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Digital Duplications (A Case for an Ichthyologist?)



Brad M. wrote:

<<Didn't some icthyosaurs re-evolve the sixth, seventh
and eigth digit?>>

and Mike Keesey wrote:

<Good point. Is that a case where existing digits were
copied, ... or is it a case where the actual primitive
digits were re-expressed.... Are the digits of
Ichthyosauria homologous to those of _Ichthyostega_,
or were some of the "crown tetrapod five" digits
duplicated?>

  I have not read the literature on polydactyly in
Ichthyosauria, and certainly must get Motani's work on
the subject, and Carroll's and Romer's general pieces
for references, unless someone wants to do a little
quick digging for me. However, I believe that Ahlberg
has bent his great intellect to the subject on
*Ichthyostega* in reference to the relative _decrease_
in digits in more advanced stem-tetrapods
(Tetrapodocladae in Headden-ese) than in crown-
tetrapods (Tetrapodocoronae). I would consider the
relive increase in digits in some "euryapsids"
(polydactyly) would have some neogenic or atavistic
origins.

  Dinogeorge suggests basal sauropods retained the
fifth pedal digit, as a plesiomorphy, if I'm to
understand the hypothesis. This might then lead to the
following statement: "All sauropods retain the fifth
pedal digit as an atavistic, plesiomorphic feature
from a basal stock of dinosaur bearing a fifth digit,
rather than as a neogenic, new feature, or reversal."
I might also switch "reversal" and "atavistic and
plesiomorphic" in the above to make a (a) and (b)
sub-theory. Without belaboring the whole shebang, I
would like to add my thoughts on the matter of having
a basal fifth toe in a larger, advanced animal.

  All known most basal dinosaurs, including
"prosauropods," theropods, and ornithischians, lack a
functional or vestigial fifth toe; herrerasaurs have a
small nubbin of the first fifth pedal phalanx (pdV-1),
and more basal ornithodirans (*Lewisuchus*,
*Marasuchus*, *Lagerpeton*) have more than one
phalanx. This is what is meant by suggesting the
sauropod fifth digit is not basal, as all bracketing
groups lack the digit (theropodans, plateosaurians and
other "prosauropods," and ornithischians) and thus
(1st level inference) sauropods should not have a
fifth pedal digit. They do. This is a suggesting for
an atavism or neogenic structure: the thing was either
held over from the embryological or faetal state, lost
in ontogeny in _all_ other dinosaurs (big bunches of
steps, since other dinosaurs would have had to have
lost the thing in separate events, not one, based on
_many_ phylogenetic and even "intuitive" analyses), or
was lost in all dinosaurs, and sauropods redeveloped
the structure whole, from scratch: it's not a pedal
digit V, its a bunch of jointed sesamoids on the end
of the fifth metatarsal, or whatever; it's pedal digit
VI, or whatever; so on....

  One could test this all to look for embryos of basal
dinosaurs (*Mussaurus* could be a start) for which
more "adult" material is known for comparison of
ontogeny. This can actually be done for some
ornithischians, as well, but not many real basal ones.

  Just a thought.

=====
Jaime "James" A. Headden

  Dinosaurs are horrible, terrible creatures! Even the
  fluffy ones, the snuggle-up-at-night-with ones. You think
  they're fun and sweet, but watch out for that stray tail
  spike! Down, gaston, down, boy! No, not on top of Momma!

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