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Re: FW: Arboreal Theropods: The prize at the bottom of the cracker jack box

Jason Brougham <jaseb@amnh.org> wrote:

> If we consider that Epidendrosaurus is very close to the last common
> ancestor of Paraves, and thought to be a hatchling, then in this context
> its odd hands may be especially interesting. The elongated finger is the
> third one: not the wing finger or the alula, both of which support large
> feathers. Thus the extreme elongation of the free finger may have provided
> a stabilizer for a hatchling scrambling about on a branch, even if just to
> fledge from the nest.

Yes, arboreality is one possible hypothesis to account for the 'odd'
morphology of _Epidendrosaurus_ (=_Scansoriopteryx_).  But this
arboreal hypothesis is difficult to square with other aspects of the
manus in _Epidendrosaurus_.  The manual unguals are only very weakly
curved - not exactly adapted for hooking branches.

> As previously noted it also has a fully descended 1st toe.

This puts the first toe of _Epidendrosaurus_ in direct contact with
the ground, alongside the other three toes.  This is also found in
therizinosaurids (although here it is achieved by an entirely
different metatarsal morphology).  So I would hesitate to endorse the
unusual pes of _Epidendrosaurus_ as a scansorial/arboreal character.
It could be; but other hypotheses are also viable.

It may just be that the pedal morphology of _Epidendrosaurus_ reflects
a decline of cursorial ability in a terrestrial theropod.
_Patagopteryx_ also had a large, cranially-directed hallux that would
have contacted the ground.  It was undoubtedly terrestrial.

> Tim has noted that birds sometimes also climb using their beaks as an
> extra point of grip. If a basal paravian were to do so we might expect
> some unusual features in its anteriormost teeth. And lo an behold,
> Epidexipteryx was noted for just that! It's anterior teeth are highly
> procumbent, which is known in only one other dinosaur, and much longer
> than the posterior ones (as in Epidendrosaurus).

The unusual, procumbent dental morphology of _Epidexipteryx_ likely
has a dietary explanation, as proposed for _Masiakasaurus_  One
hypothesis is "nit-picking", discussed by Jaime

(BTW, it wasn't me who noted that some birds use their beaks in climbing.)

> It also has an extremely shortened tail skeleton.

A shortened tail skeleton is a recurrent feature across the
Maniraptora.  I doubt if it's related to arboreality.

> Thus, to whatever extent that participants in this discussion are thinking
> that paravians show "no" or "none" or "not one" features that can be
> correlated with climbing and limited exploitation of branches, that
> opinion is mistaken.

IMHO none of these characters are arboreal adaptations.  If predatory
or terrestrial characters were mobilized for climbing or grasping
branches, that's a different matter.  My take is that
confuciusornithids and sapeornithids are the most basal theropods to
show evidence of incipient/nascent arboreal abilities.