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Re: dolphin with hind flippers
Very interesting. Notice that the hind limbs look very flipperish.
My impression is that in the last fossil whales with hind limbs
projecting beyond the body wall (basilosaurids and dorudontids: late
Eocene Archaeocetes thought to be close to the ancestry of modern
whales) the (small) hind limbs were NOT flipperlike: that the
reduction in size had NOT been accompanied by morphological changes
paralleling the changes in forelimb morphology. Then they stopped
growing externally visible hindlimbs. (Modern whales show embryonic
hind limb buds which disappear in the course of fetal development.)
So, the genetics or Evo-Devo situation is a bit of a mystery to me.
Apparently you can have a mutation that reverses the evolutionary
suppression of hind limbs*. (No surprise: you can have a mutation
that reverses the evolutionary suppression of lateral digits in
horses, etc etc etc.) But then what developes is NOT, apparently,
the hind limb as it appeared before suppression, but something whose
structure seems to be taken over from the structure of the forelimb.
People who (unlike me) actually KNOW something about Evo-Devo: is
Note (attempting to get a bit closer to dinosaur content, though not
all the way TO dinosaur content) that SaurOpsid secondarily marine
lineages-- Mosasaurs, Ichthyosaurs, Plesiosaurs, whatever those
Mesozoic marine crocodyles were called, sea turtles-- seem to
flipperize both fore and hind limbs. The route to fully aquatic
morphology taken by whales seems to have been different.
(*) Somethying I found on the WWWeb suggests that a key genetic
change involves the expression of "Sonic Hedgehog": essential to
development of both fore and hind limbs, suppressed in the
hindquarters in modern Cetaceans.
University of Melbourne