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    Published: December 17, 2013

    Morphological Integration in the Mandibles of Living Reptiles and Fossil Synapsids

    Stephanie Ware, Manager, Morphology Labs, SEM

    ARMITA MANAFZADEH

    Sophomore Integrative Biology major at University of California – Berkeley

    REU Mentor: Dr. Kenneth D. Angielczyk (Curator, Geology, S&E)

    Symposium Presentation Title: Morphological Integration in the Mandibles of Living Reptiles and Fossil Synapsids

    Symposium Presentation Abstract: The co-option of mammalian ossicles from the postdentary jawbones of their ancestors is a classic example of exaptation, and is well-documented in the fossil record through a number of transitional forms. Over the course of synapsid evolution, the articular and quadrate bones migrated from the jaw joint into the middle ear where they were coopted for hearing, the angular bone evolved into the ectotympanic, and the other postdentary bones were reduced and lost. On the contrary, sauropsids retained these bones as part of the jaw. However, despite increased studies of the capacity of systems to evolve, the potential influence of modularity as a precursor for evolvability in this transition has not been investigated. To explore this possibility, we collected two dimensional landmark-based morphometric data from mandibles of a wide range of living reptiles, as well as a test case of fossil synapsids, to examine whether all the bones of the mandible form a single integrated unit or if subgroups of bones form distinct modules. Mandibles of 446 reptile specimens across 38 families, as well as 7 specimens of the therapsid Diictodon, were photographed from the lateral view. Landmarks and semi-landmarks were digitized using Rohlf's tpsDig2 version 2.17 with focus on the dentary and two postdentary bones. We then used Klingenberg's method implemented in MorphoJ version 1.06b to test the a priori hypothesis of dentary versus postdentary modularity. We did not find a strong signal of modularity for all modern reptiles. However, modularity is apparent in some suborders (Lacertilia and Serpentes), demonstrating some degree of correlation with jaw kinesis. Comparison with fossil synapsids proved dubious, with findings biased towards very high levels of integration due to the effects of preservational artifacts. While our results do not allow for a prediction of the basal character state for sauropsids, evidence of modularity in some squamates demonstrates that the modularity we predict for synapsids is not unprecedented among amniotes.

    Original Project Title: Morphological Integration in the Mandibles of Living Reptiles and Fossil Synapsids

    Original Project Description:Living mammals have a unique mandible (lower jaw) morphology compared to other tetrapods. In mammals, each side of the mandible is formed by a single bone, the dentary, whereas all other tetrapods possess a series of postdentary bones in addition to the dentary. The reason for this difference is the fact that the postdentary bones of the ancestors of mammals have been reduced in size, detached from the jaw, and incorporated in to the hearing system of mammals. However, mammals are a subgroup of a larger group called Synapsida, and many nonmammalian synapsids possess large postdentary bones that resemble those of other tetrapods.

    Research Methods and Techniques: The REU participant will collect geometric morphometric data on the jaws of a wide range of living reptiles, all of which have post dentary bones, and will examine whether all of the bones of the mandible form a single integrated unit or if subgroups of bones form distinct modules. We will then apply similar methods to the jaws of nonmammalian synapsids with postdentary bones to determine whether they show similar patterns of modularity/integration or if there are differences that may help to explain why the ancestors of synapsids were able to co-opt their postdentary bones for hearing.


    Stephanie Ware
    Manager, Morphology Labs

    Stephanie started volunteering at the Field Museum in 1998, working with Curator John Bates in the Bird Division. In 1999, John Bates hired her as a research assistant. After that project finished in 2007, she went to work for Carl Dick in the Division of Insects helping him to complete his work on the museum's Bat Fly collection. She spent a great deal of time generating images for the Bat Fly portion of theDiptera Taxonomy Database. When Carl left for University of Kentucky in 2009, she continued her imaging work in the Insect Division. In the intervening years, she has imaged hundreds specimens, mostly types, from the rove beetle (Staphylinidae), ant and myriapoda collections.

    Stephanie began working with the Chicago Peregrine Program in 2006.  Initially, she monitored the Metropolitan Correctional Center nest in downtown Chicago.  Over time, her duties have expanded to include other nests in the metro region, emphasizing the identification of nesting adults.  In March of 2007, Stephanie created a group called Midwest Peregrine Falcons on the photo sharing website Flickr with the goal of providing a place for photographers across the country to submit their photographs of peregrine sightings in the Midwestern United States. She also has many of her own peregrine photos on Flickr as well.