[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
At 10:19 AM 7/9/97 -0400, you wrote:
> I don't have any idea what the unicorn bit was about,
Sorry. The basic idea: What is the difference between lack of information
because we haven't found it and lack of information because it isn't there?
Answer: We don't (and can't) know UNTIL we find the information we are lacking.
>but my point is
>merely that if we were to be deprived of certain key bits of information,
>i.e. the existance of volant paleognaths such as tinamous,
>lithornithiforms, and volant outgroups, we would be helpless to use
>phylogenetic bracketing to answer the question of whether ratites were
>secondarily flightless; and perhaps because of the very poor record of
Ten years ago, this statement had a lot stronger impact than now. The rate
of new Mesozoic bird discovery is phenomenal!
>a similar situation could exist in which secondarily
>flightless dinosaurs could not be revealed as secondarily flightless by
>phylogenetic means. Obviously this leaves only the functional approach.
Or some approach that hasn't been thought up yet. (Hey, if it is fair to
hypothesize that we don't know about data which we really DO know about, it
is fair to hypothesize that there are additional lines of evidence which we
haven't come up with yet...)
> And it's not true that there is "no evidence" for secondary
>flightlessness. The section in Predatory Dinosaurs provides a list of
>evidences for secondary flightlessness. Are they unambiguous?
No. They are ambiguous. They are *consistant* with a possible secondarily
flightless Maniraptoriformes, but that is not the only functional hypothesis
with which they are consistant. As such, the data do not favor secondarily
flightlessness over other functional scenarios (scansoriality or particular
forelimb-oriented prey acquisition techiniques, for two possibilities).
And there is the "problem": to adequately demonstrate secondary
flightlessness in any taxon, we need unambiguous evidence that the taxon in
question had definite volant ancestors.
>majority you can put quite a few different spins on, and invoke predation,
>tree climbing, or powered flight. But proponents of secondary
>flightlessness do not need to demonstrate that features exist in the
>skeleton of Velociraptor and other dinosaurs which could only be used for
They DO need to demonstrate this if they expect others to fairly choose
their hypothesis over the others offered.
>They merely need to demonstrate that there are enough features
>which suggest strongly enough that these animals had a volant ancestry
>that it is more likely than the alternative, i.e. the hypothesis that
>flight evolved somewhere after Archaeopteryx split with Unenlagia.
That may be sufficient to demonstrate the hypothesis to those who are
already proponents of the idea. However, the ambiguity of the evidence as
now known means that choosing one of the other hypotheses is an equally
valid choice based on the data.
I would not be terribly surprised if we do find out that dromaeosaurids,
oviraptorids, or some other advanced maniraptoriform group is secondarily
flightless. However, the data as now known to not unambiguously point to
this. To unambiguously demonstrate such, we need to find either: a)
unambiguous morphological evidence of secondary flightlessness (with the
danger, of course, that the morphology might actually support a behavior we
haven't considered yet) or b) phylogenetic evidence that the taxon is
question is bracketed by unquestioned volant animals.
Hope this help.
Thomas R. Holtz, Jr.
Vertebrate Paleontologist Webpage: http://www.geol.umd.edu
Dept. of Geology Email:email@example.com
University of Maryland Phone:301-405-4084
College Park, MD 20742 Fax: 301-314-9661