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Session Schedule & Abstracts

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Sunday 3rd July, 2016

Paleontology 6

Room: Salon F   4:30 pm–5:30 pm

Moderator(s): Henderson DM, Manafzadeh A
PAL6-1  4:30 pm  Testing the buoyancy of an immersed Spinosaurus (Dinosauria: Theropoda) with a digital model. Henderson DM*, Royal Tyrrell Museum of Palaeontology
Abstract: A recent interpretation of the fossil remains of the enigmatic, large predatory dinosaur Spinosaurus aegyptiacus proposed that it was specially adapted for an aquatic mode of life – a first for any predatory dinosaur. A detailed, three-dimensional, digital model of the animal was generated and the flotation potential of the model was tested using specially written software. It was found that Spinosaurus would have been able to float with its head clear of the water surface. However, a similarly detailed model of Tyrannosaurus rex was also able to float in a position enabling the animal to breathe freely, showing that there is nothing exceptional about a floating Spinosaurus. The software also showed that the centre of mass of Spinosaurus was much closer to the hips than previously estimated, implying that this dinosaur would still have been a competent walker on land. With regional body densities accounting for pneumatized skeletons and system of air sacs (modelled after birds), both the Spinosaurus and Tyrannosaurus models were found to be unsinkable, even with the air sacs substantially deflated. The conclusion is that Spinosaurus would still have been a competent terrestrial animal.

PAL6-2  4:45 pm  Correlated and stepwise evolution of tail weaponry in mammals, turtles, and dinosaurs. Arbour V. M.*, North Carolina Museum of Natural Sciences; Zanno L. E., North Carolina Museum of Natural Sciences
Abstract: Weaponry is a pervasive trait among amniotes that appears to evolve in predictable patterns when underpinned by similar socioecological and habitat parameters. Most animal weapons are located on either the head or the limbs, despite the trade-offs that must occur given the critical functions these parts of the body must perform. In contrast, tail weaponry is an extremely rare occurrence in amniotes, with specialized tail weaponry present in only three highly disparate clades: mammals, turtles, and dinosaurs. This suggests that 1) tail weapons evolve only under very specific selective regimes, and/or 2) there may be anatomical constraints that prevented the repeated evolution of tail weaponry in amniotes. Here we examine the evolution of stiff tail clubs in these three clades, by investigating which anatomical and ecological features are correlated with this feature. All amniotes with stiff tail clubs also bear osteoderms, are quadrupedal, and have ornamentation anteriorly on the body (horns and crests on the skull, and/or humps and spikes in the pectoral region). The tail clubs of ankylosaurids, glyptodonts, and meiolaniids all share a broadly similar gross morphology, but each evolved through unique evolutionary pathways. Ankylosaurids had modified distal vertebrae that tightly interlocked to form a rigid handle, and enlarged osteoderms at the tip of the tail forming an axe-like knob of bone. In contrast, glyptodonts and meiolaniids enveloped the tail in anteriorly in rings of osteoderms and posteriorly in an immobile distal tube of osteoderms. Similar to ankylosaurids, glyptodont tail clubs went through a stepwise acquisition of characters, with stiffening of the distal tail via encasement in an osteodermal tube preceding the expansion of the terminus of the tail, a feature only found in the doedicurine glyptodonts.

PAL6-3  5:00 pm  Modeling fragmentary dentaries as beams to test hypotheses of differing diets. Manafzadeh AR*, UC Berkeley; Holroyd PA, UC Berkeley; Rankin BD, UC Berkeley
Abstract: Differences in diet correlate with various morphological traits, permitting inferences about the feeding behavior of extinct taxa from fossil material. For mammals, most studies rely on complete cranial material or unworn teeth, but this excludes the many taxa known only from fragmentary fossils. Here we explore a method that requires only partial dentaries and provides an additional line of evidence for dietary reconstructions. This method exploits the relationship between the resistance of the dentary to bending and the capacity of an animal to process mechanically tougher food types, and places measurements of bending in a comparative context to determine to what extent different taxa may have fed on mechanically tough foods. Micro-CT scans were prepared for 16 dentaries representing ten late Paleocene to early Eocene (57 to 52 million year old) mammals from the Western Interior of North America that have been hypothesized to process prey with different mechanical properties. For each specimen, the thickness and distribution of mandibular cortical bone were analyzed at homologous cross-sections along the tooth row to calculate section moduli (geometric properties of beams) using the BoneJ plugin to ImageJ. Comparison of section moduli allowed an assessment of each taxon’s relative resistance to the bending stresses experienced during prey processing. Although members of the Pantolestidae, a group of otter-like mammals, have been hypothesized to take mechanically tougher foods, analysis of their mandibular cortical bone distribution did not reveal any specialization for resistance to dorsoventral or mediolateral bending compared to contemporaneous mammals. The biomechanical properties of the dentary can be calculated from fragmentary material, are independent of phylogeny, and reflect bone remodelling in response to forces encountered during life.

PAL6-4  5:15 pm  Specialized wear facets in mammalian dentitions. Koenigswald W*, Steinmann Institut (Paläontologie) der Universität Bonn
Abstract: Tooth wear provides important information about the life of fossil and extant mammals. Nevertheless, wear is not appreciated very much among paleontologists, for wear eliminates details essential to phylogenetic studies. Bunodont teeth, e.g. human teeth, are often worn gradually and continuously. In contrast to such functional “unspecialized” teeth, specific wear facets characterize other teeth. This paper concentrates on the three types of wear facets that are widely distributed in the dentitions of herbivorous and carnivorous mammals, fossil and recent. They are discussed in their function and compared to technical tools. A sequence of wear stages was deduced from the functional “unspecialized” teeth based on the amount of dentine exposed. As a result of continuous wear, these phases are of almost equal functional significance and can be interpreted as phases of the late ontogeny of the teeth. However, those teeth that are characterized by the discussed wear facets do not show continuous wear, but function only during one of the ontogenetic phases. Each facet can be correlated to a specific phase. This phase becomes considerably prolonged during ontogeny and the preceding or subsequent phases correspondingly shortened. Thus, the specialization marked by wear facets show a heterochronic pattern in the life history of teeth.

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