Date: Tue, 29 Jan 2002
Okay, little problem with the semiulunate I seem to note with people:
The semilunate refers only to the bone's _aspect_ as a relatively near
semicircular distal carpal block. Nothing else. The element alos contains
several other features, but these are not implicit in the discussion of
what a semilunate is. In fact, one also refers to the fifth metatarsal
bone in tetanurans as "semilunate" because it is crescentic in aspect.
There are several different qualities to the distal carpals that are
being assumed here, and one needs to be very clear about what they are
separately before one can just assume they all mean the same thing. For
one thing, they lack homologous distribution.
1. distal carpal 1 bears trochleate, "hinged" proximal surface
articulation for proximal carpals. [0 is the abscence, 1 is the presence
of this feature; a state 2 can be added for the presence of the trochlea
to persist on the second distal carpal as well]
2. distal carpals 1 and 2 fused. [0 is unfused, 1 is fused]
3. distal carpal 1 completely appressed to the proximal surface of the
first metacarpal [conversely, 0 is the presence, 1 is the absence]
4. distal carpals 1 and 2 in unity progress across the proximal surfaces
of the first _and_ second metatarsals, equalling mediolateral width [0 is
the absence, 1 is the presence; a state 2 can be added to indicate the
complete coverage of the distal carpals to the proximal surfaces of the
metacarpals flexor-extensorly as well -- note, ]
5. distal carpals 1 and 2 in unity expanded dorsally into an aspect of a
semicircle, appearing "semilunate" in profile, roughly between 120 and 180
degrees of an arc. [0 is the absence, 1 is the presence]
6. distal carpal block fused to first and second metacarpals [0 is the
absence, 1 is the presence]
Now, take all these and run down through theropods and see the
distribution for each feature.
It would be interesting to see how this would group the taxa together to
travel the course of development of the first two distal carpals relative
to the manus in these taxa. However, these are functional features, and
are relative to movement of the wrist. They may not be indicative of
evolution, but then again, they may. However, something that may be taken
from this is that the earliest theropods lacked any sort of "advanced"
wrist and that the orniothomimids have a unique wrist and my influence the
tree with this. However, they are advanced in the asscesion of the first
two distal carpals being expansive mediolaterally over their relative
metacarpals. Note, the above matrix assumes that in birds, the slc block
is comprised of a fusion of distal carpals 1 and 2.
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