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Re: Semi-lunate carpal block: Long
At 12:58 PM 7/31/97 -0400, jpoling wrote:
>><<>In _Alxasaurus_ the two carpal elements that become the so-called
>>"semilunate" >carpal in _Therizinosaurus_ are still unfused
>> Ontogenetic? Bones have been known to fuse late in ontogeny. In any
>>case, the form of the bone is clearly there. Which prosauropods have a
>>semilunate carpal block?>>
> I recently made a statement to an author that showed up as gospel in the
>resultant article, despite the words "I think I once read" (did you ever
>make a statement to a reporter that sounded reasonable at the time, but when
>you saw it in print made you cringe?). I thought I read on the list that
>the difference between the "semi-lunate" carpal block in maniraptoriformes
>and maniraptors is that in maniraptors the bones of the block are fused,
>while in m.formes they are not. Is this true?
> Also, I seem to recall somebody stating that dinosaurs outside the
>m.formes having the block, but I was not able to find it in the archive. If
>true, what dinosaurs is the block found in?
This all really depends on your definition of things (which gets towards the
bottom line of the 1995 Holtz-Sereno discussion at the SVP Theropod symposium).
*I* would require the minimal working definition of a semilunate carpal
block to be a distal carpal structure (homology with the primitive amniote
carpus not certain at present, and requiring ontogenetic work to figure out)
capping mcI and II *WITH* a marked curvature (hence "semilunate" = half moon
shaped) *AND* a pronounced trochlear groove (pulley structure).
Troodontids have this, dromaeosaurids have this, oviraptorids have this,
primitive birds have this, Therizinosaurus and other therizinosaurids
illustrated by Barsbold has this, and (according to the illustrations) the
larger individual of Alxasaurus (IVPP 88402) shows this. In Alxasaurus this
element is clearly two ossifications, in the others there is only a single
bone. However, until the ontogenetic changes in these groups is worked out
(i.e., somebody find us some baby raptor wrists!!), it is uncertain whether
the elements in these other forms are homologous to a single primitive
carpal, or to the fusion of two primitive carpals. Ornitholestes and
Coelurus seem to show this, but as noted on the net, exactly which hands
belong on which Morrison maniraptoriform is being worked out.
Tyrannosaurids and ornithomimosaurs do not show this condition: they have a
single large element in the same position, but it is flat and lacks the well
developed trochlear surface. In Tyrannosaurus proper, on this element has
been found: in more primitive tyrannosaurids (Gorgosaurus, Albertosaurus) up
to four other (more proximal?) carpals are known. The same condition is
found in ornithomimosaurs.
Allosaurids and some other non-maniraptoriforms have two elements in the
same position as the semilunate carpal block, with a moderately
well-developed trochlear surface but lacking the pronounced "half moon"
shape. These elements may well be homologous to the semilunate carpal
block, and probably represent the ancestral condition from which it was
derived. Sereno (in various publications) considers this structure
sufficient to be called a "semilunate carpal block", which is why he
considers it diagnostic of a more inclusive group than Maniraptoriformes.
As I said, a matter of definition. Hope this helps.
Thomas R. Holtz, Jr.
Vertebrate Paleontologist Webpage: http://www.geol.umd.edu
Dept. of Geology Email:email@example.com
University of Maryland Phone:301-405-4084
College Park, MD 20742 Fax: 301-314-9661