[Taxacom] Tetrapoda definition (was: Systematics Association Lecture)
kennethkinman at webtv.net
Sat Nov 26 18:37:53 CST 2011
I for one certainly welcome such a debate, especially
if deals with possible synapomorphies that are appearing in transitional
forms (like Tiktaalik). I just hope that such a discussion would not get
side-tracked and bogged down by returning to the debate (started by
PhyloCoders) over their proposal to restrict Tetrapoda to the crown
group only. Neotetrapoda (proposed by Gaffney, 1979) is a perfectly good
term for the extant (crown) group.
I think that the general view has long been that
Tetrapoda should at least include Acanthostega and Ichthyostega, but
just how much more inclusive it should now be depends on which apomorphy
(or apomorphies) one chooses. I have argued since 2006 that
consideration shouldn't necessarily be limited to characters in the
limbs (even though the name refers to the possession of four "limbs").
Paleontologists have long defined Mammalia with the transition of bones
in the jaw becoming ear ossicles (not on the possession of mammaries),
so likewise, I don't think Tetrapoda necessarily has to be based on
characters in the limbs.
After the paper on Tiktaalik came out in 2006, I suggested here on
Taxacom that it be included in Tetrapoda, and that a good candidate for
a synapomorphy defining Tetrapoda would be the loss of the bony gill
cover (operculum). This indicates that Tiktaalik was an air breather
(at least part-time) and the loss of the operculum would have also given
it a mobile neck (so important in helping tetrapods to hunt for prey on
land). And this loss seems to have happened relatively quickly, not a
drawn-out affair like the reduction of the hyomandibular (which is still
present within our bodies---the stapes). The trouble with digits is
that they are often not preserved (compared to the more robust bones of
the head and shoulder area). Anyway, the weblink below goes to my 2006
posting on the subject.
James Cotton wrote:
The Systematics Association
Annual General Meeting and President's lecture The earliest tetrapods:
What were they, and what are they? Professor Jenny Clack
University of Cambridge
The Linnean Society, Burlington House, Piccadilly, London Wednesday 30th
November 2011, 6 pm (following AGM at 5pm) The meeting is open to
visitors. Wine will be served after the lecture to members and guests.
Please advertise this lecture as widely as you can. The associations'
AGM will be held before the lecture at 5pm. Abstract:
When E.S. Goodrich coined the term tetrapod, meaning a vertebrate with
four legs (or whose ancestors had them), there was a clear distinction
between them and anything that might have been called a 'fish'. Only
extant animals were considered at that time, because the fossil record
of intermediate forms was more or less non-existent. Recent work on the
earliest limbed vertebrates, from the Late Devonian period about 375-360
million years ago, has blurred the distinction between 'fish' and
'tetrapods' as applied to fossil forms. We now know much more about the
transition between these body forms from a wealth of discoveries over
the last 15 or 20 years. But this has brought problems of definition.
What is a tetrapod, or rather how is the group Tetrapoda defined? This,
and emergent associated questions are highly controversial. This talk
will introduce some of the creatures that have been discovered, and the
different, incompatible views that have been put forward as answers.
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