Meeting Abstract
The co-option of mammalian middle ear ossicles from the postdentary jawbones of their ancestors is a classic example of exaptation, and is documented in the fossil record by 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 function as part of an impedance-matching system for hearing airborne sound. The angular bone evolved into the ectotympanic, which supports the tympanic membrane, and the other postdentary bones were reduced and lost. On the contrary, sauropsids retained a full complement of postdentary bones. Despite an increase in 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 446 living reptiles across a wide range of families, as well as a fossil synapsid test case, to examine whether all the bones of the mandible form a single integrated unit or if subgroups of bones form distinct modules. We then used the RV coefficient 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 clades (Lacertilia and Serpentes), likely correlated with jaw kinesis. Comparison with fossil synapsids proved difficult, with findings biased towards 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.
