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

[Pieter.Depuydt@rug.ac.be]

warning: no dinosaurs in the following

> On Sun, 28 Jun 1998, Earl Wood wrote:
> 
> > I would like to insert a question here........ was the original limb 
> > composed
> > of five digits
> > before going onshore , or developed after becoming terrestrial ?
> > Earl Wood
> > candles@jps.net

Jack replied: 
> This is not so simple a question as it may, at first, appear if you are
> discussing basal tetrapods.  The original tetrapod, _Acanthostega_ or
> something related, possessed as many as eight digits.  However, it has
> been suggested that this animal never left the water and only used its
> digits to help it remain still while under water or to "crawl" along the
> bottom.  I don't know much a _Tulerpeton_ (sic?), but I have seen the
> monograph on _Ichthyostega_.  This animal possessed at least six digits
> and was probably a semiterrestrial form.  Therefore, the five-digit
> tetrapod limb probably did develop on land.  

The current thinking (as expressed mainly by Ahlberg, Clack and 
Coates, sorry if I forget others) is that the tetrapod limb evolved 
primarily in an aquatic environment, more precisely, in very shallow 
lagoonal or lacustrine waters. That "urlimb", whether it was 
pentadactyl, hexadactyl, heptadactyl (and so on) evolved to 
facilitate propulsion in that shallow water, to "row" on the bottom, 
or to "propel" the animal when launching an attack (most early 
tetrapods or near tetrapods were highly predacious animals). In 
second instance, this limbs were also quite decent for locomotion on 
the land but they evolved in first instance for other purposes; 
another example of exaptation, an already existing feature is used 
for another application (as probably the feathers in theropods).
The fact that all post-Famennian tetrapods are pentadactyl just 
proves that they all happened to evolve from a common pentadactyl 
ancestor, which is not yet found (and will probably never be) but 
undoubtedly was very similar to creatures as Ichthostega, Acantostega 
and Tulerpeton. Those archaic stemtetrapods probably survived into 
the Carboniferous, as is documented by the eel-like Crassigyrinus.
There is however a real gap in the fossil record between those very 
first, "fishy-like" tetrapods and the well-known anamniotes 
("labyrinthodont amphibians") and amniotes of the Late Carboniferous/ 
Pennsylvanian with well-developed limbs.
In the Pennsylvanian some of those terrestrial amniotes already 
returned to an aquatic environment, as documented by the diapsid 
Spinoaequalis. The probably fully aquatic mesosaurs (the first 
tetrapod suspension feeders!) made their appearance in the Early 
Permian.

Regards,

Pieter Depuydt