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Re: Exaptation



In a message dated 95-12-06 12:09:16 EST, Thomas_R_HOLTZ@umail.umd.edu (th81)
writes:

>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.

Well, I follow you so far! Particularly if within "exapt" you also include
the process of vestigialization. Thus, for example, the strong first pedal
digit of small, arboreal herrerasaurians, situated alongside the second pedal
digit and used primarily for locomotor support along with the third through
fourth digits, became exapted for a grasping function when the distal end of
the first metatarsal parted company with the proximal end and moved to the
back of the second metatarsal, allowing the first digit to oppose the second
through fourth digits. In cursorial ceratosaurians, the descendants of the
small, arboreal herrerasaurians, the proximal portion of the first metatarsal
was lost through vestigialization, and the first pedal digit correspondingly
shrank as the grasping function of the foot was lost--but the digit itself
was again exapted as a dew claw.

This is just one possible scenario for the evolution of the "theropod foot,"
of course. The first digit could have become exapted through vestigialization
as a dew claw first, then the distal portion of the first metatarsal could
have moved to the back of the foot, and then the dew claw could have
re-enlarged to be exapted as an opposable first digit. I think this is less
likely, because the initial vestigialization of the digit to a dew claw would
have correspondingly vestigialized the various calf muscles and tendons that
worked the digit. These would have had to re-enlarge along with the digit
when it retroverted. In the former scenario, however, those muscles and
tendons would have been immediately available to be exapted for working the
retroverted digit.

>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.

In a different scenario, it was already part of the flight structure of
dino-birds, an adaptation to steady the wing during flight by restricting the
forewing's tendency to pronate and supinate. It continued to restrict the
motions of the hands of cursorial coelurosaurs in a similar way, without
apparent detriment to their predatory abilities.

>>>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 hypothesis.

Still, not "never heard anyone."

>>>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.

BCF and BADD phylogenies are very similar, since they work from the same
character sets and the same organisms.

>>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).

Where do I do this??

>Here's an alternative scenario:
>
>Birds are the volant ancestors of nonflying coelurosaurs.

What, have I converted you? :-)

>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.

I must be dense. I'm not following this at all.