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Re: EARLY TETRAPODS (was Re: Back-evolution of limbs)



Alan wrote:
> Concerning the idea that the basal tetrapods developed their "fingers" for
> 'rowing' along the bottom:
> 
>     All the evidence I've seen, indicates that the earliest tetrapods DID
> use their limbs to propel themselves in shallow water, however two
> additional points need to be made.

Thanks for the comments.

>     One is that at least one early tetrapod (_Hynerpeton_) had a very strong
> shoulder construction which might allow the animal to push its way out of
> the water and walk on the tips of the budding phalanges.  (Not very
> effectively, probably just enough to stick its head out of the water for
> awhile).

This is not necessarily incompatible with the aforementioned 
hypothesis. Being able to lift up the head above the water table 
might enable the animal to improve it's oxygenation by gas exchange 
through a modified swimming bladder (precursor of a true lung) and 
hence enable the colonization of very shallow, algae choked and 
hypoxic water. Of course this is only interpretation post hoc.

>     The other point is about one of the best known lobe-finned fish, the
> _Ceolocanth_.  These fish have the lobes that most agree are equivalent to
> the starting point for the tetrapod body plan.  (I am NOT counting number of
> lobes here!)  However, these fish HOVER near the bottom of the sea, barely
> moving a few inches above the bottom.  The depths are usually 300 feet or
> more.  This sort-of argues against the shallow water only start for the
> tetrapods.

There is indeed absolutely no proof that the primitive tetrapod limb 
did evolve in response to colonization of lagoonal environments. 
Yet as far as I know, the sediments in which  various earliest 
tetrapod fossils (Ichthyostega, Acanthostega, Tulerpeton) were 
found, have been interpreted as estuarine or lacustrine deposits. But 
of course, the invasion of these habitats by the near- and early 
tetrapods may have been enabled by their modified fins/primitive 
limbs, which already had evolved in other circumstances, maybe in 
deep water. I think there is however no direct fossil evidence for 
this scenario.

Regards,

Pieter Depuydt