Higher Classification Notes:

Chordata  -   Urochordata
                -   Name?            - Cephalochordata
                                           -  Vertebrata

------------------------------

Date: Thu, 2 Nov 2000 06:52:53 -0800
From: "Billie J. Swalla" <bjswalla@u.washington.edu>
To: tunicata@jiscmail.ac.uk
Subject: <tunicata> Deuterostome Phylogenetics
Message-Id: <v03007804b626796cb9ab@[128.95.68.172]>

Hi Everyone,
 Ronald Jenner's introduction as a person interested in "various
aspects of higher level animal phylogenetics" and the interesting
discussion of the ascidian "pituitary" made me begin to think again about
the higher classification of chordates.
 As you all know, for over a hundred years, Urochordata have been
classified as a sub-phylum of Chordata, along with the Cephalochordata and
Vertebrata.  I recently disputed this classification in a manuscript on the
phylogeny of the deuterostomes, in which even my co-authors were conflicted
on that particular point:

Cameron, C.B., Garey, J.R. and Swalla, B. J. (2000)  Evolution of the
chordate body plan: New insights from phylogenetic analyses of deuterostome
phyla. Proc. Natl. Acad. Sci.97: 4469-4474.

I think on the basis of adult morphology (including neural innervation),
life history traits, and the huge genetic divergence within the
urochordates, that the urochordates should be a separate phylum from the
vertebrate/cephlachordata clade.

For instance, do you know any cephalochordates or vertebrates that survive
after their brain is removed as an adult?
Mackie, G. O. and Wyeth, R. C. (2000) Conduction and coordination in
deganglionated ascidians. Can. J. Zool. 78: 1626-1639.

However, when discussing this issue at meetings, in summer courses, and in
casual exchanges with other invertebrate biologists, I have encountered a
wide range of opinions.  These have ranged from:

Should the urochordates be a separate phylum from the
cephalochordates/vertebrates?
1) Absolutely NOT.  They all have a notochord and the same body plan, the
chordates are one phylum.
2)  Of course they should be a separate phylum.  Everyone who works with
them thinks so.
3)  Who cares?
 

So I am asking those of you that really know and love the urochordates.
What do you think?
 

There are no formal rules for creation or subtraction of animal phyla.  I
am told conventional wisdom must "trickle down" to the invertebrate
textbook writers.  However, I am currently in the process of writing
several review articles and I am curious about your opinion.  Please, let
me know how you think and what morphological/molecular/life history data
that backs up your opinion on the matter.

I personally believe that this question is at the heart of understanding
chordate evolution.
Best Regards,
Billie

"To be what we are, and to become what we are capable of becoming, is the
only end in life." Robert Louis Stevenson

Dr. Billie J. Swalla
Assistant Professor
Box 351800
Department of Zoology
University of Washington
Seattle, WA       98195-1800

Phone: 206-616-9367
Fax:    206-543-3041
E-mail: bjswalla@u.washington.edu
 
 
 

------------------------------

Date: Thu, 2 Nov 2000 21:38:17 +0100
From: "Thomas Stach" <thomas_stach@web.de>
To: tunicata@jiscmail.ac.uk
Subject: Re: <tunicata> Deuterostome Phylogenetics
Message-Id: <200011022038.VAA22106@mailgate3.cinetic.de>

this is a direct reply to Swalla's inquiry.

Since the fifties we have a more or less reliable, but at least coherent, consistent and scientific methodology to assess evolutionary relationships (Hennig 1950). And allthough his "Konsequent phylogenetische Systematik" in English dubbed 'cladistics' was and still is altered and developed, the basic thoughts and principles are not seriously challenged. Among other very thoughtfull and deep discussions Hennig discusses the significance of higher taxonomic categories. He correctly points out that the relevant question on ALL taxonomic levels is the search for monophyletic groups, which may be discovered (or made plausible) by synapomorphies. This procedure leads to the establishment of monophyletic groups ('clades') and is the hard backbone of all evolutionary argumentation, questions and research.

There are no rules for the designation of taxonomic categories, especially not for higher ranks. It definitely should NOT be the subjective differentness of a group of animals, otherwise we would have to exclude the Lernaeidae (to quote only one well known example) from the Crustacea. Hennig suggested a paleontological criterion to make the assignment of higher ranks less ambiguous. (He refered to the absolute age of a taxon deduced from the fossil record as 'terminus ante quem'.) With that criterion and the fossil record of appendicularians (Lohmann 1956, Zhang 1987) and conodonts (e.g., Clark and Miller 1969, Mueller and Hinz-Schallreuter 1998) the 'terminus ante quem' the three monophyletic chordate lineages leading to todays Tunicata, Cephalochordata, and Craniata separated, has to be the middle Cambrian period (or slightly earlier: Zhang dates his fossil as Lower Cambrian). This would, given the example of other 'phyla', justify the usage of the phylum category for all !
 three chordate clades. But to come to the main conclusion: since the monophyly and the interrelationship of the three chordate 'subphyla' as it is traditionally perceived is not challenged, there would be NO (NOT A SINGLE ONE) difference in any answers to any evolutionary questions.

Though it would be nice to unify the designation of higher ranks and Hennig's suggestion is the most sober I can think of, but it is not commonly used. And since NOTHING would change when giving phylum instead of subphylum rank to the chordate 'clades' I suggest to go with the traditional view, unless we want to unify the procedure of designation of higher ranks.

(by the way: although there is a (or rather many) species concept(s) the designation of this lower category to a natural entity of living individuals with some similarities is far from being uniform. Hennig considered all categories higher than the species level 'Kategorien hoeherer Ordnung')
 

tunicata@jiscmail.ac.uk schrieb am 02.11.00:
> Hi Everyone,
>  Ronald Jenner's introduction as a person interested in "various
> aspects of higher level animal phylogenetics" and the interesting
> discussion of the ascidian "pituitary" made me begin to think again about
> the higher classification of chordates.
>  As you all know, for over a hundred years, Urochordata have been
> classified as a sub-phylum of Chordata, along with the Cephalochordata and
> Vertebrata.  I recently disputed this classification in a manuscript on the
> phylogeny of the deuterostomes, in which even my co-authors were conflicted
> on that particular point:
>
> Cameron, C.B., Garey, J.R. and Swalla, B. J. (2000)  Evolution of the
> chordate body plan: New insights from phylogenetic analyses of deuterostome
> phyla. Proc. Natl. Acad. Sci.97: 4469-4474.
>
> I think on the basis of adult morphology (including neural innervation),
> life history traits, and the huge genetic divergence within the
> urochordates, that the urochordates should be a separate phylum from the
> vertebrate/cephlachordata clade.
>
> For instance, do you know any cephalochordates or vertebrates that survive
> after their brain is removed as an adult?
> Mackie, G. O. and Wyeth, R. C. (2000) Conduction and coordination in
> deganglionated ascidians. Can. J. Zool. 78: 1626-1639.
>
> However, when discussing this issue at meetings, in summer courses, and in
> casual exchanges with other invertebrate biologists, I have encountered a
> wide range of opinions.  These have ranged from:
>
> Should the urochordates be a separate phylum from the
> cephalochordates/vertebrates?
> 1) Absolutely NOT.  They all have a notochord and the same body plan, the
> chordates are one phylum.
> 2)  Of course they should be a separate phylum.  Everyone who works with
> them thinks so.
> 3)  Who cares?
>
>
> So I am asking those of you that really know and love the urochordates.
> What do you think?
>
>
> There are no formal rules for creation or subtraction of animal phyla.  I
> am told conventional wisdom must "trickle down" to the invertebrate
> textbook writers.  However, I am currently in the process of writing
> several review articles and I am curious about your opinion.  Please, let
> me know how you think and what morphological/molecular/life history data
> that backs up your opinion on the matter.
>
> I personally believe that this question is at the heart of understanding
> chordate evolution.
> Best Regards,
> Billie
>
> "To be what we are, and to become what we are capable of becoming, is the
> only end in life." Robert Louis Stevenson
>
> Dr. Billie J. Swalla
> Assistant Professor
> Box 351800
> Department of Zoology
> University of Washington
> Seattle, WA       98195-1800
>
> Phone: 206-616-9367
> Fax:    206-543-3041
> E-mail: bjswalla@u.washington.edu
>
>  >
>
> )

------------------------------------------
Dr. Thomas Stach
Department of Biological Sciences
019 West Avenue Annex
University of Arkansas
Fayetteville, AR 72701
USA
Tel.:  501-575 4846
FAX:   501-575 4010

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------------------------------

Date: Thu, 2 Nov 2000 14:19:12 -0800
From: "Billie J. Swalla" <bjswalla@u.washington.edu>
To: tunicata@jiscmail.ac.uk
Subject: Re: <tunicata> Deuterostome Phylogenetics
Message-Id: <v03007818b62787ff4c0e@[128.95.68.172]>

Dear Dr. Stach,
 Thank you for your thoughtful reply.

>This procedure leads to the establishment of monophyletic groups
>('clades') and is the hard backbone of >all evolutionary argumentation,
>questions and research.

 I am familiar with Hennig's work, which is why we have been directly
looking for monophyletic groups within the deuterostomes.  In fact, the
urochordates are definitely monophyletic, whereas the Cephalochordates and
Vertebrates make a separate clearly monophyletic group.

Swalla, B.J., Cameron, C.B., Corley, L.S. and Garey, J.R. (2000)
Urochordates are   monophyletic within the deuterostomes.
Systematic Biology 49: 122-134.

Obviously, it matters where you make the distinction because the
deuterostomes are also clearly monophyletic!  Would you make them a single
phylum?  Hemichordates and Echinoderms also are monophyletic.  Should they
be a single phylum?

As far as the fossil record, all of the major phyla appear during the
Cambrian period, so I do not see how this will help one sort out higher
level relationships.

>But to come to the main conclusion: since the monophyly and the
>interrelationship of the three chordate >'subphyla' as it is traditionally
>perceived is not challenged, there would be NO (NOT A SINGLE ONE)
>>difference in any answers to any evolutionary questions.

Actually, I respectfully disagree with you.  Cephalochordates have somites,
segmented mesoderm, and a developed peripheral nervous system, similar to
vertebrates but unlike tunicate tadpole larvae.  I think the question of
whether the small, simple ascidian tadpole lava was an ancestral form or
was derived secondarily from a free-living ancestor is still unresolved.
It appears that ascidians have a modular life-history, with a heterochronic
shift in genes expressed in neuroectoderm in larvae, but not in the
endoderm until after metamorphosis (Hinman & Degnan, 2000 Dev. Genes Evol.
210: 129-139; (Hinman, Becker & Degnan, 2000 Dev. Genes Evol. 210:
212-216.)  It is still a mystery whether this in an ancestral or derived
life history.

Thanks very much for the input!
Best regards,
Billie

"To be what we are, and to become what we are capable of becoming, is the
only end in life." Robert Louis Stevenson

Dr. Billie J. Swalla
Assistant Professor
Box 351800
Department of Zoology
University of Washington
Seattle, WA       98195-1800

Phone: 206-616-9367
Fax:    206-543-3041
E-mail: bjswalla@u.washington.edu
 
 
 

------------------------------
 
 
 
 

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Precedence: list

A recent classification of Hydrozoa is found here:
Bouillon, J. & Boero, F. 2000 The Hydrozoa: A new classification in the
light of old knowledge. Thalassia Salentina 24, 1-45.
Bouillon, J. & Boero, F. 2000 Phylogeny and classification of
Hydroidomedusae. Thalassia Salentina 24, 1-296.

I also have a recent publication dealing with Hydrozoan phylogeny:
Collins, A. G. 2000 Towards understanding the phylogenetic history of
Hydrozoa: hypothesis testing with 18S gene sequence data. Scientia Marina
64 (Supl. 1), 5-22.

A classification of the larger hydrozoan groups that is consistent with my
phylogenetic findings is as follows:

Hydrozoa
 Trachylina
  Limnomedusae
  Narcomedusae
  Trachymedusae
 Hydroidolina
  Capitata
  Filifera
  Hydridae
  Leptomedusae
  Siphonophorae

I would be happy to provide a reprint, or a PDF file for your own printing,
if you would like.

Best wishes,
--Allen G. Collins
======================================================
Allen G. Collins
Museum of Paleontology
Department of Integrative Biology
University of California
Berkeley, CA 94720
URL: www.ucmp.berkeley.edu/people/agc/agc.html
Lab: 510 642-1607    Fax: 510 642-1822
======================================================
Reply-To: CNIDARIA@UCI.EDU
Sender: owner-CNIDARIA@UCI.EDU
From: "Allen G. Collins" <allenc@socrates.Berkeley.EDU>