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Session Schedule & Abstracts

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Friday 1st July, 2016

Symposium: The many faces of the skeleton: a tribute to the achievements of Brian K. Hall 2

Room: Salon A   11:30 am–1:00 pm

Moderator(s): P. E. Witten & M. Vickaryous
HAL2-1  11:30 am  Ontogeny and homology of the vertebrate skull. Hanken J.*, Harvard University
Abstract: Embryological and other developmental data have been used for well over a century to infer the homology of morphological structures and substantiate claims of evolutionary relatedness. Darwin, for example, relied heavily on embryological development as evidence for his claims of common ancestry among disparate adult forms. A growing body of empirical data, however, demonstrates divergent patterns of development of otherwise homologous structures in independent lineages. Moreover, such divergence frequently occurs with little or no concomitant change in adult phenotype, a phenomenon termed "developmental system drift." These findings caution against the use of ontogenetic data as an infallible let alone exclusive criterion for evaluating homology. Instead, they reinforce recent claims that routine application of a so-called ontogenetic criterion of homology may be unjustified, if not downright misleading in specific instances, and thereby obscure, rather than reveal, important trends in comparative and evolutionary biology. Examples will be provided from recent comparative studies of the development of the vertebrate skull. These studies employ sophisticated cell-labeling techniques, which show that the embryonic derivation of individual bones may vary according to lineage and is thus subject to evolutionary change. According to these data, longstanding homologies for at least some bones of the cranial vault may be incorrect and should be reevaluated.

HAL2-2  12:00 pm  Cranial morphology in the earliest shark-like fishes (Chondrichthyes). Maisey JG*, American Museum of Natural History
Abstract: The head skeleton (splanchnocranium, viscerocranium) of modern sharks has been widely used as a model for the generalized condition in jawed vertebrates. But how reliable is this "primitive" vertebrate paradigm? New information gleaned from the fossil record suggests that splanchnocranial and viscerocranial morphology in modern elasmobranchs is highly derived, whereas early/basal chondrichthyans resemble osteichthyans more closely in their basicranial, labyrinth, and visceral arch morphology. Additionally, the "boneless" condition is almost certainly derived, considering (1) the widespread presence of site-specific ossifications in the dermal and chondral skeletons of non-chondrichthyan gnathostomes, and (2) recent phylogenetic resolution of acanthodians as stem chondrichthyans, basal to "euchondrichthyans" (forms possessing tessellated calcified cartilage). Cranial morphology is now known in two Early Devonian euchondrichthyans, Doliodus and Pucapampella. However, these taxa present profoundly different morphologies, creating a dilemma for phylogenetic analysis. Doliodus is essentially a "conventional" Paleozoic chondrichthyan bearing acanthodian-like dermal spines. Pucapampella combines extreme autapomorphies with ontogenetically primitive features that may be neotenic, obfuscating its phylogenetic position within the euchondrichthyan total group. Either taxon is thus a candidate for "most primitive euchondrichthyan" status.

HAL2-3  12:30 pm  Cartilage regeneration and diversity in lizards. Subramaniam N, University of Guelph; McDonald RP, University of Guelph; Jacyniak K, University of Guelph; Vickaryous MK*, University of Guelph
Abstract: Cartilage is an avascular skeletal tissue that not only acts as a transient model during bone development, but is also retained as a permanent tissue type contributing to the skull and the surface structure of joints. In lizards, cartilage is also found in the regenerated tail. Regenerated tail cartilage is unusual in several respects. It replaces an original tail skeleton made of bony vertebrae with an unsegmented hollow cone, and not an articulating series of block-like elements. In addition, the cartilage is distinctly cell-rich and extracellular matrix poor. To better understand regeneration-mediated cartilage formation, and the diversity of lizard cartilages, we conducted a spatiotemporal characterization of chondrogenesis using the leopard gecko (Eublepharis macularius). Our data reveals that regeneration-mediated cartilage formation is distinct from the embryonic process of chondrogenesis. Prior to tail loss, cartilage in the tail is restricted to articular surfaces of joints, intervertebral discs, and segments of the persistent notochord. Loss of the tail initiates a wound healing process that leads to the formation of a blastema—an aggregation of proliferating cells. The earliest sign of cartilage formation is a cone-like condensation of cells encircling the regenerating spinal cord that begins to secrete type II collagen and glycosaminoglycans. Cartilage cells activate the transforming growth factor beta signaling pathway, and express the principal regulator of chondrogenesis, the transcription factor Sox9. However, compared to embryonic chondrogenesis Sox9 expression appears to be relatively delayed. These findings reveal that regeneration-mediated cartilage formation is a unique process, and demonstrates an unexpected skeletal diversity in lizards. Funding source: Natural Sciences and Engineering Research Council (NSERC) Discovery Grant 400358 (MKV).

HAL2-4  12:45 pm  Integrative biology, evo-devo, and Brian Hall. Wake M. H.*, University of California, Berkeley
Abstract: Brian Hall's research, his primary literature papers, and his several books present a progression in the development of new ways of doing science. The work, focused on the biology of the vertebrate skeleton, has spanned the hierarchy of biological organization, from molecules and cells to evolution over time. The presentation of the research reflects Hall's detailed knowledge at several levels of that hierarchy, and of a diversity of taxa. In part because of his breadth, and also his energy, Hall has been a major leader in the development of new syntheses that mesh several of the sub-disciplines of biology. His integrative scope has resulted in foundations for "evo-devo", the inclusion of development in the study of extinct vertebrates, and new ways of looking at life history strategies. Selected examples illustrate these advances, and Hall's role in them.

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