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Re: tiny-armed theropods



Jason Brougham <jaseb@amnh.org> wrote:

> I am interested that theropod arm reduction and loss is sometimes also 
> concomitant with
> herbivory. This happened in Limusaurus, the basal ceratosaurid, and 
> apparently reached its
> zenith in the complete loss of the pectoral appendage in the moa.


I suspect the situation vis-a-vis forelimb truncation in theropods is
very complicated.  A short forelimb is actually fairly standard for
ceratosaurs, as noted by Xu et al. (2009) in the description of
_Limusaurus_.  Having said that, _Limusaurus_'s forelimbs (including
the forearms and manus) are proportionally shorter than those of
_Ceratosaurus_, a carnivore.


I'm a little skeptical about the comparison with moa, because the
latter are secondarily flightless crown birds, and so evolved from
forms in which a grasping/prehensile manus had been lost long ago.
Most terrestrial flightless birds show highly reduced forelimbs, with
moa representing an extreme condition.  _Xenicibis_ is one of the few
flightless birds that retained and co-opted long forelimbs for another
purpose.


> In Limusaurus arm reduction occurred along with reduction of the head size, 
> which is to say
> that the arms and head got proportionately smaller concurrently.


_Limusaurus_ reminds me of shuvosaurids like _Effigia_ more so than
ornithomimosaurs.  The latter have very long forelimbs that appear to
have been adapted for hooking foliage and bringing it to the mouth.
Therizinosaurs also had long forelimbs, and were most likely
herbivores - but the large manus does not seem well adapted for
hooking or gripping branches.


>From another post...

> Has anyone mentioned the cases of arm reduction in dromaeosaurids?


Yes.  I did.  ;-)


> Tianyulong, Mahakala and Austroraptor all independently arrived at short 
> pectoral limbs. This
> means that pectoral limb reduction was selected for in the tiny (65cm long) 
> Mahakala, and also
> in the 5 or 6 meter long Austroraptor, implying totally different ecological 
> scenarios.


Yes, forelimb reduction has a highly erratic distribution within
Maniraptora.  Not just the dromaeosaurid taxa you mention, but the
forelimbs of _Caudipteryx_ appear truncated, and derived troodontids
have fairly short forelimbs... just to name a few.


> Austroraptor apparently had some dental specializations as well.


The teeth are rather spinosaurid-like - although similar dentition
(small, conical, unserrated) is also found in _Buitreraptor_, a
long-snouted unenlagiid like _Austroraptor_.  However, _Buitreraptor_
had a very long and slender humerus, in contrast to the very short and
stocky humerus of _Austroraptor_.





Cheers

Tim