[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
At 06:39 PM 10/18/97 -0700, you wrote:
>Fascinating. It's been quite a while since I saw a reply to one of my
>posts in which the replier did such a comprehensive job of jumping to
>wrong conclusions and substantively misinterpreting just about every
I am sorry you feel this way. Perhaps this might inspire you to
construct your future posts in a manner which more accurately reflects your
>The only photograph I have of _Protoavis_ is in Lessem's book. It shows
>skull, forelimb, a nearly complete spine, most of a hindlimb below the
>knee, and a partial sternum. Nothing else.
Perhaps, then, you ought to avail yourself of better sources before
positing such questions to the list.
>Incidentally, I did say "_IF_ no details of the shoulder anatomy are
>known". If. Conditional. As in, "I am not sure this is true, but
>assuming it is, then . . ."
This is a rather weak excuse, don't you think? I shan't quibble over
the implications of the post, except to say that if this was the intent of
your post, it is certainly not clear. It rather sounds like a rhetorical
"if". If you are unclear on how I mean "rhetorical", you might ask Caitlin
or "Charger"... :)
>It would have been relatively simple to
>just say "we do know something of the shoulder anatomy," without getting
>snotty about it.
I hardly think that enumerating the known portions of the shoulder
girdle is "snotty". As for the rest, perhaps you might in future look into
the animal in a little more depth before positing imponderables to the list. :)
>My point exactly. I was not questioning the bipedal reconstruction; I
>was stating it as a precondition for a sequence of reasoning.
This seems to be a natural sequitor to the line of questioning you
were persuing in the shoulder girdle. I am glad that you concur.
>Birds are a discrete subgroup of dinosaurs. In vernacular usage, saying
>"bird or dinosaur" is quicker and easier to understand than saying
>"avian theropod dinosaur or non-avian theropod dinosaur."
If we are to persue a menaingful scientific dialoug, we must be sure
that we have commonality of definition and precision of usage. This is
especially true in the discussions of phylogeny, where it can mean the
difference between a cladogram and one of those "mushroom-o-grams" of taxa
arising from nowhere, relateing to nothing, and slipping silenty into
extinction which invert paleo-types like so much.
So, while the "vernacular sense" may be quicker ("non-avian" adds
two whole syllables to your statement. Hardly crippling...), it is hardly
easier to understand, and it detracts from the overall utility of your
statements. Especially in an area such as this, where terminology is
frequently twisted to obscure the biological implications of theories. As
far as _Protoavis_ is concerned, excessive reptition of a supposed
bird-dinosaur "dichotomy", whether real or as an artifact of speach, can
only result in misrepresentation and misinterpretation. As I have pointed
out in past posts, this has indeed already occurred.
>I think my meaning was clear: there is at least a possibility that
>_Protoavis_ is the sole known member of a lineage of tetrapods for which
>no other member is yet known, so nothing can be clearly known of its
>evolutionary history or its relationships to other tetrapods.
And I would say that this is not possible in the manner in which you
mean it, and almost certainly true in a more broad interpretation.
The very fact that this animal was preserved suggests that it was a
part of a radiation which may have included other taxa more closely related
to it than to all other known taxa. Thus, it is certainly possible that it
is the "sole known member of a lineage of tetrapods for which no other
member is yet known". This, however, has nothing to do with whether we can
say what other taxa are most closley related to it.
Remember that "relatedness" is a *relative* phenomenon, not an
absolute. Despite what you may read in invertebrate biology textbooks,
groups of animals are probably *never* "unrelated". This idea seems to
result from a mapping of evolutionary theory onto Linnean taxonomy. The
confusion which follows results in statements such as the one you made in
your original post, statements which sound rational until you really think
_Protoavis_ is a member of, if nothing else, the tetrapoda. It is,
in fact, almost clearly a member of the Diapsida, and most likely the
Archosauria. As that group is founded on the Ornithosuchia/Pseudosuchia
dichotomy, it is therefore a member of one of those two lineages.
The idea that they might be a member of an "unclassifiable" group is
simply not a part of a modern, phylogenetic approach to systematics. Once
the appeal to "groups unknown to science" is delimited, your point becomes moot.
_Protoavis_ is a member of a known lineage of tetrapods, and its
relationships to other tetrapods may be evaluated to determine where it fits
into the overall evolutionary scheme. Perhaps it is a member of an as yet
unknown radiation within the Archosauria. However, this is immaterial to the
point you are making. It's derived characteristics will give us an idea of
which group it is most closely related to.
>> Ok, last time: bird => theropod
>Semantic quibbles again.
When communicating ideas, it is vital that there be commonality in
the meaning and usage of words. Otherwise, as you seem to be discovering,
there can be no certainty that the idea has been communicated correctly.
>Avian theropod vs. non-avian theropod or chimaera. Satisfied?
Yes, this is both more lucid and more useful. However, you seem to
overlook the possibility that _Protoavis_ might be a non-theropod dinosaur,
a non-avian ornithosuchian, etc.
>I don't need a lesson in basic stratigraphy, Professor Wagner.
>In Britain, Cretaceous mammals and other small animals are often found
>in infilled cracks in Carboniferous strata.
While this may fit the definition of "reworking", the depositional
setting of Post seems to pretty much discount the possibility of crevasse
fills, etc. It was perhaps improper of me to assume that you meant the
reworking of bones as sedimentary particles, for which I apologize. However,
you could certainly have been more clear as to what you meant.
>Why is such a scenario impossible? Or even implausible?
Read Chatterjee's paper. Or for that matter, Lehman's work on the
Dockum. You tell me.
>Does the local stratigraphy rule it out?
As I recall, and don't quote me since it's been a while since I went
out to Post, the fossils are found in coarse-grained lenses in the redbeds
of the Dockum group. Redbed above them, redbed below them, redbed on all
sides of them, etc. The depositional of the fossils was regarded as being
contemporaneous with the surrounding strate.
Dr. Chatterjee is a competent geologist with years of experience, a
wealth of literature available for his use at the TTU library (just so you
know, I'm being sarcastic about the library), and a department full of other
competent professionals, including Dr. Thomas M. Lehman, who has also worked
on the Dockum. It is highly unlikely that either of them would have missed
evidence of the scenario you present.
>You're jumping to conclusions again. I am not saying that _no_ fossil
>is acceptable as a bird unless it had feathers. I am saying that in my
>mind, PROTOAVIS is not acceptable as a bird unless somebody shows it had
>feathers. Extraordinary claims require extraordinary evidence. A bird
>in the Cretaceous is not an extraordinary claim. A bird in the Triassic
This is unacceptably poor reasoning. Is the presence of birds
delimitied by the oldest occurance of feather preservation? This is not the
way paleontology proceeds. Whether or not _Protoavis_ is hypothesized to be
a member of the Avialae is not directly related to whether feathers are
preserved, but to a suite of derived characters, one of which might be
feathers. Other characters are especially important since the distribution
of feathers in Ornithosuchia is in doubt, and since they are found preserved
only in unusual cricumstances.
We should not allow our concept of the stratigraphic relationships
to affect our understanding of the phylogenetic relationships. As I just
finished pointing out to someone else, we use phylogeny and stratigraphic
position independently to evaluate the stratigraphic range of organisms. If
we allow stratigraphy to dictate our concepts of phylogeny (which is then
extended to the stratigraphic ranges of fossils, we are engaging in circular
Further, in order to accept your objections, we must postulate a
much better fossil record for terrestrial vertebrates than what we actually
observe. This is why scientists use osteological evidence to evaluate the
relationships of animals. Once this is done, then we can suggest how well or
how poorly our hypothesis fits the stratigraphic record.
_Protoavis_ should be evaluated on the basis and biases of the
evidence, not on a priori assumptions of the stratigraphic range of birds.
Certainly "it's not a Triassic bird until I see feathers" is not a
Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
"There's a fine line between stupid and clever." -- Spinal Tap