Re: On the Issue of Sprawling Dromaeosaurs (long)

[David Marjanovic <david.marjanovic@gmx.at>]

----- Original Message -----
From: "Jaime A. Headden" <qilongia@yahoo.com>

Thank you -- your message allows me to write a much shorter post than I
intended. I'm short of time at this time of the year... :-)

> Kris proposes that the leg may have been held sideways
> with a medial femur that has been rotated
> about its long axis to allow the tibia to project laterally. To do this,
> however, the femur must also dislocate.

The question was asked whether marsh wrens are special. Yes, they are, and
they're well known for it (I just didn't know that this was their English
name, or I'd have casually mentioned them much earlier :o) ). I was told
they can sprawl; judging from the photo, they don't do this, instead they
rotate the femur and/or the tibiotarsus about the long axis. This is
impossible in any ordinary dinosaur, including *Microraptor zhaoianus*; the
soft tissue would have to dislocate the hip and/or knee joints and relocate
them later, without ripping the joint capsule apart -- this is not plausible
in any way.

The bones determine the maximum possible amount of movement in a joint. Soft
tissue can only restrict it, not increase it. Our skeletons allow _us all_
to hyperextend our fingers and wrists by 90° at the same time, and to do
both sorts of splits; our soft tissue doesn't, if we don't have enough
practice. (I can't make my left thumb touch the lower arm anymore; it never
quite worked with the right one, but there the range of motion has decreased
in the last 10 years, too... and I can't get myself into the lotus seat
anymore.) This is further demonstrated by another fact: I can hyperextend my
fingers by almost 90° (again, left more than right), but only by pressing
them against something; hyperextension by muscular action stops at some, I
don't know, 30°. I can't even hyperextend my toes further than that with my
muscles, even though otherwise 120° sounds realistic (though that hurts
soon).

Amazing lots of people can hyperextend their elbows. I've never heard of
knee hyperextension... but I'm sure all this has never occurred in a
reasonably healthy theropod. Our elbows and knees are built to allow being
fully extended, and whether this means 180° or 190° may be individual
variation. Theropod elbows and knees are quite far away from allowing 180°.

I mentioned *M. zhaoianus* above... sure *M. gui* is not *M. zhaoianus*. But
if their hindleg apparatuses were so different -- lots of muscles would have
to be rearranged, lots of processes on the bones rearranged or lost --, this
would imply that all these differences evolved in no time, after all, the
two species are still very similar. This looks improbable to me. Indeed, the
general shape of the ilium of *M. gui* (including *Cryptovolans* or not)
indicates that no novel muscle enlargements had happened.

>   If the leg feathers were confined to the lower leg, this would be easier
> to use having the leg tucked; Paul argues that because NGMC 91 has femoral
> feathers, this is unlikely, but NGMC 91 also lacks metatarsal feathers --
> the reverse is true in *Microraptor gui*, though preservation isn't all
> that helpful in this regard.

*Cryptovolans* -- and *M. zhaoianus* -- clearly have long femoral feathers.
So, without digging up the description, I'm sure they were there in *M.
gui*; they -- _or_ the lower leg feathers -- couldn't have an aerodynamic
function if the animal was able to adopt the position of the marsh wren in
the photo. Not that I could think of a position in which they would have
one.

>   There are other ways to hold the legs, but I argued a while back that
> only those that place the leading edge forward and form an aerodynamic
> foil would be effective and at all useful. This would also have to form a
> plane of the entire wing. My tucked-leg theory

Mine works from the ad hoc assumptions that the thigh and lower leg feathers
pointed caudally while those on the foot pointed laterally. This would turn
the animal into a very stable biplane -- a real biplane, with one pair of
wings above the other. :o) Perhaps there would even be enough space to let
the upper pair flap; if aerodynamically feasible (don't ask me!!!), this
would maybe provide enough lift to reconcile the rather narrow forewings
with active flapping flight. I hope I can do a sketch of this, and of at
least one other idea of mine, this weekend. Suffice to say that I'm not
pleased with any of my ideas on this creature.

>   1) provide a means for the metatarsal and tibial feathers to act in a
> single plane;

Probably this isn't the case. The fossils are two-dimensional; the feathers
could have pointed laterally, caudally, perhaps even medially (OK, I don't
believe this -- there wasn't enough room between the legs), and perhaps they
could have been mobile between lateral and caudal positions. More or less
microscopic examinations of the actual specimens might tell us the answers
to some, but probably not all of these problems.

>   3a) match the inability of the femur to evert laterally,

The hip joints of *M. gui* including *Cryptovolans* have never been
illustrated or described. But see above on why I think that this probably
doesn't matter.