Tetrapoda (and the new *tetrapod* genus _Tiktaalik_)

John Grehan jgrehan at SCIENCEBUFF.ORG
Mon Apr 10 08:13:28 CDT 2006


Just an aside to Ken's phylogenetic speculations, one might recast the
"invasion" of land by marine organisms as an "escalation" in the sense
that it is very possible (as shown by numerous biogeographic examples)
that terrestrialization occurred through tectonic uplift like an
escalator, rather than organism themselves invading the land for some
imagined selective or other teleological 'advantage'. 

John Grehan

Dr. John R. Grehan
Director of Science and Collections
Buffalo Museum of Science1020 Humboldt Parkway
Buffalo, NY 14211-1193
email: jgrehan at sciencebuff.org
Phone: (716) 896-5200 ext 372 

Panbiogeography
http://www.sciencebuff.org/biogeography_and_evolutionary_biology.php
Ghost moth research
http://www.sciencebuff.org/systematics_and_evolution_of_hepialdiae.php
Human evolution and the great apes
http://www.sciencebuff.org/human_origin_and_the_great_apes.php
 

> -----Original Message-----
> From: Taxacom Discussion List [mailto:TAXACOM at LISTSERV.NHM.KU.EDU] On
> Behalf Of Ken Kinman
> Sent: Sunday, April 09, 2006 11:00 PM
> To: TAXACOM at LISTSERV.NHM.KU.EDU
> Subject: Re: [TAXACOM] Tetrapoda (and the new *tetrapod* genus
> _Tiktaalik_)
> 
> Dear All,
>       I have been thinking about this all weekend, and the transition
from
> lobe-finned fish to tetrapods is a relatively gradual development in
many
> areas of the skull and also the forelimbs.  Some of these changes
could be
> quantified to draw a line between them, but I think the primary
division
> should be something which occurred relatively quickly (in evolutionary
> terms).  Frankly, it is no surprise that those who described Tiktaalik
> refer to it as a fish or a "fishapod", but I have to disagree with
this
> characterization.   We need a clear definitional (synapomorphic) line
> between fish and tetrapods, and waffling on the issue (with a
"fishapod"
> label) does not inspire confidence or stability.
> 
>      I propose that the loss of the bony gill cover is just such a
> (relatively) quickly evolved characteristic, AND the fact that it is
> functionally associated with the origins of a mobile neck is just
icing on
> the cake!!!  Therefore, Panderichthys would remain in the
sarcopterygian
> Order Osteolepiformes, and Tiktaalik (which has lost the bony gill
cover)
> would be classified as a primitive tetrapod.  I am still not sure if
we
> know for sure whether Elpistostege had a bony gill cover or not.  It
> certainly resembles Tiktaalik in the expansion of bones in the snout,
but
> are they really sister groups?  Therefore, Elpistostege is the one
genus
> which I am not sure which side of the line to put it.
> 
>      Everything else can be easily placed by possession or loss of the
> bony gill cover.  Other associated (but gradual) characteristics can
be
> quantified to show their more gradual evolution before and after this
> event.  This event apparently occurred well before tetrapods could
spend
> any appreciable time on land, and walking along shallow water bottoms
was
> the main activity of the earliest tetrapods (including Acanthostega).
It
> is becoming increasingly clear that the origin of Tetrapoda, as well
as
> the slightly later tetrapods with real independent digits, preceded
the
> true invasion of land.  Hopefully the inclusion of Tikaalik in
Tetrapoda
> will make it even more abundantly clear to everyone that tetrapody
came
> first, and that the complete weight-bearing adaptations for land
invasion
> followed later.
>      ---Cheers,
>              Ken Kinman




More information about the Taxacom mailing list