Re: Transitional fish

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From:  Pim van Meurs <pimvanmeurs@yahoo.com>
To:  David Opderbeck <dopderbeck@gmail.com>
CC:  asa <asa@calvin.edu>
Subject:  Re: Transitional fish
Date:  Mon, 10 Apr 2006 11:09:39 -0700
>David Opderbeck wrote:
>
>>Showing some ignorance here probably, but why is this so
>>dramatically different than the mudskippers that live today?
>>
>A very good question
>
>Several reasons
>
><quote>It has long been clear that limbed vertebrates (tetrapods)
>evolved from osteolepiform lobe-finned fishes^3
>http://www.nature.com/nature/journal/v440/n7085/full/440747a.html#B3

 

Frankly, although I find suggestive corroboration in morphologically similarities, it has never been clear to me -- nor to many others -- that limbed vertebrates (tetrapods) evolved from osteolepiform lobe-finned fishes.  (I believe "tetrapod" is too general a term, as tardigrades are tetrapods, are they not, yet hardly derived from fish.)

I am not willing to pay the fee to read the article.  My words are free.  and they are my own.

>, but until recently the morphological gap between the two groups
>remained frustratingly wide. The gap was bounded at the top by
>primitive Devonian tetrapods such as /Ichthyostega/ and
>/Acanthostega/ from Greenland, and at the bottom by /Panderichthys/,
>a tetrapod-like predatory fish from the latest Middle Devonian of
>Latvia (Fig. 1

 

Are you saying that Greenlander Ichthyostega and Acanthostega, and Latvian Panderichthys MIGRATED!?

><http://www.nature.com/nature/journal/v440/n7085/full/440747a.html#f1>).
>/Ichthyostega/^4
><http://www.nature.com/nature/journal/v440/n7085/full/440747a.html#B4>
>and /Acanthostega/^5
><http://www.nature.com/nature/journal/v440/n7085/full/440747a.html#B5>
>retain true fish tails with fin rays but are nevertheless
>unambiguous tetrapods with limbs that bear digits^6
><http://www.nature.com/nature/journal/v440/n7085/full/440747a.html#B6>
>. /Panderichthys/^7
><http://www.nature.com/nature/journal/v440/n7085/full/440747a.html#B7>
>is vaguely crocodile-shaped and, unlike the rather conventional
>osteolepiform fishes farther down the tree, looks like a
>fish–tetrapod transitional form. The shape of the pectoral fin
>skeleton and shoulder girdle are intermediate between those of
>osteolepiforms and tetrapods, suggesting that /Panderichthys/ was
>beginning to 'walk', but perhaps in shallow water rather than on
>land^8
><http://www.nature.com/nature/journal/v440/n7085/full/440747a.html#B8>
>.</quote>
>
>http://www.nature.com/nature/journal/v440/n7085/fig_tab/440747a_F1.html
>
>
>
>
><quote>
>
>Into this gap drops /Tiktaalik/. The fossils are earliest Late
>Devonian in age, making them at most 2 million or 3 million years
>younger than /Panderichthys/. With its crocodile-shaped skull, and
>paired fins with fin rays but strong internal limb skeletons,
>/Tiktaalik/ also resembles /Panderichthys/ quite closely. The
>closest match, however, is not to /Panderichthys/ but to another
>animal, /Elpistostege/, from the early Late Devonian of Canada.
>/Elpistostege/ is known only from two partial skulls and a length of
>backbone, but it has long been recognized as a fish– tetrapod
>intermediate^11,
><http://www.nature.com/nature/journal/v440/n7085/full/440747a.html#B11>
>^12
><http://www.nature.com/nature/journal/v440/n7085/full/440747a.html#B12>
>, probably closer to tetrapods than is /Panderichthys/. This
>impression is now confirmed: the authors^1,
><http://www.nature.com/nature/journal/v440/n7085/full/440747a.html#B1>
>^2
><http://www.nature.com/nature/journal/v440/n7085/full/440747a.html#B2>
>demonstrate convincingly that /Elpistostege/ and /Tiktaalik/ fall
>between /Panderichthys/ and the earliest tetrapods on the
>phylogenetic tree.
>
>So, if /Tiktaalik/ is in effect a better-preserved version of
>/Elpistostege/, why is it important? First, it demonstrates the
>predictive capacity of palaeontology. The Nunavut field project had
>the express aim of finding an intermediate between /Panderichthys/
>and tetrapods, by searching in sediments from the most probable
>environment (rivers) and time (early Late Devonian). Second,
>/Tiktaalik/ adds enormously to our understanding of the
>fish–tetrapod transition because of its position on the tree and the
>combination of characters it displays.</quote>
>
>So why are lungfish or mudskippers not relevant? Because hundreds of
>millions of years have passed..
>
><quote>An impediment to understanding the fin–limb transition has
>been the nature of available evidence fromthe sister group of
>tetrapods. The closest living relatives of tetrapods—lungfishes and
>coelacanths— either lack homologous elements to distal limb bones or
>are so specialized that comparisons with tetrapods are
>uncertain.</quote>
>
>Mudskippers belong to a different branch

 

This sounds like you're saying mudskippers are not evolving into terran quadrupeds (or tripeds, to see the way some of them gambol in our aquarium). 

>
>See
>http://tolweb.org/tree?group=Actinopterygii&contgroup=Gnathostomata
>
>http://tolweb.org/Gnathostomata/14843
>
>sarcopterygians lobe finned fish and four-legged vertebrates
>(lungish, mammals, reptiles, amphibians, birds etc)
>actinopterygians ray finned fish (mudskipper) see also
>http://en.wikipedia.org/wiki/Perciformes