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Re: Origin of Birds
At 11:27 AM 12/8/97 -0800, Ralph Miller III wrote:
>As I understand it, cladistics are based on the synapomorphies (shared
>derived traits) between animals, inherited from common ancestors. Cladists
>don't necessarily concern themselves with precisely who begat whom. How
>could we determine whether some theropods descended directly from other
>non-avian theropods or were secondarily flightless descendants of flying
>forms? In other words, how might we determine the likely anatomy or
>identity of the aforementioned common ancestor? I suppose the jury will be
>out until more fossil evidence comes in, but, in that event, precisely what
>evidence would we need to settle the matter (assuming that we could set
>aside our biases)?
You can check the dinosaur list archives for my (numerous) posts in the past
on how true ancestors might be recognizable in the fossil record using
However, as you note (and stemming from various pre-cladistic hypotheses of
speciation), it is unlikely that members of the actual population of animals
which gave rise to a given species or lineage will be a) entombed, b)
fossilized, c) eroded out in historic times, d) collected, and e) recognized
So, out of the few individuals of the few populations that do get entombed,
fossilized, erod in historic times, collected, and described, the best we
can do is discover which are most closely related to which other ones.
Cladistics does this by sorting the distribution of derived character states
among the taxa analysed.
We can be relatively secure that the immediate ancestor of two sister taxa
shared every single derived trait found in both sister taxa, so we can use
the known distribution of features to hypothesize about the ancestral condition.
As to the particular case of whether any given taxon is secondarily
flightless: the method which would give the greatest support for such an
hypthesis would be to demonstrate that the form in question (say,
dromaeosaurids) were phylogenetically nested among known fliers. (This is
how we can be secure that _Hesperornis_ is secondarily, rather than
primarily, flightless: birds more primitive than it (_Archaeopteryx_,
_Iberomesornis_, most enantiornithines, etc. are known fliers)).
Alternatively, one can find some trait which you considered demonstrative of
pre-existing flight. However, this method's main weakness is that we may
not have yet thought of other factors which could produce the same
structure, but that doesn't mean those factors aren't real. Also, many
features are not limited to a single life habit (i.e., highly curved claws
can be arboreal traits, or raptorial traits, or grooming traits, or
combinations of any two, or combinations of all three, or...).
Thomas R. Holtz, Jr.
Vertebrate Paleontologist Webpage: http://www.geol.umd.edu
Dept. of Geology Email:firstname.lastname@example.org
University of Maryland Phone:301-405-4084
College Park, MD 20742 Fax: 301-314-9661