Re: the transverse ligamental groove

[Nick Longrich <nrlongri@midway.uchicago.edu>]

>While compiling my theropod synapomorphy list, I found two characters with
>separate distributions that seem to be the same.  Can anyone help me out?
>The first is from Sues and Makovicky's Microvenator paper.  It is "humeral
>head offset and emarginated ventrally from pectoral and bicipital crests by
>groove".  It is said to be present in Microvenator, alvarezsaurs,
>dromaeosaurs, troodonts and birds.  This seems very similar to the "distinct
>transverse ligamental groove" that is used as an ornithothoracine
>synapomorphy in the Rahonavis paper, the Shuvuuia paper and the
>Caudipteryx/Protarchaeopteryx paper.  I've checked the literature and
>Deinonychus and Patagonykus, for instance, seem to have this condition, as
>do birds like Ichthyornis.  Can someone please tell me if these characters
>are indeed referring to the same condition?  If so, why the different
>interpretations, and if not, what is a transverse ligamental groove?  Thank
>you in advance.

        I think  Makovicky and Sues' character refers to the emargination
of the humeral head seen on the posterior aspect of the humerus, which is
is present in a variety of maniraptorans, and is especially well-developed
in modern birds. "Ventral" would therefore refer to the humerus oriented
with its long axis pointing downwards. I suspect that Chiappe and others
are referring to a groove found on the anterior face of the humerus, I'm
not sure what its distribution is.
        I don't know that anybody really knows for sure what Archaeopteryx'
sternum looked like- there are disagreements about Wellnhofer's
reconstruction. It seems unlikely that Archaeopteryx would have such a
small sternum compared to dromaeosaurs and oviraptorids.


>Thanks for the ref. Jaime. On reading it a third time, (things take a bit
>longer to sink in than they used to!),I see that Dr. Holtz interprets it
>more as a force transducing structure.....(a shock absorber?). But, before
>risking further misinterpretation, perhaps I`d better wait to get the
>details straight from the horse`s mouth....(hopefully when he trots back to
>the office Mon morning ).
>PS, do any modern birds have this condition? Or are the metatarsals totally
>fused?

        A shock absorber would deform elastically, straining over a
distance to absorb energy (think of the travel on the shocks in a car or
bicycle). So the limited amount of movement possible in the metatarsals
would probably make it a relatively poor shock absorber, as I understand
things. I think the idea is that the third metatarsal transfers force to II
and IV.

-N