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Date: Tue, 25 Feb 2003 09:03:07 -0500
From: "Thomas R. Holtz, Jr." <tholtz@geol.umd.edu>
To: "peter a. cejchan" <cej@gli.cas.cz>, PhyloCodeList <PhyloCode@ouvaxa.cats.ohiou.edu>
Subject: RE: Note 9.4.1
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Node- and stem-based definitions are most certainly useful tools in
taxonomy! This is especially true for those of us who work in fossil forms
and on large scale evolutionary biology issues.
For example, complimentary stem-defined taxa have exactly the same time of
origination *by definition*; in contrast, node-defined taxa within these
stems could have radically different times of origins (as, similarly, would
apomorphy-defined taxa within these stems).
Let's take the example of the two major clades of Amniota: Synapsida (for
the moment, let's use the definition "Homo sapiens and all taxa sharing a
more recent common ancestor with H. sapiens than with Lacerta viridis") and
Sauropsida (Lacerta viridis and all taxa sharing a more recent common
ancestor with L. viridis than with H. sapiens). Synapsida and Sauropsida
thus began at the same moment in time. However, the basalmost population of
synapsids and the basalmost population of sauropsids would have acquired
none of the distinctive synapomorphies by which the more derived forms are
recognized.
In contrast, Mammalia (H. sapiens, Didelphis virginanus, Ornithorhynchus
anatinus, their most recent common ancestor, and all of its descendants) and
Reptilia (L. viridis, Chelonia mydas, Alligator mississipiensis, their most
recent common ancestor, and all of its descendants) can have radically
different times of origination: Jurassic in the former case, Permian in the
latter. Similarly, if you chose apomorphy-based definitions for Mammalia
and Reptilia you would still find that these two clades (which among living
forms are sister taxa) do not begin at the same moment in time.
Thus the stem-defined concept can be used to encompass a more inclusive,
older clade than the node within it. However, you can never diagnose the
stem-defined clade itself, as (in principle, if not in practicality) the
basalmost members of the clade will be essentially identical to the
basalmost members of the sister taxon. At best you can recognize the
diagnostic character states of the most inclusive node within that stem, but
turn over another slab of rock and you may find an even more basal taxon
that falls within the stem-defined clade.
Taxonomic Disclaimer: the particular definitions used herein are not, as far
as I know, currently in the literature. Instead the currently published
literature tends to use more inclusive taxa as the anchors for the
definitions)
Thomas R. Holtz, Jr.
Vertebrate Paleontologist
Department of Geology Director, Earth, Life & Time Program
University of Maryland College Park Scholars
College Park, MD 20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone: 301-405-4084 Email: tholtz@geol.umd.edu
Fax (Geol): 301-314-9661 Fax (CPS-ELT): 301-405-0796
-----Original Message-----
From: peter a. cejchan [mailto:cej@gli.cas.cz]
Sent: Tuesday, February 25, 2003 8:35 AM
To: PhyloCodeList
Subject: Note 9.4.1
Do we really need node- and stem-based definitions? Perhaps molecular
phylogenies (seem to) force us to use these. However, adhering solely to
apomorphy-based definitions would simplify Art. 13. Are there other cases
for node- and stem-based d's than are the distance-based phylogenies? Just
my opinion, as usually...
++pac
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<DIV><SPAN class=3D869344213-25022003><FONT face=3DArial color=3D#0000ff =
size=3D2>Node-=20
and stem-based definitions are most certainly useful tools in =
taxonomy! =20
This is especially true for those of us who work in fossil forms and on =
large=20
scale evolutionary biology issues.</FONT></SPAN></DIV>
<DIV><SPAN class=3D869344213-25022003><FONT face=3DArial color=3D#0000ff =
size=3D2></FONT></SPAN> </DIV>
<DIV><SPAN class=3D869344213-25022003><FONT face=3DArial color=3D#0000ff =
size=3D2>For=20
example, complimentary stem-defined taxa have exactly the same time of=20
origination *by definition*; in contrast, node-defined taxa within these =
stems=20
could have radically different times of origins (as, similarly, would=20
apomorphy-defined taxa within these stems).</FONT></SPAN></DIV>
<DIV><SPAN class=3D869344213-25022003><FONT face=3DArial color=3D#0000ff =
size=3D2></FONT></SPAN> </DIV>
<DIV><SPAN class=3D869344213-25022003><FONT face=3DArial color=3D#0000ff =
size=3D2>Let's=20
take the example of the two major clades of Amniota: Synapsida (for =
the=20
moment, let's use the definition <EM>"Homo sapiens</EM> and all =
taxa=20
sharing a more recent common ancestor with <EM>H. sapiens</EM> than =
with=20
<EM>Lacerta viridis"</EM>) and Sauropsida (<EM>Lacerta viridis</EM> and =
all taxa=20
sharing a more recent common ancestor with <EM>L. viridis</EM> than with =
<EM>H.=20
sapiens</EM>). Synapsida and Sauropsida thus began at the same =
moment in=20
time. </FONT></SPAN><SPAN class=3D869344213-25022003><FONT =
face=3DArial=20
color=3D#0000ff size=3D2>However, the basalmost population of synapsids =
and the=20
basalmost population of sauropsids would have acquired none of the =
distinctive=20
synapomorphies by which the more derived forms are=20
recognized.</FONT></SPAN></DIV>
<DIV><SPAN class=3D869344213-25022003><FONT face=3DArial color=3D#0000ff =
size=3D2></FONT></SPAN> </DIV>
<DIV><SPAN class=3D869344213-25022003><FONT face=3DArial color=3D#0000ff =
size=3D2>In=20
contrast, Mammalia (<EM>H. sapiens</EM>, <EM>Didelphis virginanus</EM>,=20
<EM>Ornithorhynchus anatinus</EM>, their most recent common ancestor, =
and all of=20
its descendants) and Reptilia (<EM>L. viridis</EM>, <EM>Chelonia =
mydas</EM>,=20
<EM>Alligator mississipiensis</EM>, their most recent common ancestor, =
and all=20
of its descendants) can have radically different times of origination: =
Jurassic=20
in the former case, Permian in the latter. Similarly, if you chose =
apomorphy-based definitions for Mammalia and Reptilia you would still =
find that=20
these two clades (which among living forms are sister taxa) do not begin =
at the=20
same moment in time.</FONT></SPAN></DIV>
<DIV><SPAN class=3D869344213-25022003><FONT face=3DArial color=3D#0000ff =
size=3D2></FONT></SPAN> </DIV>
<DIV><SPAN class=3D869344213-25022003><FONT face=3DArial color=3D#0000ff =
size=3D2>Thus=20
the stem-defined concept can be used to encompass a more inclusive, =
older clade=20
than the node within it. However, you can never diagnose the =
stem-defined=20
clade itself, as (in principle, if not in practicality) the basalmost =
members of=20
the clade will be essentially identical to the basalmost members of the =
sister=20
taxon. At best you can recognize the diagnostic character states =
of the=20
most inclusive node within that stem, but turn over another slab of rock =
and you=20
may find an even more basal taxon that falls within the stem-defined=20
clade.</FONT></SPAN></DIV>
<DIV><SPAN class=3D869344213-25022003><FONT face=3DArial color=3D#0000ff =
size=3D2></FONT></SPAN> </DIV>
<DIV><SPAN class=3D869344213-25022003><FONT face=3DArial color=3D#0000ff =
size=3D2>Taxonomic Disclaimer: the particular definitions used herein =
are not, as=20
far as I know, currently in the literature. Instead the currently=20
published literature tends to use more inclusive taxa as the anchors for =
the=20
definitions)</FONT></SPAN></DIV>
<DIV> </DIV>
<P> =20
<FONT size=3D2>Thomas R. =
Holtz,=20
Jr.<BR> =20
Vertebrate=20
Paleontologist<BR>Department of Geology =20
Director, Earth, Life & =
Time=20
Program<BR>University of Maryland =20
College Park=20
Scholars<BR> =20
College Park, MD =20
20742 <BR><A=20
href=3D"http://www.geol.umd.edu/~tholtz/tholtz.htm"=20
target=3D_blank>http://www.geol.umd.edu/~tholtz/tholtz.htm</A><BR><A=20
href=3D"http://www.geol.umd.edu/~jmerck/eltsite"=20
target=3D_blank>http://www.geol.umd.edu/~jmerck/eltsite</A><BR>Phone:&nbs=
p;=20
301-405-4084 Email: tholtz@geol.umd.edu<BR>Fax=20
(Geol): 301-314-9661 Fax =
(CPS-ELT):=20
301-405-0796<BR></FONT></P>
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solid; MARGIN-RIGHT: 0px">
<DIV class=3DOutlookMessageHeader dir=3Dltr align=3Dleft><FONT =
face=3DTahoma=20
size=3D2>-----Original Message-----<BR><B>From:</B> peter a. cejchan=20
[mailto:cej@gli.cas.cz]<BR><B>Sent:</B> Tuesday, February 25, 2003 =
8:35=20
AM<BR><B>To:</B> PhyloCodeList<BR><B>Subject:</B> Note=20
9.4.1<BR><BR></FONT></DIV>
<DIV><FONT face=3DArial size=3D2>Do we really need node- and =
stem-based=20
definitions? Perhaps molecular phylogenies (seem to) force us to use =
these.=20
However, adhering solely to apomorphy-based definitions would simplify =
Art.=20
13. Are there other cases for node- and stem-based d's than are the=20
distance-based phylogenies? Just my opinion, as =
usually...</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>++pac</FONT></DIV>
<DIV><FONT face=3DArial =
size=3D2></FONT> </DIV></BLOCKQUOTE></BODY></HTML>
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