Which was not one of the changes in development and specialization that primitive mammals underwent specialization of teeth increase in overall body size increase in stomach capacity development of bo?

The change that was not part of the development and specialization of primitive mammals is the "development of bone." Primitive mammals primarily underwent changes such as specialization of teeth, an increase in overall body size, and an increase in stomach capacity, which were adaptations to their environments and dietary needs. Bone development was already a characteristic of early vertebrates and not a specialization unique to mammals.