

Re: Osteology of Balaur bondoc

Jaime Headden <qi_leong@hotmail.com> wrote:

> European, short-footed, double-sickle claws, maybe two-fingered, stocky, 
> robust, unique. Phylogeny speculates a near-*Velociraptor mongoliensis*
> placement, but is preliminary. Specimens are not particularly juvenile (there 
> are two, one larger than the holotype and possibly not a specimen of
> *Balaur bondoc*).
> http://digitallibrary.amnh.org/dspace/handle/2246/6419

Yes, this description really brings out just how interesting this
dromaeosaur is.  Because _Balaur_ is so autapomorphy-heavy, it's
difficult to untangle (1) those characters that might be related to
its island habitat from (2) those characters that represent a
culmination of trends within the Dromaeosauridae.

For example, the extremely short, wide and coossified metatarsus of
_Balaur_ might be an insular trait, conferring increased stability
during locomotion across "rocky, uneven terrain, which is often
widespread in insular environments."  But this trait might also be
related to conferring stability when the predator is atop large,
struggling prey - exactly as envisioned in Fig 49, where a bunch of
_Balaur_ are bringing down a small-ish hadrosaur.  For the latter, the
morphology of the foot of _Balaur_ would be in keeping with its
ancestral eudromaeosaur role in predation directed at large prey.  (It
might also explain the function of wings: "stability flapping" during
prey immobilization, as posited by Fowler et al., 2011.  Both the
wings and feet would therefore be involved in keeping the predator
steady, against a thrashing and bucking hadrosaur underfoot.)

Although the phylogeny is "preliminary" (then again, no phylogeny is
ever final) I see no reason to regard _Balaur_ as a secondarily
flightless bird, as some people have avered.  A fused carpometacarpus
and enlarged first metatarsal of _Balaur_ are also seen in birds, but
the morphology is quite different between the two.  A fused
carpometacarpus is seen in several non-avian maniraptorans, especially
oviraptorosaurs (e.g., _Avimimus_, _Heyuannia_, _Ingenia_ [referred
specimen]), and might reflect the loss of mobility/prehensility of the
manus, rather than representing a novel specialized function (i.e., a
loss of some ancestral function of the manus, rather than gaining a
new function).  Therizinosaurs have an enlarged first metatarsal,
accompanied by a short metatarsus.  So _Balaur_, therizinosaurs, and
derived avialans each enlarged the hallux for a new (and very
different) function.