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Re: pelvis/sacrum question
David Peters (firstname.lastname@example.org) wrote:
<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.>
I am not so sure coossification is coincident, to use an alliteration.
However, I am curious what your source data is on the ossification of sacral
ribs in dinosaurs and pterosaurs? Sacral ribs are present even on the
relatively unfused sacra of many dinosaurs, and a completely fused sacrum will
simply seem to be more rugose in texture with more extremely developed rims of
the contact surfaces as cartilage becomes increasingly ossified and fused to
the original vertebral bone. Similarly, ligaments connecting bones in the hip
will fuse with age and case iliac/sacral fusion even after intersacral fusion
occurs, and will become increasing fused as spinal ligaments join those binding
the ilia to the sacral vertebrae, ribs, neural arches and cause elaboration of
the sacral structures and even binding ligaments and tendons into sacral
laminae and fusing the zygapophyses, as well as obliterating, as in birds and
pterosaurs and several dinosaurs, all indications of separation of the elements
themselves. In dinosaurs, sacral ribs are also present even in the very young
dinosaurs, as evidenced from juveniles preserved in this way. Sacral ribs in
dinosaurs appear to fuse earlier in ontogeny than they do in other reptiles. I
am not sure the data has been published on the juvenile pterosaurs that are
known, though I know Peters rejects these as juveniles, and thus may not accept
their use as benchmarks for ontogeny in pterosaurs.
Jaime A. Headden
"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)
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