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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 3

Room: Salon A   2:30 pm–4:00 pm

Moderator(s): E. Rega, M. Dean, & T. Owerkowicz
BON3-1  2:30 pm  3D virtual bone histology reveals the life history of the early tetrapod Acanthostega . Sanchez S.*, Uppsala University and SciLife Lab; Tafforeau P., European Synchrotron Radiation Facility; Clack J. A., University of Cambridge; Ahlberg P. E., Uppsala University
Abstract: How did tetrapod adapt to a life on land? This question has been a focus of interest for geologists, palaeontologists and evolutionary biologists for more than a century. Although various hypotheses have been put forward about the selective pressures of terrestrialisation, we actually know almost nothing about the life histories and reproductive strategies of the earliest tetrapods. Here we show 3D virtual histological data based on the long-bone microstructure of the Devonian (360 million-year-old) tetrapod Acanthostega to elucidate its development and life history. We discovered that the Greenland mass-death locality, which yielded the specimens of Acanthostega, was dominated by, and might in fact consist exclusively of, juveniles. This predominance suggests that the juveniles gathered in schools and were separated from the adults at least at certain times. The late onset of limb ossification indicates that the juvenile Acanthostega were exclusively aquatic. The palaeoecology of adults however would remain unknown. The juveniles grew almost to full observed size before their humeri began to ossify. This developmental trait is primitive for tetrapods as it aligns with the lobe-fin development of the Devonian fish Eusthenopteron. As well, Acanthostega proved to have a long pre-reproductive growth period (spanning much more than 6 years, probably at least a decade) as in lobe-finned fishes and Palaeozoic tetrapods (Discosauriscus). Such a prolonged juvenile stage in this early tetrapod is considerably longer than in most extant amphibians. This study highlights the utility of synchrotron microtomography as a non-destructive tool to shed new light on the palaeobiology and life history of key fossils. Even a single limb bone can, in principle, provide crucial information. Not only useful for its non-destructive aspect, virtual bone histology also permits 3D modelling, thereby yielding a more complete overview of fossil bone evolution.

BON3-2  2:45 pm  Adaptive patterns in aquatic amniote bone microanatomy: was it more complex than previously thought? Houssaye A*, CNRS/Museum National d'Histoire Naturelle Paris; Klein N
Abstract: Numerous amniote groups are secondarily adapted to aquatic life. This change of habitat naturally leads to numerous convergences (e.g., streamlined body, limb transformation into paddles, modification of sense organs, reproductive mode, diving adaptations, inner bone structure specializations). The various adaptive traits vary pending on the degree of adaptation to aquatic life, between shallow water taxa still able to occasionally locomote on land and open-marine forms totally independent from the terrestrial environment, but also between surface swimmers and deep divers, freshwater and marine forms. As a consequence, despite convergences, there is a high diversity within aquatic amniotes in shape, size, physiology, swimming mode. Bone microanatomy is considered to be strongly associated with bone functional requirements, thus is a powerful tool to understand bone adaptation to functional constraints and to make functional inferences on fossil taxa. Two major microanatomical specializations have been described in aquatic amniotes, referred to as bone mass increase and a spongious "osteoporotic-like" organization respectively. However, between extremes in these specializations, numerous intermediary (and some contradictory) patterns occur. Moreover aquatic taxa display various distributions of these specializations in their skeleton. Here we propose, based on the analysis of the various microanatomical patterns observed in long bones, vertebrae and ribs of a large sample of aquatic amniotes, to illustrate and discuss this variability and the distinct types of microanatomical adaptations to various aquatic ecologies.

BON3-3  3:00 pm  Bone histology of osteoderms of archosauriform diapsid reptiles (Sauropsida: Archosauriformes). Scheyer T. M.*, University of Zurich, Palaeontological Institute and Museum, Karl Schmid-Strasse 4, 8006 Zurich, Switzerland; Desojo J. B., CONICET, Sección Paleontología Vertebrados, Argentina y Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, Av. Angel Gallardo 470, C1405DJR, Buenos Aires, Argentina; Cerda I. A., CONICET, Argentina y Instituto de Investigaciones en Paleobiología y Geología, Universidad Nacional de Río Negro, Museo Carlos Ameghino, Belgrano 1700,Paraje Pichi Ruca (predio Marabunta), 8300 Cipolletti, Río Negro, Argentina
Abstract: The presence of osteoderms (dermal armor) within the integument is widespread in many vertebrate lineages, hence the integumentary skeletogenic potential has been proposed to represent a case of deep homology of vertebrates. The presence of osteoderms can be seen as a plesiomorphic trait in archosauriforms, constituting a very important skeletal component, besides the endoskeleton, which is useful for taxonomy and systematics and is known to serve multiple functional aspects. Among archosauriforms, some stem group members (Doswelliidae, Proterochampsia, Phytosauria) and many crown group archosaurs (Pseudosuchia, Avemetatarsalia) show stunning examples of osteoderm formation, such as bucket-sized osteoderms of titanosaurian sauropods, stegosaur plates and spikes, ankylosaur tail clubs, or hand-sized paravertebral shield osteoderms of giant crocodylians. It is surprising that until recently, only few studies (mostly focusing on Avemetatarsalia) on the development, microanatomy, and histology of osteoderms were available, especially when compared to histological studies focusing on long bones. Here we compare and review the osteoderm histology of several Archosauriformes, including doswelliids and proterochampsians, as well as crown group members (aetosaurs, rauisuchians, and few crocodylians). Results reveal a diversity of histological structures, but the mode of growth of all osteoderms is intramembraneous or metaplastic ossification. Besides crocodylian osteoderms, at least some members of the each group presents woven or fibrolamellar bone tissue, indicating faster bone deposition rates, whereas lamellar-zonal tissue in crocodylian osteoderms indicates a reversal to lower growth rates. Further, doswelliid osteoderm histology bears closer resemblance to phytosaurs and pseudosuchians than to proterochampsians and osteoderm histology in fossil archosauriforms has become an important tool for age determination of individuals, and even for growth curve reconstruction.

BON3-4  3:15 pm  In search of the basal amniote condition of Lines of Arrested Growth (or something along those lines...). Sumida S.S.*, California State University of San Bernardino; Segovia B., Western University of Health Sciences; Mathew N., Western University of Health Sciences; Rega E.A., Western University of Health Sciences   
Abstract: Lines of arrested growth ("LAGs") are normally interpreted as cessation or slowing of growth between more favorable growing seasons. In fossil specimens this is confounded by animals that lived in environments with little seasonal variation, experienced little resource variation, or in endothermic animals whose physiology may have buffered formation of clearly visible LAGs. Prolific recent advances have been seen in dinosaurian archosaurs, and within the varying grades of Synapsida, but little has been done to determine whether patterns seen in these two groups are homologous or convergent. The Late Paleozoic Diadectomorpha have been suggested as basal-most amniotes, or as the sister group to Amniota as traditionally defined. Regardless, definitive LAGs in the group could provide a clearer understanding of the basal condition for LAGs for all Amniota. Diadectomorphs present significant potential, as concentric bony ridges mark the zygapophyseal surfaces in a pattern that radiate from their innermost aspect to the distal edges. These ridges have been proposed as indicators of annual or seasonal growth. Universally considered as ectotherms, diadectomorphs have been recovered from deposits reflective of seasonally variable environments. If the ridges could be shown to correspond to internal LAGs, a basal amniote condition for LAGs might be developed. Virtually all such assertions that LAGs are equivalent to annual growth lines for derived members of Synapsida and Dinosauria have been made in the absence of a testable models tying LAGs to documented age. Studies involving crocodilians could bear on such assertions in dinosaurian archosauromorphs, but may be less reliable for the more distantly related Synapsida. Understanding the condition in diadectomorphs could set a baseline standard for LAG characteristics and/or polarity of LAG features for all Amniota, and whether the formation of LAGs in Synapsida and Reptilia should be considered as independently acquired.

BON3-5  3:30 pm  Novel insight into the growth dynamics of sauropodomorph dinosaurs. Cerda I.A., CONICET-Instituto de Investigaciones Paleobiología y Geología; Chinsamy-Turan A.*, University of Cape Town; Pol D; Apaldetti C, 4CONICET- Instituto y Museo de Ciencias Naturales; Otero A, CONICET- Instituto y Museo de Ciencias Naturales; Powell JA; Martinez R
Abstract: The Sauropodomorpha comprise of the more basal members, the non-sauropod sauropodomorphs, and the more derived members of the clade, the sauropod dinosaurs. The basal sauropodomorph dinosaurs are reported to have cyclical growth dynamics (evidenced by the cyclical deposition of growth marks), while the sauropods, are considered to have uninterrupted rapid rates of growth (inferred from the lacks of growth marks). These deductions pertaining to the growth dynamics of the Sauropodomorpha have largely been derived from histological studies of only a few basal taxa, and several more derived sauropod taxa (Neosauropoda). The current study examines a comprehensive sample of the bone microstructure of thirteen sauropodomorph dinosaur taxa, which includes seven basal non-sauropod sauropodomorphs. Our findings revealed that except for Mussaurus, growth marks occur throughout the cortex in all basal sauropodomorphs, but were also found to occur in the sauropod, Lessemsaurus. Additionally, a single growth mark was recorded in Volkheimeria, while several poorly defined annuli where observed in the outer cortex of Patagosaurus. Our results agree with the current consensus that the plesiomorphic condition for the sauropodomorpha is cyclical growth dynamics. However, our findings show that the uninterrupted and sustained rapid growth (the so called "sauropod pattern"?) also occurred in the basal taxon, Mussaurus. Furthermore, we found that the basal sauropod, Lessemsaurus exhibited the cyclical growth strategy generally associated with basal sauropodomorphs. Thus, our study reveals that the "typical" sauropod growth pattern arose more than once during the evolution of Sauropodomopha and that such a growth pattern was not exclusive to the Sauropoda.

BON3-6  3:45 pm  Intraskeletal growth dynamics and functional maturation in the limb bones of “dinobirds”. Prondvai E*, Evolutionary Morphology of Vertebrates, Ghent University, Ghent, Belgium; Hu D-Y, Paleontological Institute, Shenyang Normal University, Shenyang, China; Godefroit P, Royal Belgian Institute of Natural Sciences, Directorate 'Earth and History of Life', Brussels, Belgium; Adriaens D, Evolutionary Morphology of Vertebrates, Ghent University, Ghent, Belgium
Abstract: Growth rate and functional maturity of a tissue are known to be inversely related because cells capable of fast proliferation and thereby ensuring fast growth are undifferentiated, while differentiated cells of mature tissues specialized to perform a certain task cannot proliferate at a high rate. Differing energy allocation into growth vs. maturation results in diverse ontogenetic strategies, such as the altricial – superprecocial spectrum in birds. To get insight into the degree of precociality in five extinct paravian dinosaurs (dinosaur-bird transitional forms), Anchiornis, Aurornis, Eosinopteryx, Jeholornis and a yet unnamed taxon, we inferred dynamics of postnatal development of their limb bones from intraskeletal histovariability because many osteohistological traits (e.g. vascularity, osteonal development, secondary remodelling) are indicative of growth rate and functional maturity of the bone tissue. Differential growth rates as well as histological traits implying differences in functional maturity among limb elements of the same skeleton were detected in all specimens. Except in the fully grown Aurornis and the subadult specimen of the unnamed taxon, humeri appear to mature functionally the latest among all limb bones, whereas radii and ulnae exhibit histological signs of more extensive functional demands already in earlier stages of development. Hand bones generally show high functional maturity even in the juvenile Eosinopteryx specimen. Degree of maturity of femora appears to correspond with that of radii and ulnae, except in the unnamed taxon where it is the least mature element. Thus, in most taxa femora could have enabled precocial bipedal cursorial locomotion, while the apparent proximodistal gradient of increasing functional maturity in the forelimb elements may relate to different ontogenetic onset and extent of arm-assisted grasping, climbing, or in some taxa even gliding or flying. E.P. is funded by BOF (grant nr. 01P12815)

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