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Session Schedule & Abstracts

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Thursday 30th June, 2016

Symposium: New insights into skeletal microstructure of vertebrates, extant and extinct 4

Room: Salon A   4:30 pm–5:15 pm

Moderator(s): E. Rega, M. Dean, & T. Owerkowicz
BON4-1  4:30 pm  Microstructure isn't enough: Additional diagnostic criteria to test among hypotheses of bone tissue identity. Werning S.*, Des Moines University; Schweitzer M. H., North Carolina State University; Padian K., University of California, Berkeley
Abstract: Histologists describe skeletal tissues using many anatomical, compositional and developmental characters. Paleohistologists use these to infer the identity, function and evolution of fossil bone, but lack consensus on which characters are most useful, and often examine only a few microstructural features. This makes comparison difficult, leads to misidentification, and calls the resultant functional and evolutionary inferences into question. These problems worsen when fossil tissues share some but not all characters with extant bone, the range of extant variation is unknown, or etiologies cannot be linked unambiguously to bony features. We identify eight diagnostic criteria that test hypotheses of tissue identity: skeletal distribution, hormonal stimulus, development, duration, timing in context of life history, chemical composition, gross morphology and microstructure (using explicit aspects of fibrillar, vascular and osteocyte arrangement and morphology). All but stimulus can be evaluated in subfossil bone; most can be evaluated in fossils. We demonstrate the need for more comprehensive diagnoses using the examples of avian osteopetrosis (AOP; pathology) and medullary bone (MB; reproductive marker), common alternative hypotheses for endosteally-derived bone in bird-line archosaurs. Their correct identification informs questions of disease and life history evolution. We described the range of MB variation in extant birds, compared MB histology to that of genetically-diagnosed AOP, and re-evaluated every reported case of MB and AOP outside crown birds. Using the broader suite of characters, AOP can be rejected in most cases. Adopting common criteria and reporting more microstructural features provides greater diagnostic power and enables comparison among paleohistological studies. Additionally, clarifying definitions and diagnoses eliminates tautology by positing a test of an etiological hypothesis (definition) by independent lines of empirical evidence (diagnosis).

BON4-2  4:45 pm  Localized resorption spaces in femoral cortical bone of a mature Tyrannosaurus rex (Theropoda) are adaptive response to muscle traction. Rega E.A.*, Western University of Health Sciences; Mathew N., Western University of Health Sciences; Weis B., Western University of Health Sciences; Noriega K., Western University of Health Sciences
Abstract: Skeletochronology and growth curves have necessarily been the preoccupation of the majority of histological studies in Dinosauria. Because of specimen scale, rarity and the necessity of sampling areas most likely to produce lines of arrested growth (LAGS), research involving productive and expendable bones such as ribs and fibulae are over-represented in the literature. In this study, an unusual complete cross-section of the proximal femoral bone of a mature Tyrannosaurus rex was created for exhibit in for the Los Angeles County Museum of Natural History. This complete cross section affords the opportunity to evaluate bone response to localized loading conditions. The cranial, lateral and medial surfaces of the femoral cortical bone are composed of the typical fibrolamellar bone normally observed in this location in other specimens. However, numerous large resorption spaces throughout the posterior cortical bone demonstrate a localized osteopenia which is initially difficult to reconcile with the stresses placed on this limb. Hypotheses including pathology and bone depletion to support egg development are considered. However, traction of the m. caudofemoralis could provide the best explanation. These resorption spaces are lined with 1-2 concentric lamellae of reactive bone after cavity formation, The circular nature of the lacunae and the shell of bone would serve to resist crack propagation and lamellar separation which would otherwise occur under conditions of tension produced by muscle inserting over the area.

BON4-3  5:00 pm  Preliminary results on the bone histology of hadrosaurs (Ornithopoda, Dinosauria) from the Latest Cretaceous of Far Eastern Russia. Stein Koen*, Earth System Science - AMGC, Vrije Universiteit Brussel; Royal Belgian Institute of Natural Sciences, Directorate 'Earth and History of Life', Rue Vautier, 29, 1000 Brussels, Belgium; Bolotski Yuri, Russian Academy of Sciences, Far Eastern Branch, Blagoveschensk, Russian Federation; Bolotski Ivan, Russian Academy of Sciences, Far Eastern Branch, Blagoveschensk, Russian Federation; Claeys Ph., Earth System Science - AMGC, Vrije Universiteit Brussel; Godefroit Pascal, Royal Belgian Institute of Natural Sciences, Directorate 'Earth and History of Life', Rue Vautier, 29, 1000 Brussels, Belgium
Abstract: Hadrosaurs such as Olorotitan and Edmontosaurus, were among the most abundant and successful Cretaceous dinosaurs. They spread to all continents and some reached body masses rivaling those of the giant sauropods. However, some hadrosaur groups disappeared from the North American continent 5 to 6 million years before the extinction event, whereas the same groups thrived in Asia until the event. The Maastrichtian hadrosaur communities from Far Eastern Russia are a prime example to study the biology of late surviving lambeosaurine hadrosaurs. The bonebeds from which these taxa were excavated show a high species diversity and a high abundance of various skeletal elements, most notably long bones. We sampled long bones from different size classes of the lambeosaurines Olorotitan and Amurosaurus. Both taxa show highly vascularized cortical bone consisting mainly of a woven-parallel fibered complex with plexiform to laminar vascularization, indicative of high growth rates. Only few lines of arrested growth (LAG) could be observed in even the biggest specimens (femur length ~1 m). It remains unclear if this indicates an uninterrupted juvenile growth phase, or if mass increase was extremely high in the first years, but the high average growth rate is in accordance with high growth rates reported for North American hadrosaurs (e.g. Maiasaura). We also sampled the tibia of a large tyrannosaurid theropod (corresponding femur length ~ 0.92 m) which was recovered from the same layers as Olorotitan, however, its growth is in sharp contrast with that of the hadrosaurs. The thin section taken from the shaft of the proximal tibia shows a woven-parallel fibred complex with plexiform to laminar vascularization, however, it also shows numerous (>15) LAGs, indicating a much slower average growth and higher chance of survivorship than for hadrosaurs. These data add to a more global perspective on diversity and ecology of Latest Cretaceous dinosaur faunas.

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