Re: Transitional fish

So where does the Darwin Fish fit in all of this? Sorry I couldn't resist :)
Brent

---- Pim van Meurs wrote:

=============
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

It has long been clear that limbed vertebrates (tetrapods)
evolved from osteolepiform lobe-finned fishes^3
/www.nature.com/nature/journal/v440/n7085/full/440747a.html#B3> ,
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
/www.nature.com/nature/journal/v440/n7085/full/440747a.html#f1>).
/Ichthyostega/^4
/www.nature.com/nature/journal/v440/n7085/full/440747a.html#B4>
and /Acanthostega/^5
/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
/www.nature.com/nature/journal/v440/n7085/full/440747a.html#B6> .
/Panderichthys/^7
/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
/www.nature.com/nature/journal/v440/n7085/full/440747a.html#B8>
.

http://www.nature.com/nature/journal/v440/n7085/fig_tab/440747a_F1.html

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,
/www.nature.com/nature/journal/v440/n7085/full/440747a.html#B11>
^12
/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,
/www.nature.com/nature/journal/v440/n7085/full/440747a.html#B1>
^2
/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.

So why are lungfish or mudskippers not relevant? Because hundreds of
millions of years have passed..

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.

Mudskippers belong to a different branch

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
Received on Mon Apr 10 16:54:17 2006