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Re: Anzu wyliei, near-complete caenagnathid oviraptorosaurian



Jay <jayp.nair@yahoo.com> wrote:

> Congrats to Matt Lamanna and the other authors on this awesomeness; this 
> paper presents the first decent look into the complete morphology of 
> caenagnathids:



I'll second that.  It even has an eminently cool name.  It's amazing
that our knowledge of the anatomy of derived caenagnathids has
advanced so far, in a large part thanks to _Anzu_.  We've come a long
way since the first _Caenagnathus_ jawbone was regarded as coming from
a bird (Sternberg, 1940); and all we had to go on for the postcranial
skeleton of caenagnathids was tantalizing hand- and foot-bones (the
latter often called "elmisaurids").

A couple of studies (including this one by Lamanna et al., as well as
Longrich et al. [2013]) have mentioned the odd pedal morphology of
caenagnathids - especially their long toes and relatively elongated
penultimate phalanges.  One suggestion is that the proportions of the
non-ungual phalanges, especially the relatively long penultimate
(ungual-bearing) phalanges, made the pes more adapted for grasping
than running.  So it follows that the pes might have been used for
prey capture or even climbing.

However... this idea shouldn't be taken too far.  Hopson (2001) found
that, among birds, the phalangeal proportions of the third toe were a
fairly reliable indicator of habits.  Using the pes of the
caenagnathid _Chirostenotes_ as an example (based on the specimen
originally named _Macrophalangia_), it is true the penultimate phalanx
of the third toe is quite long by non-avian theropod standards (about
31.5% the length of the total non-ungual digit length).  Also, the
first phalanx is relatively short (~40.5%) - quite unlike what is seen
in highly cursorial birds.  But the phalangeal proportions are a long
way from the feet of specialized grasping birds, where the penultimate
phalanx is typically over 40% of the non-ungual digit length (in the
osprey and swifts it's over 50%).

The phalangeal proportions of _Chirostenotes_'s third toe are not too
different from those of _Coelophysis_.  So rather than representing a
shift to a 'better' grasping foot, caenagnathids might represent a
reversal toward a more generalized, less cursorial type of foot. The
long toes and phalangeal proportions of _Chirostenotes_ are similar to
those of the moorhen ("swamp chicken"), which might suggest a
preference for wetlands.  Lamanna et al. mention that the depositional
environments of caenagnathid fossils suggest that these theropods were
"adapted to wetter, more humid surroundings".  This fits with an old
hypothesis that the long hindlimbs (especially the metatarsus) and
large feet of caenagnathids were adaptations for wading (Currie and
Russell, 1988).  However, I wouldn't read too much into the pedal
proportions (and long hindlimbs) in inferring wading habits for
caenagnathids, including _Chirostenotes_ and _Anzu_.  It may just be,
as Lamanna et al. suggest, that caenagnathids were ecological
generalists.  So they may occasionally have ventured into shallow
water to feed, without being specialized waders.  Either way, the
evidence points to a very different ecology to oviraptorids, as
Lamanna &c make clear.

Caenagnathids also had an arctometatarsalian foot, which may seem
strange for non-cursorial animals.  Nevertheless, although this
feature is found in highly cursorial theropods, it may not be a
cursorial adaptation per se, and be more concerned with energy
transduction.  (A near-arctometatarsalian condition is seen in
_MIcroraptor_, which was also distinctly un-cursorial.)






Cheers
Tim