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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 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