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Re: Dinofest Report #2 (and final)
At 08:17 PM 4/24/98 -0400, you wrote:
>His heavy accent makes it a bit difficult, and I've paraphrased some
things, >but that is what I understood him to be saying.
Ok, it was you whose name I forgot. Sorry! :)
>Let me see if I understand this correctly:
>1. Arctometatarsalia was named after, and defined based primarily on, a
>specific synapomorphy, the pinched 3rd metatarsal. This feature was thought
to >indicate a close evolutionary relationship among these taxa. The name
was >therefore chosen to reflect this relationship.
1) In some cases, the "purpose" of a taxon can be pretty hard to
figure out. My interpretation (and this is *only* my interpretation) is that
Arctometatarsalia was named in order to recognize that certain coelurosaurs
(tyrannosaurs, bullatosaurs, etc.) form a clade distinct from other
coelurosaurs, rather than being dispersed variously across the family tree.
Now, as all three taxa share an arctometatarsus, that character is a
synapomorphy supporting the clade, and the phylogenetic significance of that
character was a big point in the paper which outlined the hypothetical
relationship, the taxon was named for the character.
However, phylogenetic taxa are independant of proposed phylogeny (at
least theoretically), and the "taxon concept" the namer had in mind is
irrelevant to their validity. If you think out the definition of
Arctometatarsalia ("_Ornithomimius_ and all taxa sharing a more recent
common ancestor with _Ornithomimus than with modern birds") you will see
that this taxon will always exist, regardless of anyone's hypothesized
phylogenetic structure for theropods, or when exactly they think certain
characters evolved. The *content* of the taxon changes with changing
phyolgenetic hypotheses, and as a result some taxa with an arctometatarsus
may not be included, while some which lack that structure may actually be
members. The name *was* chosen based on the author's interpretation of the
phylogeny, but a rose by any other name...
2) That said, there is still room to wonder, because, as I noted
earlier, the original definition was more or less character-based ("the
first animal to evolve an arctometatarsus and all of its descendants"). As
such, the character was important to the original definition. However, it
has been noted that this is not a very stable means of defining a taxon. For
example, if two clades evolved an arctometatarsus independantly
("convergently" or whathaveyou), how would you determine which one evolved
the character first?
So, Holtz (1996), in accord with the revisor principle (which
certainly has a place in phylogenetic taxonomy, as he demonstrated)
redefined the taxon as _Ornithomimus_ and all taxa more closely related to
_Ornithomimus_ than to modern birds. This formulation retains the original
intent of the taxon, assures that at least one member taxon will have an
arctometatarsus, and keeps questions of character transformations out of the
direct determination of taxon composition. This definition, however, is not
like an old Linnean definition. It does not insist that all taxa included
must possess an arctometatarsus, nor that taxa lacking one be excluded, nor
is anyone obliged to include any of the taxa Dr. Holtz included except
>2. This feature is apparently not synapomorphic among the taxa included in
3) Synapomorphy does not require that the character evolved only
once. A synapomorphy is a shared derived character. A character which
evolved in the common ancestor of a certain clade, and seperately in the
common ancestor of another clade may be synapomorphic for each of those
clades independantly but not for both together.
4) If it is most parsimonious to interpret certain taxa as forming a
clade, and they all possess an arctometatarsus while the immediate outgroups
of the clade do not, then the arctometatarsus may be synapomorphic for these
taxa, despite its occurance in other taxa which show up in a different part
of the tree.
5) That the character evolved more than once does not imply that
some or all of the taxa formerly included in the group don't still
constitute a clade exclusive of other taxa.
6) The "taxa included in Arctometatarsalia" changes with changing
hypotheses of phylogeny. True, the arctometatarsus may not be synapomorphic
for *all* of the taxa Holtz originally included in the taxon. However, to my
knowledge, no one has proposed that it is not synapomorphic for at least
*some* of those taxa, perhaps within two or more clades.
>Therefore, these taxa are not more closely related to each other than any
of >them is to other theropods.
7) Not really. If the arctometatarsus were the only character
uniting these taxa, then perhaps this would be the case. Of course, even if
it were the only character uniting them, they could still form a clade even
if other clades also possessed arctometatarsi, if that were the most
parsimonious interpretation (unlikely, I suppose). As it turns out, there
are some other characters which unite several of the taxa originally
included in the Arctometatarsalia.
8) If one were to make the statement you make above, one would be
required to demonstrate *which* other theropods each group is more closely
9) Having done so, you could still apply the name Arctometatarsalia,
given the definition above. It might only include ornithomimids, heck it
might only include _Ornithomimus_. Still, the taxon would remain, only the
composition would change.
>3. Even though the relationship after which the clade was named does not exist,
>the clade is still valid.
10) What is then "the relationship after which the clade was named"?
As noted below, a clade cannot be named based on a set of taxa, because
those taxa and those taxa alone almost certainly do not constitute a clade
exclusive of all other life. Constraining it to being a clade exclusive of
other taxa in the study is fine for discussion, but useless for taxonomy as
other taxa will come along later and you must decide what to do with them
Of course, if you again read carefully the definition of
Arctometatarsalia you will see that there *is* a relationship implicit in
the definition. This relationship will *always* exist, as long as there are
taxa _Ornithomimus_ and Neornithes.
11) Validity of a clade is an odd concept. Recall that a clade is an
ancestor and all of its descendants. No clade we recognize based solely on
its content is likely to be a true clade. If I say A, B, and C form a true
clade, I am likely wrong because there are taxa D, E, and F which I do not
know about but which are descendants of the common ancestor of A, B, and C
(or are that common ancestor). So, calling a group of taxa a clade is
imprecise. It is possible to talk about taxa forming a clade exclusive of
other taxa within a study. Now, if we precisely identify a clade as being an
ancestor and all of its descendants, that clade will *always* exist.
12) I believe you are confusing clades and phylogenetic taxa. P.
taxa are clades, but all clades are not P. taxa. A phylogenetic taxon is the
recognition of a clade with a name. The definition of a phylogenetic taxon
is the formula by which you describe the clade by determining which taxa are
members. A clade, when *properly* recognized as an ancestor and all of its
descendants is a real entity which exists in nature and can never be
"invalid". Therefore, a P. taxon can never be "invalid" (unless we goof up
the process of defining it somehow.
>That makes absolutely no sense to me.
Hopefully this disorganized little missive will help. I'm sorry its
not more lucid, but I am currently attempting to summarize 50 years of the
oxygen isotope paleontology of planktic foraminifera in twelve pages.
>Oh, wait, I forgot. It's cladistics. It doesn't have to make sense.
Perhaps it would be more productive for you to seek resolution of
your confusion, rather than resorrting to this sort of arrogant posturing.
Lack of understanding is not a crime, and is nothing to be ashamed of.
However, flaunting your ignorance in a flourish of self-righteous melodrama
is downright rude. It says to the world "hey, I'm not ready to be taken
seriously by anybody."
How about trying this instead: "This makes absolutely no sense to
me. Could someone please help?"
Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
"...To fight legends." - Kosh Naranek