suuwassea is an important taxon for the understanding of diplodocoid sauropod evolution. Although it has been variously recovered as a member of either the primarily Laurasian diplodocids or the Gondwanan dicraeosaurids, the precise relationship between those lineages and Suuwassea has remained uncertain. Here we describe a recently discovered dentary assignable to the holotypic specimen of Suuwassea emilieae (Academy of Natural Sciences 21122). This dentary possesses important character data that suggest dicraeosaurid affinities for Suuwassea; as a consequence, Suuwassea is potentially the only recognized Laurasian member of Dicraeosauridae. The description of this dentary also adds to our scarce knowledge of sauropod dentaries. Institutional Abbreviations AMNH, American Museum of Natural History, New York City, U.S.A.; ANS, Academy of Natural Sciences, Philadelphia, U.S.A.; CM, Carnegie Museum of Natural History, Pittsburgh, U.S.A.; CMC, Cincinnati Museum Center, Cincinnati, U.S.A.; MB, Museum fur Naturkunde der Humboldt-Universitat zu Berlin, Berlin, Germany; MNN, Musee National du Niger, Niamey, Niger; MOR, Museum of the Rockies, Bozeman, U.S.A.; USNM, United States National Museum, Washington, D.C., U.S.A.

Phylogenetic relationships of the Eocene percomorph fishes †Priscacara and †Mioplosus

ABSTRACT The early Eocene great lakes of the Green River system preserved a rich fauna, allowing detailed study of the paleoecology of the area. Two genera, †Priscacara and †Mioplosus, are interesting because they are among the earliest well-known representatives of Percoidei sensu lato, and so offer a chance to explore the evolution of the North American fish fauna. A phylogenetic analysis including †Priscacara, †Mioplosus, and representatives of some ‘basal’ percoid families suggests that †Priscacara is a member of Moronidae, and that †Mioplosus has affinities with Lateolabrax, Siniperca, and Lates. †Priscacara is recovered as paraphyletic relative to the rest of Moronidae, primarily on the basis of meristic characters; in the absence of more robust evidence, both †P. serrata and †P. liops are retained within †Priscacara. The phylogenetic position of †Priscacara suggests a trans-Atlantic connection, because Moronidae contain genera from eastern North America and Europe. †Mioplosus is suggestive of an Asian link to the Eocene Green River system, due to its relationship with the Western Pacific Lateolabrax, the Asian Siniperca, and the African/Asian Lates.

Was Diplodocus (Diplodocoidea, Sauropoda) capable of propalinal jaw motion?

Citation for this article: Whitlock, J. A. 2017. Was Diplodocus (Diplodocoidea, Sauropoda) capable of propalinal jaw motion?. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2017.1296457.

A Juvenile Specimen of Barosaurus Marsh, 1890 (Sauropoda: Diplodocidae) from the Upper Jurassic Morrison Formation of Dinosaur National Monument, Utah, USA

ABSTRACT We describe five partial middle and posterior dorsal vertebrae from an upper Kimmeridgian exposure of the Upper Jurassic Morrison Formation at the Carnegie Quarry of Dinosaur National Monument (Utah, USA), and refer these vertebrae to the poorly known diplodocid sauropod dinosaur genus Barosaurus Marsh, 1890. The small size and unfused neurocentral junctions of these vertebrae indicate that they belonged to a subadult individual and the osteologically youngest specimen yet described for the genus. We also provide corroborating evidence for the pneumatic hiatus previously hypothesized for Barosaurus and identify a previously unrecognized autapomorphy of the genus, the presence of an anterodorsallyoriented accessory lamina arising from the spinodiapophyseal lamina on dorsal vertebrae. Furthermore, we document a temporal uncoupling of neural spine lamination and pneumatization of the centrum, such that full development of the laminae preceded development of sharp-lipped pleurocoels in dorsal vertebrae.