Department or Program
Primary Wellesley Thesis Advisor
The mammalian sternum has undergone a reduction in relative size and complexity over evolutionary time. This transformation was highly variable across species, generating multiple, controversial interpretations. Some authors claim that the evolutionary reduction led to loss of presternal elements, while others believe that all, or some, presternal elements were fused into a structure ambiguously referred to as the “manubrium”, a term adopted from the post-interclavicular unit of pre-mammalian ancestors. Previous work on the Paramylodon harlani presternum revealed a composite presternum and identified three elements - mediocranial, mediocaudal and lateral - each with a different developmental origin and marginal articulation. This project used medical and micro CT scans to determine if these elements are conserved across species and if there is a characteristic histology associated with each. All three elements were identified in humans based on their locations and articulations. Their fusion during ontogeny was documented. Elements could not be associated with a characteristic histology. Instead, histology appears to reflect the mechanical forces to which different regions of the presternum are subjected, whatever their developmental origin. This opens the possibility that the presternum can be used to infer the limb use and locomotor style of extinct taxa. These lines of evidence indicate that the reduction of relative sternal size seen over evolutionary time has not led to element loss. Instead, three conserved elements with unique developmental origins and articulations are present in a composite presternum. Further, this reasoning suggests that the use of the term “manubrium” should be abandoned as ambiguous and instead when referring to specific areas on the presternum the terms interclavicle, sternal bands, and lateral elements should be used.