Re: On the Issue of Sprawling Dromaeosaurs

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

> Many birds that climb have the ability to laterally extend their lower
legs to some degree. And just so we have at least one other example
supporting this, here are multiple photographs of bitterns that might be of
some interest...

Those pics show that bitterns, too, can rotate their tibiotarsi about their
long axes, and seemingly their femora, too. Putting *Microraptor* in such a
position would deny its femoral feathers any function. Putting *M.
zhaoianus*, at least, in such a position would be impossible without
disarticulating the hips (and most probably the knees, but I haven't
looked).

> Two is that this particular posture was most likely induced by the bird's
environment,

Sure -- to me it looks like a special adaptation to living in reeds, where
horizontal branches are not available.

> much in the same manner that the arboreal hypothesis suggests selective
pressure that would have been applied to the basal dromaeosaurs in question
(something that Ralph Miller also noted).

I still don't know why everyone seems to assume that *Microraptor* and the
like were arboreal. The 1st toe was a bit longer than usual, but still
short, and its position is unknown; the phalangeal proportions are still on
the terrestrial side; the body size means nothing; and the foot feathers
could or could not have hindered running, we don't know, because we don't
know if the feathers were foldable (among other things).

> I am also betting that there is little in the way of alterations to the
anatomy that facilitates this ability when compared to most birds of similar
sizes and overall build. This can be tested rather easily with a little
comparative analysis, though I admit the results have little impact on the
issue.

I'd expect rather odd hip and knee joints. Anyone have an illustration of a
skeleton of such a bird...?

> Also, I did not state conclusively that these dromaeosaurs possessed the
ability to laterally sprawled or not. What I did do was ask for exactly what
prevented them from doing so:

For *M. gui* respectively *Cryptovolans* nobody knows, except those
(perhaps) who've seen the fossils. For *M. zhaoianus* we know: the hips just
don't allow it.

> > If anyone really sees the need for a massive change in the shape of the
femoral head that would be required in order to allow this type of motion,
please speak up and explain.

The femoral head of *M. zhaoianus* is a cylinder sticking in a cylindrical
hole. It can only, to any significant extent, rotate about the long axis of
the cylinder.

http://research.amnh.org/users/sunny/hwang.et.al.2002.pdf
I think it's fig. 8.

> Is a radical manipulation of the soft tissues also required in order to
facilitate this movement? Again, if so, please explain.

Depends on what you call "radical". Muscles that twist the leg would
probably have to evolve from others which changed their insertion points...
radically. :-)

> A claim was made that a soft tissue's purpose is only to restrict joint
movement. This is indeed the case in many ways, but the suggestion taken as
it was stated is quite misleading. If one has a joint that is deep, like
many hip joints are in birds for example, the thicker the cartilage is, the
more shallow the acetabulum becomes. This can, in many cases, free up the
range of motion of the femur a great deal.

Really?

> It is almost a less cruel version of "Arbeit macht frei", in which instead
of "Work will set you free",

(Actually just "work makes free"; a moderately old proverb.)

> Given the replies already received, saying that there is zero evidence in
the fossils that prevents a sprawling posture from being possible after
taking variations in the form of soft tissues into account, has yet to be
sufficiently countered.

I'd say the burden of evidence is on you. Is unfair, because you haven't
seen the fossils either, but in principle, when you suggest that a dinosaur
was able to sprawl, it's you who's making a departure from the null
hypothesis.

This also holds for the photo of that drone that someone mentioned to show
that sprawling below 180° can be aerodynamically effective, too. The angle
between those wings is around 90°; this is the upper limit of what I can do
(for human measures, I'm quite bad at sprawling). *Saturnalia*, a very early
dinosaur, only managed 70° -- more than any non-neornithine dinosaur I know
of:

Max Cardoso Langer: The pelvic and hind limb anatomy of the
stem-sauropodomorph *Saturnalia tupiniquim* (Late Triassic, Brazil),
PaleoBios 23(2), all 40 pages (22 September 2003)

This paper is chock full of meticulous descriptions of muscle attachment
sites and of muscle reconstructions.

Until someone can show that the fossils tell otherwise, I'll maintain that
*M. gui* was likely just as incapable of sprawling as *M. zhaoianus*.