Re: On the Issue of Sprawling Dromaeosaurs

[Dino Guy Ralph <ralph.miller@alumni.usc.edu>]

In a post to the Dinosaur Mailing List, Kris (MariusRomanus@aol.com)
recommends we think outside the box with respect to fossil taxa which don't
match extant animal anatomy.  The point being that if _Microraptor gui_ (or
shall we call it _Microraptor pauli_?) possessed large asymmetrical "flight
feathers" on its hind limbs -- as it clearly did -- that one ought to
carefully consider what additional anatomical and locomotory novelties might
have permitted the animal to deploy these extravagant feathers in some way.
The hind limb feathers are apparently adapted for some aerodynamic function,
and their length and extensive distribution could be maladaptive in a purely
nonvolant function (such as display).

In addition, one should consider that this theropod possessed a skeleton
with features suggestive of an arboreal lifestyle (as described in the
preliminary description in _Nature_, January 23, 2003), and, indeed, it may
have been more adept in the trees than on the ground.  Hence, there would
have been adaptive advantages to atypical flexibility in the pelvis and legs
of this derived theropod.

Unfortunately, the current understanding of theropod pelves and hind limbs,
concerning such factors as the perforate acetabulum, in-turned femoral head,
the outer condyle of the knee, and the inflexible ankle, would appear to
prevent such flexibility.  After all, birds are theropods, and they can't
sprawl with their hind limbs.

Or can they?

Please consider the following photograph of a marsh wren:
http://friendsofsherwoodisland.org/Photos/Hand/Land/AJH-marsh%20wren%20supersplit.jpg.

OK, so now we can say that whereas many bird species could not perform such
a maneuver, it is none-the-less possible for a marsh wren to do so, and one
can see in this picture why this would be a useful capability for this
animal in this environment.  Are the marsh wren's legs and hips uniquely
specialized for this degree of flexibility, and if so, what are the
osteological markers for this trait?

Could not _Microraptor gui_ have done something similar?

I welcome your comments.

--------
"Dino Guy" Ralph W. Miller III
Docent at the California Academy of Sciences
proud member of the Society of Vertebrate Paleontology