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Re: tetrapoda



Adam Yates wrote:
> 
> On Thu, 13 Nov 1997, T. Mike Keesey wrote:
> 
> > On Thu, 13 Nov 1997, Jack wrote:
> >
> > > Anyone have the formal cladistic definition of Tetrapoda handy?
> >
> > Judging from what I see at the On-Line Tree of Life, it's something like
> > "all descendants of the most recent common ancestor of amphibians (narrow
> > definition -- all animals sharing more recent ancestry with frogs,
> > salamanders, and caecilians than with amniotes) and amniotes". This
> > excludes a lot of primitive forms like _Ichthyostega_. The more inclusive
> > clade Tetrapodomorpha would contain these.
> >
> 
> Yes the narrow phylogenetic definition of the Tetrapoda does exclude a
> number of basal taxa such as Ichthyostega, Acanthostega etc.. If Laurin's
> new tetrapod phylogeny is correct, then most of the Palaeozoic "tetrapods"
> (eg. temnospondyls, baphetids, embolomeres, gephyrostegids and
> Seymouriamorphs) would actually be excluded from this taxon. Nevertheless
> there is great resistance amongst early tetrapod workers to applying this
> crown group definition of the Tetrapoda, it is analogous to kicking
> Archaeopteryx, Ichthyornis etc. out of Aves because they don't belong to
> the crown group. I think what will settle is either a stem based
> definition like "all taxa more closely related to modern Amphibia and
> Amniota than to Elpistostegids" (Elpistostegids are the closeset known
> taxa to Tetrapoda that still lack limbs and digits) or a node based
> definition like "Elginerpetontidae and all modern amphibians, amniotes and
> all descendants of their common ancestor". The problem with the crown
> group definition of Tetrapoda is that a) people are very used to calling
> limbed vertebrates such as Ichthyostega, temnospondyls and
> seymouriamorphs," tetrapods" and b) there isn't anything else we can call
> them at the moment beyond limbed, near-tetrapod osteolepiforms.
> Tetrapodomorpha does not just apply to these limbed
> near-tetrapods + tetrapods, it is
> infact phylogenetically defined as a stem group including all vertebrates
> more closely related to modern tetrapods than to modern lungfish.
> Therefore such "fishy" taxa as Eusthenopteron are tetrapodomorphs. Whew! I
> seem to have gone on far longer than I intended, I hope this helps clear
> things up.

OK, time for another of those stupid questions for which I am probably
becoming infamous: what is the point of the arguments over different
cladogram-based definitions of taxa?  As I understand it, "crown-based"
definitions say "all animals closer to modern X than to modern Y." 
"Node-based" definitions say "the last common ancestor of groups X and
Y, and all descendants of that ancestor."  "Stem-based" definitions say
"X and all organisms closer to X than to Y," where X and Y may be living
or extinct species.  All depend on cladograms, which means the
definition is only as good as the cladogram is.  All also depend totally
on having a good fossil record to work with.  

For example, we can't build a proper crown- or node-based definition of
mammals because nobody knows when monotremes branched off from the basal
mammal line, so we don't know the last common ancestor of modern
mammals.  We could probably put together a decent stem-based definition
of mammals, but it would wind up including many animals that most people
probably wouldn't think of as mammals.  Also, it seems like cladograms
and cladists have a logical contradiction in their thinking.  They
depend on cladograms to tell them relationships -- but cladograms never,
ever say "X is descended from Y."  If cladograms don't tell you what's
descended from what, then how can you base a time-spanning, evolutionary
definition of a taxon on a cladogram?

I don't understand why people got away from the plain old-fashioned idea
of basing definitions on physiology:  if X and Y have features 1, 2, and
3 in common, then X and Y are in the same taxon.  And taxa are defined
as "the first organism to have features 1, 2, and 3, and all of its
descendants."  Tetrapoda then would be the first four-legged partially
land-dwelling animal and all of its descendants.  Amniota would be the
first animal that laid hard-shelled eggs, and all of its descendants. 
Tyrannosauria would be the first theropod to have the shrunken forelimbs
and whatever other autapomorphies distinguish tyrannosaurs, and all of
its descendants.  Aves would be the first animal to have flight
feathers, and all of its descendants.  Ceratopsia would be the first
animal to have the parrot beak and neck frill, and all of its
descendants.  And so on.  This is close to the "node-based" definition,
but it's based on hard facts, not theorizing about "relationships."

Can anyone on the list explain this to me in words of ten syllables or
less? <g>

-- JSW