Re: dolphin with hind flippers

["A.P. Hazen" <a.hazen@philosophy.unimelb.edu.au>]

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 
this plausible?

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.

--

Allen Hazen
Philosophy Department
University of Melbourne