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[In connection with the requests of various folks on the list, I have tried
to weed out the jargon, or at least explain it. Hopefully, this post will be
of some use to folks who are trying to figure out cladistics. I am a little
out of it today, though, so if something is hard to understand, please just
e-mail me (off list if you like) and I'll try to explain...]
On Wed, 26 Mar 1997 15:24:42, Jeffrey Martz wrote:
> It made the point that the longitudinally oriented
>and keeled sternum is a feature that is shared covergently between
>digging animals and flying birds, and this may be the case with
>It also suggested that the fibula
>not articulating with the tarsus was another covergent feature.
It is ironic that _Mononykus_ has been singled out by Ostrom and
others as an example of the inadequacy of cladistics. _Mononykus_ is better
taken as an example of the inadequacy of traditional systematics.
I am reasonably certain that no individual involved in the debate
over this animal would deny that a keeled sternum is present in some digging
animals. Further, there is nothing that I can see in the avian origin theory
which implies that _Mononkus_ was not a digging animal. Simply pointing out
that this *could be* a convergence is meaningless. What is important is
whether it is *more likely* to be a convergence. Here we see another example
of traditional systematicists determining their character-state
transformations before they develop their phylogenetic hypothesis.
Non-jargon translation: They are making decisions about what is and
isn't convergent *before* they determine which animals are most closely
related. While, on the surface, this sounds like a good idea, in the end it
is defeating the purpose of science.
As I noted to several fellow observers at a presentation on
"Creation Science" (yes, it was by a Physics PhD!) yesterday, making an a
priori assumption and then attempting to prove it (especially by accepting
that assumption as part of your proof) as a rule is inconsistant with the
scientific method (and specifically, circular reasoning).
The only minimally-circular, reproducable, consistant method we have
currently at our disposal to resolve these issues is the evaluation of
shared dervied characteristics, and the postulation phylogeny based on that
data (collectively cladistics/phylogenetic systematics). Only after we have
developed our hypothetical tree can we suggest whether a character is truly
convergent or not.
What it comes down to is whether or not the keeled sternum of
_Mononykus_ evoloved to support digging muscles from the non-avian dinosaur,
or whether it is an exaptation (modification) of the avian sternum. Ceteris
paribus it is more parsimonious to consider that within Coelurosauria the
keeled sternum evolved only once (contra the implications of Hou, Zhou,
Martin, and Feduccia 1996). The splint like fibula and derived state of
fusion in the manus (as noted by Dr. Holtz earlier) *support* this hypothesis.
Let's put it another way: What if I showed you a fossil terrestrial
animal that doesn't look like a whale, but looks a lot like it's common
ancestor. Now what if I demonstrated that this animal has four features
(say, nose on the top of the head, heavy elongate tail, etc. etc.) in common
with advanced whales that it doesn't share with _Ambulocetus_ (the most
primitive whale I can remember) or whale's closest fossil relatives, and one
in common with those relatives. Should I then say that, based on the fact
that this animal is terrestrial, or "looks" like mesonychids, that these
four characters are convergent? Which makes more sense?
Olshevsky, in his many posts on this topic, has decried the
application of phylogenetic systematics to this situation on the basis that
one cannot simply claim the presence of homoplaisy (convergences, reversals,
etc) without an explanation drawn from functional morphology. Indeed, Ostrom
(in his enjoyable paper in the Paleo Society's short course on vertebrate
evolution) seems to aggree with him. As noted above, an explanation based on
functional morphology a priori to the analysis is not required before
proceeding, and is detrimental to a scientific approach to systematics.
However, certainly such an explanation is useful once the best explanation
of the data has been arrived at. I do not see where such a thing is lacking
in this case: _Mononykus_ as a very primitive, possibly fossorial flightless
bird is both functionally and phylogenetically supportable.
Ostrom states that _Monoykus_ looks more like a (non-avian) dinosaur
than a bird (or something to that effect). This is, of course, not
meaningful phylogenetically. _Archaeopteryx_ could be said to look more like
a non-avian dinosaur than a (modern) bird too. Certainly, the lack of wings
makes the situation worse in _Mononykus_, but this underscores an important
point. _Mononykus_ looks like a non-avain dinosaur due to shared primitive
characteristics. OTH, some of these characters may be reversals, as is
suggested by the currently published phylogeny of Mononykus (such as the
presence of Haemal arches which Olsevsky claims as not being present in
birds, however these are (dammit where's the ref, I just read it) present in
However, with the exception of reversals, the primitive characters
which make _Mononykus_ look like a N-A. D. are not useful for determining
it's relationships. How could they be? Oversimplification: should we say
that we are more closely related to lizards than to horses because our scaly
"cousins" have five fingers just like us?
Anyone wishing to dispute this in a scientific manner should begin
by demonstrating that a parsimonioius (the simplest) distribution of
character states within the Coelurosauria supports the conclusion that
_Mononykus_ is more closely related to a "non-avian" dinosaur than it is to
an avialan dinosaur. It is certainly not valid to merely suggest that it is
a N.-A. D. of "uncertain affinities" without doing this when other workers
have published such a study. How often do you hear physicists declaring that
new subatomic particles simply do not exist without performing an experiment
to find out if it does or not?
>**Dinosaur Bob rips the underpants off guys we hate!!**
Didn't everyone spend some time a while back torching poor Kelly for
this sort of thing? ;)
Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock TX 79409
Web Page: http://faraday.clas.virginia.edu/~jrw6f