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RE: dolphin with hind flippers



A.P. Hazen wonders why
> 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.

There are a couple of reasons I can think of. The main hydrodynamically
relevant difference in initial conditions is that mammals undulate in the
sagittal plane (i.e. like mythical sea-serpents, not real serpents) while
the sauropsids retain (unless secondarily rigidified) the plesiomorphic
lateral undulatory mode of locomotion, so they're either limb-propelled (and
the more limbs the better) or get the limbs out of the way by holding them
tight against the sides.  With the hipular region [thanks Alexei Sayle for
that term] flapping up and down, the hindlimbs are just a drag.  Also, many
of those sauropsids (call 'em reptiles, why not?) are oviparous and may need
those hindlimbs for getting up and down a beach and digging a nest. 

Of course, snakes completely deleted the fore-flipper stage along with the
shoulder girdle and the whole cervical-thoracic boundary, but otherwise
parallel some early whales likie Basilosaurus, keeping similarly vestigial
and unflipperized hindlimbs until the rest of the body attained quasi-modern
form. Limblessness works fine for lateral undulators, but whales have never
disarmed.
 
-----------------------------------------------
Dr John D. Scanlon
Palaeontologist, 
Riversleigh Fossil Centre, Outback at Isa
19 Marian Street / PO Box 1094
Mount Isa  QLD  4825
AUSTRALIA
Ph:   07 4749 1555
Fax: 07 4743 6296
Email: riversleigh@outbackatisa.com.au
http://tinyurl.com/f2rby


> -----Original Message-----
> From: A.P. Hazen [mailto:a.hazen@philosophy.unimelb.edu.au]
> Sent: Monday, November 06, 2006 7:55 PM
> To: dinosaur@usc.edu
> Subject: 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