Figure 4.

The principal changes in development leading to the evolution of mouse and vole molar cusp patterns. In the mouse lineages (left), inferred development of the ancestral morphology (Fig. 1A) is altered early in development (enamel knots in black) by heterotopic shifts in the molecular prepatterns that change the location within primary enamel knot giving rise to the protoconid (Prd) next to the metaconid enamel knot (Med). In the vole lineages (right), no large changes in cusp topography are needed, but the number of times that the diagonal cusp pattern is iterated is increased.