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Re: pelvis/sacrum question

Typically in pterosaurs, dinosaurs and other reptile types when the sacrum coosifies so then is the sacrum coosified to the entire ilium (or at least the sacrum sends out sacral ribs), but in mammals this doesn't seem to be the case. Only two(?) sacral ribs seem to be present high on the ilium and the rest just sort of hang there extending out to form a tail.

In functional terms, why the difference?

Mammal...imorphs went through a size squeeze during which they actually lost sacrals. At the size of a mouse hardly any sacrum is needed. Additionally, our main muscle for walking is some iliofemoralis phenomenon called the gluteus; it inserts near the proximal end of the femur and originates near the preacetabular end of the ilium! The existence of a postacetabular process is therefore impossible. (OK, that depends on the assumed homology, but nothing can extend caudally beyond the acetabulum without crossing this muscle.)

An elephant skeleton is quite a sight!