Instead of having a perpendicular pubis-ischium arrangement…

Instead of having a perpendicular pubis-ischium arrangement (like many saurischians), ornithischians have a rearward-facing pubis that lies parallel with the ischium (also mis-characterized as “bird-hip”). This arrangement (indicated by the red arrow in the figure below) is technically referred to as a/an:

As we learned in class, the head crests of some Ornithopods…

As we learned in class, the head crests of some Ornithopods (e.g. Lambeosaurinae) were not exaggerated through their evolution.  What is the possible explanation for this?  Support this argument using another evolutionary trend seen in different members of Ornithopods.