evolution of Tetrapods

[spuriousmonkey]

from science 8 august 2003 vol 301 page 766-767


Apparently evolution worked in a different way than we originally thought. In 1928 they discovered the Ichthyostega fossil, is a combination of fish and amphibian structures. The theory was then developed that these transitional species were the ones that 'invaded' the land. The drying up of the Devonian ponds forced these species to adapt a terrestial lifestyle.

More recently there were otehr discoveries though. They discovered for instance a tetrapod that was adapted to an aquatice lifestyle extremely well, with having gills and flipper-like limbs. They claim that the fingers and toes therefore first appeared in a paddle of an aquatic organism than on a foot or hand of a terrestial one.

Then they found out that there was much more diversity in the tetrapods than perviously thought, with many graduation between terrestial and aquatic adaptations.

therefore we have to revise our idea of how fins turned into limbs and the origin of fingers and toes. And these major structural changes can therefore precede ecological ones, in this case the move from water to land.

 

[Dr Lou Natic]

I just watched a special about this on the history channel and found out I made about my first mistake in guessing the natural history of all living things I still have a good record.
I assumed the common ancestor of all land animals would have been similar to todays mudskipper.

But a mudskipper still has fins and apparently our ancestor developed legs before it ever left the water. They said on this documentary(entitled "the missing link") that they used them to paddle and navigate through thick weeds in swamps. I got the rest right. I knew they must have evolved in swamps or ponds as opposed to streams or rivers.

How did I know this? First our ancestor needed the ability to breathe air, it could have all the legs it wants but it can't stay on land if it needs water constantly running through its gills. Luckily before it ever evolved legs it developed a labyrinth system, which is a primitive lung, as well as gills. Many legless fish have this, and they didn't develop it so they could live on land, they must have developed it because their water environment is prone to deoxygenation(thats probably not a word but you know what I'm saying). Ponds and swamps become devoid of oxygen during long droughts, so fish that evolved in those environments developed a way to gulp air of the surface that they could extract oxygen from. You'll find that all labyrinth fish live in swamps, ponds, rice paddies, etc.

I figured for our ancestor to survive the move to land it must have had a well developed air breathing adaptation first, thus it must have evolved in ponds or swamps.
Obviously not all swamps and ponds completely dry up, there are still legless labyrinth fish alive today, but many would have and still do and ichthyostega must have been in one of those ones.

In an almost suspicious instance of luck, he had already developed legs and a lung for completely different purposes and as we all know legs and lungs work pretty good on land so after he figured that out and found another pond to remoisten himself he would have started going for liesurely strolls more often. Finding no competition on land whatsoever it is natural that over time he became better adapted to land branching out to make amphibians and later reptiles who didn't need to moisten themselves at all. Interestingly reptiles still breathed like labyrinth fish, by which I mean they have a "breathe-hold-hold-hold" system.

Only mammals and birds became as dependent on air as the original fish were on water, needing air working though them practically every second. And of course now some reptiles, mammals and birds are going back to the water becoming less and less dependent on air all over again.

 

[valich]

Yeah, this is a very contoversial subject with many theories. The scientific community consensus says that tetrapods evolved from either lungfishes or coelacanths or a now extinct group. For more in depth info go to the Tree of Life page at http://tolweb.org/tree?group=terrestrial_vertebrates

Also, check out this recent article posted on CNN.com Science and Nature:

"Study: First land animals may have shuffled not walked

Friday, September 2, 2005; Posted: 12:03 p.m. EDT (16:03 GMT)

Scientists from Uppsala University in Sweden and McGill University in Canada came to these conclusions after studying the fossils of the Ichthyostega -- a four-legged fish that lived around 360 million years ago.

They noted that the construction of the creature's backbone ruled out wriggling in a fish-like side-to-side motion, but would have instead suggested a ripple-like forwards movement.

The creature would have used its hind legs to pull its tail forward, arching the spine as it did so. It would then have smoothed out the arch by using its forelegs to pull the front section forward, the scientists wrote.

"Two gaits seem theoretically possible: 'walking' with diagonally synchronized limb movements and rigid elevated trunk; and a bilaterally symmetrical 'shuffling' or 'inchworm' movement," they said, adding that more research was needed.

"Ichthyostega appears to be an early and ultimately unsuccessful attempt at adapting the tetrapod body plan for terrestrial locomotion, divergent but not very remote from the lineage that ... ultimately gave rise to all living tetrapods," the scientists wrote.

Copyright 2005 Reuters. All rights reserved.This material may not be published, broadcast, rewritten, or redistributed."