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Exaptation



>The semilunate carpal is but one feature related to flying that appears in
>_Archaeopteryx_. If the semilunate carpal were all there was, then I wouldn't
>even try to concoct a different scenario for bird origins from the orthodox
>one. But BADD expects me to swallow the conjunction of many different
>supposed "exaptations," _each_ arising for some reason unrelated to flying,
>in a single lineage where, miraculously, they produce a flying dinosaur! This
>is nuts. It makes much more sense to me to envision these "exaptations"
>arising in turn in a lineage as incremental improvements to a volant
>lifestyle of some kind (just what kind--climbing, gliding, lousy flying--is
>not yet clear).

Brace yourselves, folks:  :-)

There are only two possibilities in the evolution of a structure, if we
accept that all species evolve from previous species.  Either a) the
structure is a neomorph, such as the rostrals of ceratopsians, or (vastly
more common) b) the structure is exapted from a previously existing one.
That's it: either there is a whole new structure, or it is a modified
version of a pre-existing piece of anatomy.

Bones might be great for support on land, but they are exaptations of
mineral storage devices in basal verts.  The reduced postdentary jaw bones
of basal synapsids was exapted into the middle ear structure of mammals.  The
wings of penguins are exapted as flippers. The spike in the hand of
iguanodontians, whatever its purpose, was exapted from the previous usage in
more primitive ornithopods. And, in one scenario, the semilunate carpal of
coelurosaurs, used for folding (or rapidly unfolding?) the limb, was exapted
into the flight structre of birds.


>>I may be ignorant here but I have never heard anyone claim that the
>>ancestors of birds weighed 20-50 kg - only that LATER non-avian maniraptors
>>did.
>
>I saw an article in _Discover_ several years ago in which they illustrated
>flying birds evolving directly from _Tyrannosaurus rex_! Good grief!

Hardly a main aspect of the BAAM hyothesis.

>>Why couldn't both lines - the avian and non-avian - have evolved from
>>a smallish ancestor?
>
>Because "both lines" is not necessarily the best picture. There weren't
>simply two lines--avian maniraptors and non-avian maniraptors; there were
>lots of different lines of both. There were archaeopterygid maniraptors,
>dromaeosaurid maniraptors, avimimid maniraptors, mononykid maniraptors,
>troodontid maniraptors, tyrannosaurian maniraptors, enantiornithine
>maniraptors, and so forth. BCF says that the ancestral forms of all these
>lines themselves form a line, the line leading to the more modern avian
>maniraptors.

Actually, certain BAAM theorists would not disagree here, with the
modification that some of the lineages share more recent common ancestry
with others not shared by the third.
>
>
>It is simple to envision in general terms the evolution of a large,
>flightless bird such as _Diatryma_ or an ostrich from a smaller, volant form,
>even though the details may not be available in the fossil record. What
>evolutionary steps would something like a _Diatryma_ or an ostrich have to
>undergo in order to produce a (small) flying descendant? Can you document an
>occurrence of this kind of evolution? If you can, I'm certainly interested in
>hearing about it, because then I would be much more willing to accept the
>evolution of a (small) flying descendant from a larger theropod.

You are using your conclusions (that dromaeosaurids, tyrannosaurids, and so
on are secondarily flightless) to justify your conclusion (that
dromaeosaurids, tyrannosaurids, and so on are secondarily flightless).
Here's an alternative scenario:

Birds are the volant ancestors of nonflying coelurosaurs.

The question of flight is based on functional data: we are all agreed that
Velociraptor and its kin were not flyers (I hope!).  However, the
conclusions of the above scenario are NOT based on the observation that
dromaeosaurids were not fliers, but is instead based upon the distribution
of derived character states in known taxa.

Granted, I know what you think of the phrase after the comma above, but I'd
just like to make sure I put my comments into perspective.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist
Dept. of Geology
University of Maryland
College Park, MD  20742
Email:Thomas_R_HOLTZ@umail.umd.edu (th81)
Fax: 301-314-9661
Phone:301-405-4084