Re: Prolacerta/Prolacertaform Sources


"Alan Kellogg" <mythusmage@xxxxxxxxxxxxxx> wrote in message
news:mythusmage-E4FA8C.06040808042006@xxxxxxxxxxxxxxxxxxxxxxxx
In article <KJwZf.65266$Jd.56320@xxxxxxxxxxxxxxxxxxxxxxxxxx>,
John Harshman <jharshman.diespamdie@xxxxxxxxxxx> wrote:

cliff_lundberg@xxxxxxxxxxx wrote:

John Harshman wrote:

Developmental anomalies have often been studied
as a guide to developmental processes. I don't know of any duplicated
limbs, only bifurcated ones, i.e. extra fingers, the occasional extra
hand, and perhaps a spare radius and ulna. But never a complete and
separate arm. I could be wrong.


With Siamese-twinning you can get a complete and nearly separate
whole body (although these are often somewhat reduced), so why
not a complete and separate limb? Is the question then whether
Siamese-twinning could ever be a heritable mutation? Why not?

Because it's not a mutation at all. Anyway, the reason this is difficult
is that to be functional, a front limb has to be attached to a pectoral
girdle, and has to have a particular relationship to other bones: the
rib cage, sternum, clavicles, vertebrae, and to the muscles that attach
to them. If you put the limb in a different spot, none of these
relationships are there any more. Do you duplicate the whole upper
torso? That causes developmental problems too, because the internal
organs are using the same locational cues. It's not a simple matter.

Consider the difficulty Philip Jose Farmer had in designing a centaur,
for example (in The Maker of Universes).

(I could be not even wrong.)

Cliff


And then have an animal that can fare well enough to have descendents. I
don't know enough about the problems to speculate, which is why I'm not
even going to handle the problem in the setting. The scientists there
have their ideas about how it all began, but they don't know. Even the
dragons have no idea, because it might have happened in the Early
Jurassic at the latest, and even as far back as the Late Triassic. (You
try keeping a family journal for over 100 million years.)

If you went back to the age of fish you might have a fish with three fin
pairs end up making the grade rather than tetripods.


.

Relevant Pages

  • Re: Prolacerta/Prolacertaform Sources
    ... as a guide to developmental processes. ... limbs, only bifurcated ones, i.e. extra fingers, the occasional extra ... a front limb has to be attached to a pectoral ... Do you duplicate the whole upper ...
    (sci.bio.paleontology)
  • Re: Prolacerta/Prolacertaform Sources
    ... as a guide to developmental processes. ... limbs, only bifurcated ones, i.e. extra fingers, the occasional extra ... a front limb has to be attached to a pectoral ... Do you duplicate the whole upper ...
    (sci.bio.paleontology)
  • Re: Prolacerta/Prolacertaform Sources
    ... as a guide to developmental processes. ... limbs, only bifurcated ones, i.e. extra fingers, the occasional extra ... a front limb has to be attached to a pectoral ... Do you duplicate the whole upper ...
    (sci.bio.paleontology)