Morphological Integration in the Mandibles of Living Reptiles and Fossil Synapsids
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.
