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



From: Tim Williams <tijawi@gmail.com>

> the aforementioned arboreal scenario requires that small

>theropods went from being terrestrial bipeds --> arboreal quadrupeds
>--> arboreal bipeds.  This transitional phase of "arboreal quadruped"
>is often glossed over.  But it entails a major ecomorphological shift.


> So although I don't rule
>out some small theropods being arboreal quadrupeds, this ecology would
>be in spite of (not because of) their morphology.

Well it's a good thing you didn't rule it out because this is what
happened. Because small theropods ARE arboreal quadrupeds. Birds (small
theropods) in many groups, especially anatids and galliforms but even
including passerines, many of which are tree - nesters, have one or two
manual ungual claws when they are hatchlings, and the development of the
distalmost primaries is delayed. Later these same animals lose the claws
and the primaries develop fully, as the bird switches to the  more flight
- based strategy that adults use in trees. Traditional textbooks, which
are casually adaptationist as a rule, state that these claws are used for
scrambling about within, and for climbing out of, the nest. Ken Dial's
films make it look like they are also used in climbing by day old chukars.

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. As previously noted it also has a fully descended
1st toe. How fascinating it would be to see if the allometry of that
finger changed as Epidendrosaurus grew, its primaries developed, and the
animal adopted a more flapping - stabilized strategy, as in basal birds
today. It is almost a minor metamorphosis, as the unguals vanish into the
wing.

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). It also has an extremely
shortened tail skeleton. Epidexipteryx is recovered as very, very close to
the last common ancestor of the Paraves by Turner et al. 2012.

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.