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Re: Scales, hair, integumentary structure relationships?



David Marjanovic (david.marjanovic@gmx.at) wrote:

<I'll have to look that up. However, there are cases where the 3rd ungual is 
still there (like *Jibeinia*), so fusion (as it happened in therapsid 
evolution) seems likely.>

  This is where it gets tricky for me (at least). I do not actually see why 
this is likely. I see this as an either/or situation: The phalanges are either 
fused, or they are not. If they are, then there should be primary evidence to 
confirm this, such as evidence of suturing or the condition seen in abnormal 
development of some limbs in which the bones literally fuse, producing an 
element that is much longer than any one single part of it would be (that is, a 
phalanx of 1cm fusing to a phalanx of 3cm should result in a phalanx of ~4cm, 
so that the parts of these would be smaller than the whole). If they are not 
fused, then there are alternate arguments to be made, such as anteriorization 
or posteriorization of the digits, so that they develop as though they were one 
or two steps up or down the road, or that distal elements of the bones are 
lost, but development still favors distal elements for whatever reason. (I am, 
of course, talking about
 frame-shift.) This issue is also relevant to, say, arguments about "fusion" of 
pedal phalanges in some fossil birds, especially what is essentially the 
biggest reason some have argued for a generic (much less specific) identity for 
the Solnhofen specimen of *Archaeopteryx lithographica* (as I consider it).

  Consider, for example, the megalancosaur manus. The phalangeal formula for 
*Megalancosaurus preonensis*'s holotype and a couple of referred specimens is 
2-2-3-3-3. Each of these digits has two things in common: an ungual, and an 
antepenultimate phalanx with a peculiar morphology. Moreover, each of these two 
phalanges per digit are nearly the same size. Following the logic of fusion, 
and taking the basal diapsid phalangeal formula of 2-3-4-5-3 (or so), we should 
see in digits II and III each a loss of 1 phalanx, in digit IV a loss of two, 
and in digit V a loss of none, despite digit V, unlike other basal diapsids, 
being particularly large. In this, if we expect fusion, there should be an 
irregular level of fusion along each digit that ignores the unguals, and in 
fact the proximal phalanx. It can only fuse intermediate phalanges, and give 
them all identical morphologies. If we applied this theory to, say, avian manus 
and pes phalangeal formulae, we would
 get some odd feet. Especially if they had supernumeral digits, but that's 
beside the point. Alternately, we can argue that these digits have undergone a 
homologous development, whereby the digit identities were subsumed and "cloned" 
into copies of preceeding digits. Digit II copies digit I, and digits IV and V 
are copies of digit III. (This is on top of the peculiarly mobile metacarpals, 
which, if not all actual metacarpals but some as phalanges, still complicates 
the issue and should still argue for developmental issues that do not 
accomodate a fusion theory.)

  Back to birds. It is reasonable to argue for fusion, but there does not 
actually seem to be a primary principle arguing for fusion, rather than just 
phalangeal loss. Size of a phalanx may change. As modern birds use the first 
phalanx of the third digit differently than do, say, fully-phalanxed theropods, 
we can expect that different constraints and function have affected the bone's 
history. To argue fusion is the "likely" explanation for this element is to 
ignore a wealth of alternate, and just as likely, explanations. And this is 
hardly parsimonious. However, I've talked a blue streak on this issue, and I 
haven't even waited for David to jog his memory on phalangeal fusion for birds.

  Cheers,

Jaime A. Headden
http://bitestuff.blogspot.com/

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)