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Re: Dinosaur Genera List update #196

I wrote:

<<But whales have, as in all mammals, median elements that were originally
not paired,>>

David Marjanovic (david.marjanovic@gmx.at) wrote:

<Wrong. The mammalian sternum is, as detailed in the description of 
*Zhangheotherium*, a composite of interclavicle + manubrium sterni + 
sternebrae (+ xiphisternal stuff);>

  Okay, maybe I should clarify my statement. No mammal has bilaterally
paired sternal elements. Period. The espisternum is formed from the
interclavicle, and this is similarly not paired, comprising a singular,
originally Y-shaped cartilage that bound the clavicles, coracoids, and
non-mammalian paired sternals. Before the development of crown mammals,
the manubriim and all other sternal cartilages (xiphisterni) were singular
elements and median, not bilateral. No mammal has bilaterally paired
sternal elements. I would like to see where you get this observation,
either in photograph, or otherwise, and wether you are referring to
division of the episternum or manubrium either sagittally, or
transversely. Cynodonts show a trend towards singularlization of sternal
elements and unless I'm mistaken, this is enforced through a reduction of
the true sternals to absence in place of the sternal cartilages along with
the interclavical, which became the only "true" sternal bone that existed
earlier to crown mammals.


Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

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

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