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Re: pelvs/sacrum question



Dave Peters (davidrpeters@earthlink.net) wrote:

<But getting back to mammals, perhaps the answer is in the size squeeze that
David M. wrote about, because Thrinaxodon and Probelesodon have the full lineup
of sacrals, but Morganucodon has the mammalian pattern in which the posterior
ones are disarticulated from the ilium. I was wondering if frogs and their
bounding behaviour can offer any sort of analogy. Or perhaps if egg passage
might have loosened those last few to some benefit that was carried on.>

  In regards to the squeeze he wrote about, the extant mammalian "sacrum" is
not homologous to the ancestral amniote sacrum. As for frogs, they ancestrally
posses a single sacral vertebra and the identity of this vertebra is apparent
even in juveniles and fossils. The fused element of the urostyle is a case of
fusion of proximal caudals, as is apparent when observing the ontogeny of
Urodela, to the sacral to anchor the hip muscles, not the presence of a fused
sacrum itself (hence the term urostyle, from _uro_ "caudal" + _style_ "rod").

  Cheers,

Jaime A. Headden
http://bitestuff.blogspot.com/

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)

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