Re: pelvis/sacrum question

[David Marjanovic <david.marjanovic@gmx.at>]

> 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!