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Session Schedule & Abstracts

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Friday 1st July, 2016

Paleontology 3

Room: Salon F   9:30 am–11:00 am

Moderator(s): Griffin C, Moustakas-Verho J
PAL3-1  9:30 am  Intraspecific variation and the evolution of the ancestral dinosaurian growth condition. Griffin C. T.*, Virginia Tech
Abstract: Understanding growth patterns of extinct clades has been a persistent problem in vertebrate paleontology, especially for dinosaurs and other archosaurs, which possess the widest range of body sizes and growth rates of any group of terrestrial vertebrates. Determining the ontogenetic stage of an individual is important for paleontological interpretation, but there are few non-destructive methods for determining skeletal maturity. Understanding the ancestral dinosaurian growth condition is vital to interpreting the evolution of this clade. To better understand this question, I analyzed 29 ontogenetically variable characters in the early theropod dinosaurs Coelophysis bauri and Megapnosaurus rhodesiensis. These taxa are temporally and phylogenetically close to the origin of dinosaurs and are known from large growth series from single populations. I used ontogenetic sequence analysis to reconstruct growth pathways and quantify the amount of intraspecific variation in growth, then used non-metric multidimensional scaling (NMDS) to test if these characters vary continuously. These data suggest a high level of intraspecific variation, with >50 equally parsimonious developmental sequences found for these taxa. NMDS analysis found a single cluster of individuals with no evidence of bimodality, suggestive that these characters are not dimorphic. In these results, size is a poor predictor of skeletal maturity. Variation in growth patterns is widespread among early dinosaurs and other dinosauriforms, suggesting that this high level of variation in growth is the ancestral dinosaurian condition. Given that strong variation in ontogeny is absent in more derived theropods (e.g., Coelurosauria), this intraspecific variation was lost during the evolution to living birds. Such variable characters should be used with care in cladistic analyses. High variation should be assumed in early dinosaur taxa and may not be indicative of taxonomic diversity or sexual difference.

PAL3-2  9:45 am  Wing-bone thickness and bending resistance in pterosaurs. Martin-Silverstone E*, University of Southampton/University of Bristol
Abstract: Pterosaurs were both the first true flying vertebrates, and the largest animals to fly, with wingspans up to 11m. They have long been studied for their biomechanics and flight capabilities, especially the larger species. Classical and recent literature describe pterosaurs as having extremely thin-walled bones, particularly in proportion to the bone diameter, (except for known outliers like dsungaripteroids), as well as having the highest degree of pneumaticity of any animals. A new large-scale comparative dataset was analysed to test this proposition. Over 60 pterosaur wing bones from several families were studied. Air/marrow space proportion (ASP/MSP) and radius/thickness (R/t) values were calculated and compared to previous studies. Pterosaur ASP and MSP values range from 0.12 in apneumatised bones to 0.90 in highly pneumatised bones. The lower ASP and MSP values are similar to those seen in extant birds, while higher values remain greater than those seen in any other animal. Contrary to previous studies, there is a high occurrence of low R/t values in pterosaurs (R/t between 1 and 7.99) with 75% of values falling within this range, more similar to R/t values seen in birds, with smaller animals having the lower R/t values. R/t values and diameter are related to the Second Moment of Area (I), a geometrical property correlated with bending stiffness, an important property of pterosaur wing bones supporting flight loads. I varies with the fourth power of diameter, with values ranging from as low as 30 mm4 for small diameter thick-walled pterosaur bones to 17,000 mm4 for large thin-walled pterosaur wing bones. The allometry of scaling dictates that mass reduction becomes increasingly important with size. Consequently as pterosaurs become larger, the bone wall thickness is reduced relative to overall diameter, providing adequate bending stiffness in combination with relatively low mass, at the cost of increased bone fragility.

PAL3-3  10:00 am  Evolutionary increases in vertebral regionalization within the mammalian lineage: evidence from fossil synapsids. Jones K. E.*, Harvard University; Polly P. D., Indiana University; Head Jason, University of Nebraska-Lincoln; Angielczyk K. D., Field Museum of Natural History; Pierce S. E., Harvard University
Abstract: Tetrapods primitively have a vertebral column that is divided into distinct neck (cervical vertebrae) and trunk (dorsal vertebrae) regions. In contrast, strong regionalization of dorsal vertebrae into thoracic (ribbed) and lumbar (ribless) regions is thought to be an independently derived feature of mammals and archosaurs. Recently, however, subtle regionalization has also been found in the trunks of lepidosaurs that corresponds to HOX expression boundaries, suggesting that underlying regionalization patterns may be more universal than previously thought. To resolve the evolutionary origin of the strongly differentiated trunk in mammals, we determined the degree of regionalization in their extinct stem-group, the non-mammalian synapsids. Axial regionalization was measured in non-mammalian synapsids, extant mammals and sauropsids, using a likelihood-based method that requires no a priori knowledge of region boundaries. Vertebral shape was quantified using linear measures, and multivariate segmented regressions were used to model variation in all possible vertebral regions. Finally, the ‘best’ model for region boundaries was selected from the data using the Akaike Information Criterion based on regression residuals. Among extant species, we find support for four presacral regions in mammals, archosaurs and lepidosaurs, reflecting cervical, cervicothoracic, anterior dorsal and posterior dorsal modules; irrespective of the presence of a ribless lumbar region. Conversely, non-mammalian synapsids show more variable regionalization patterns, with a minimum of two regions in basal “pelycosaurs” and up to four in advanced non-mammalian cynodonts. Thus, despite the ubiquity of regionalization among extant amniotes, our data suggest increasing regionalization through synapsid evolution, and raises questions about the ancestral amniote condition for axial patterning.

PAL3-4  10:15 am  The cave bear story: integrating paleontological and developmental evidence. Moustakas-Verho JE*, Institute of Biotechnology, University of Helsinki; Jernvall J, Institute of Biotechnology, University of Helsinki
Abstract: The fields of paleontology and developmental biology contribute the significant element of time to evolutionary theory. Though these timescales differ greatly, the integration of these disciplines is key to understanding mechanisms of morphologic expression and evolutionary change. Paleontological data, especially fossil collections, provide our only direct evidence of past biodiversity and how individual species, communities, and ecosystems respond to environmental change on evolutionary and long-term ecological timescales. The development of proxies and models for predicting environmental or genetic stress associated with extinction is a major research challenge and has important economic and societal implications. The European cave bear, Ursus spelaeus, was part of the Pleistocene megafauna that went extinct during the Last Glacial Maximum. Why the cave bear went extinct, whereas other bears such as the brown bear did not, has been a central question in evolutionary biology. In our study of the evolution and development of the dentition in bears, we try to understand not only morphological variation, but also extinction. The shape of the mammalian dentition, which is determined prior to eruption and modified only by wear, is strongly correlated with diet. Tooth shape can be described by the patterns of cusps that compose the crown, or chewing surface, of the tooth. We use a metric called dental complexity to reconstruct the diet of cave bears and develop a system whereby dental variation can be measured as an indicator of environmental and genetic stress in a population.

PAL3-5  10:30 am  Morphometry and behavioural Biology: As seen in the humerus of Pleistocene tiger (Panthera cf. tigris Pocock 1929) from Manjra Valley, India. Sathe Vijay*, Deccan College Post Graduate and Research Institute; Chakraborty P., Deccan College Post Graduate and Research Institute
Abstract: The Palaeobiogeography of large predators in India is largely based on scanty and fragmented fossil materials. In fact, carnivore fauna in Indian Pleistocene is mostly elusive and as a result any interpretation of their behaviourial biology tends to be theoretical and conjectural. Presently in India, the niche of apex predator is almost always filled by large felids, while canids have occupied a secondary level in the trophic system. Therefore, the study of predator biology and behaviour in Pleistocene fauna must look extensively at large felid fossils, at least at this initial stage. It is here that the recent discovery of a well-preserved, almost complete humerus of Panthera cf. tigris from Pleistocene formations in the Manjra River valley near Harwadi (Latur dist. Maharashtra), is of particular interest, since it provides a hitherto unprecedented potential for systematic palaeontology and morphometric analysis. Its state of preservation has allowed a near-complete set of measurements, which have hitherto not been available given the paucity of the fossil record of carnivore specimens in India. The availability of a humerus is an additional convenience since the humerus is an important element in the biomechanics of any animal, particularly one as mobile and active as the tiger. Studies have been carried out on the connection between humerus morphometry and behavioural biology of various felid species, such as the recent study by Meloro and his team, which compared the humerus measurements of several different felid species worldwide. This was done as an attempt to correlate the morphology of various parts of the humerus with certain activities and behaviour involving predation and hunting patterns of the respective felids. The present study utilises a similar methodology in measuring the aforementioned specimen from Harwadi and offers some insights into the behaviourial biology of Pleistocene tiger in India.

PAL3-6  10:45 am  Evolution and function of the angular process in early mammalian jaws. Grossnickle D. M.*, University of Chicago
Abstract: The lower jaws of mammaliaforms of the Mesozoic Era (252-66 million years ago) have shown disparate morphologies, and it is expected that such major differences correlate with modifications in biomechanical functions. To address this issue I use fossil data to analyze morphological and functional changes in lower jaws of mammaliaforms and early mammals. In particular, I focus on the angular process, which is functionally important as the insertion site for two primary jaw adductor muscles: the medial pterygoid and the superficial masseter. Morphometric analyses are used to establish macroevolutionary trends. A key result is that the angular process of cladotherians (i.e., eutherian-placentals, metatherian-marsupials, and eupantotherians) does not appear to be homologous with the angular process of non-mammalian mammaliaforms (e.g., Morganucodon), and the process is more posteriorly positioned in cladotherians. The results of the morphometric analyses are used as the framework for three-dimensional modeling to examine the functional effects of evolutionary changes to the angular process. The position and size of the process is artificially manipulated to mimic evolutionary changes, and relative mechanical advantages of jaw adductor muscles are calculated. Results indicate that the posterior extension of the cladotherian angular process reduces the mechanical advantage for orthal jaw moments. However, molar morphologies of cladotherians suggest that the chewing cycle is not purely orthal, and instead includes mediolateral movement, which is likely produced by rotation around a vertical axis of rotation. When the jaw model is restricted to this type of rotational movement, the presence of a posterior angular process increases the mechanical advantages by lengthening the moment arms of the jaw adductor muscles. These results suggest that molar and jaw morphologies evolved in concert to produce a more complex chewing cycle involving increased mediolateral movement.

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