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Re: Phyl tax misunderstandings (was Re: tetrapoda) (LONG)



At 07:14 PM 11/14/97 -0800, Johnathon Woolf wrote:

>> Nope.  Time to do the mind-shift.  Accepting, for the moment, the reality of
>> evolution, there will ALWAYS be some clade which can be described by these
>> different definitions.  
>
>Always?  I suppose so, but you have to guard against two
>always-significant possibilities.  One is that the anchor species are
>more closely related than you think they are, and the definition turns
>out to be much too limited.

The taxon definition may no longer describe a group you consider important,
but it will always describe a real, monophyletic group.

>The other is that the two anchor species
>are more distantly related than you think, and the definition turns out
>to be grossly paraphyletic or even polyphyletic.

[*Sigh*]  I guess I'm just not making myself clear.  Phylogenetic taxonomy
sets up definitions which CANNOT yield para- or polyphyletic taxa.

>For example, suppose
>you define Mammalia as the last common ancestor of the platypus and
>{insert placental-of-choice here} and all descendants of that ancestor. 

[small snip]

>OTOH, suppose you define Mammalia that way and then find that the
>monotremes actually descend form a line that branched off from cynodonts
>in the Late Triassic, several million years before even _Morganucodon_
>lived.  Do you then include cynodonts in the Mammalia, despite the fact
>that they had few or none of the apomorphies that are used to
>distinguish either modern or fossil mammals?

Yes!  That's it!  You've got it! It is the DEFINITION which is given the
emphasis, not the diagnosis or the composition.

>> >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.
>> 
>> Sure we can, and it has been done!  
>
>No, it hasn't, because we can't.

Yes, it can.  The only "problem" is getting people to agree on which
definition to use.  Once that is settled, we can concentrate on the more
interesting aspects of the evolution and origins of these groups.

>The most recent work on mammal
>classification that I've read, Hopson's 1994 paper in MAJOR FEATURES OF
>VERTEBRATE EVOLUTION,

There are several more recent, but that's a good one.

>> Real World Example: Dinosauria was once defined by upright limbs,
>> asymmetrical hands, three or more sacral vertebrae, and an open acetabulum.
>> This worked fine when Pisanosaurus, Thecodontosaurus, and Coelophysis were
>> among the most primitive dinosaurs known.  Now we have Lagerpeton,
>> Lagosuchus, etc. (which have upright limbs), Eoraptor (with upright limbs,
>> asymmetrical hand, three sacrals, but uncertain acetabulum), and
>> Herrerasaurus (upright limbs, asymmetrical hands, only two sacrals, and an
>> open acetabulum).  Which ones are dinosaurs?
>
>Simple (IMO): if it doesn't have _all_ the features that have been used
>to define Dinosauria, it ain't a dinosaur.  Two or three out of four
>isn't enough.  

But what if Sereno and Novas are correct, and Herrerasaurus and Eoraptor are
true theropods.  Then either a) one or more of these features evolved
independantly in ornithischians, sauropodormorphs and/or theropods, and we
are stuck defining our taxa on convergent characters or b) some of the
basalmost members of the lineage lost these features really early.

>Ichthyostega wouldn't be a tetrapod under this scheme.  "Leggy" means
>"capable of supporting the animal's weight out of water" -- which
>_Ichthyostega's_ weren't.  "Partial" means "it could spend more than a
>few minutes at a time there" -- which _Ichthyostega_ apparently couldn't
>do, since it had no lungs.

Be careful here: lungs are actually a primitive feature for a large group of
vertebrates.  Some developmental evidence, for example, suggests that swim
bladders of actinopterygians are secondarily derived from lungs!

>My understanding is that whether we have actual fossil eggs or not,
>there are definite differences in the skeleton between an animal that
>lays soft eggs like an amphibian does and an animal that lays
>hard-shelled eggs. 

This is a misunderstanding.  There are, as yet, no hard-tissue correlates
between amniotic egg layers and "amphibians".  (Would that there were!)  At
present, to decide if a form like Limnoscelis, Diadectes, or Seymouria laid
amniotic eggs, we have to establish whether it fits among the phylogenetic
bracket for living amniotes or if it doesn't.

>> >Aves would be the first animal to have flight
>> >feathers, and all of its descendants.
>> 
>> But feathers are only preserved in a VERY limited environmental setting.
>> (We are damned lucky Archaeopteryx lived near lagoons!!).
>
>OTOH, archy is identified as the first bird ONLY because it has those
>feathers.  There is not one other feature that definitively places archy
>within the birds.

Not true, but the other avian features of its skeleton are admitedly obscure
and would not be recognized as such if Archie wasn't found in a lithographic
limestone.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661