RE: Thou Shall Not Climb!

[Tim Williams <twilliams_alpha@hotmail.com>]

Kris (mariusromanus@aol.com) wrote:

> Could someone be so kind as to explain to me why is it, that a theropod is 
> automatically banned from the trees unless its first toe is reversed???

It isn't.  Refer to the description of _Pedopenna_ as an example:

Xing Xu and Fucheng Zhang (2005).  A new maniraptoran dinosaur from China 
with long feathers on the metatarsus.  Naturwissenschaften 92: 173-177.

The authors argue that in _Pedopenna_ the feet were suitable for "an 
arboreal habit", but the hallux was not yet adapted for grasping or 
perching.

> On that note, what about the curved phalanges in the manus of 
> Archaeopteryx? Why can't these be interpreted as an indication of climbing? 
> After all, monkeys and squirrels have the same trait.

In general, monkeys and squirrels have many more adaptations for arboreality 
(or climbing in particular) compared to _Archaeopteryx_ - or any other 
non-ornithothoracean maniraptoran for that matter.

> Also, the orientation of the unguals on the manus, as well as an increased 
> range of motion for the shoulder joint in basal birds and their kin, look 
> to me like something that would come in handy when climbing... And what 
> about the pecs? Primates = big pec selection, so why not the same in some 
> smaller theropods??? Big pec selection in theropods = pre-evolved state for 
> powering flight.

These are all good ideas, but they need to be tested (e.g., 
biomechanically).  Some work has already been done in this area, especially 
in terms of the "predatory stroke" hypothesis.

> And furthermore, WHY would the 1st toe reverse if the theropod wasn't 
> already in the trees???

For prehension.

Note that a "perching pes" is not one character but actually several 
characters combined.  Many (but not all) of these characters pertain to the 
hallux (first pedal digit).  The hallux has to be more than just reversed in 
order to effectively oppose the other toes; it has to be the right length, 
and be positioned low enough on the foot.  Xu and Xhang (2005) make the 
point that these characters may have evolved in a gradualistic manner.

> I mean seriously... did some fluffy little monster decide one day to give 
> birth to other fluffy little monsters with reversed 1st toes so that they 
> could climb a tree even though the parent couldn't? Is that what we are 
> saying here?...

I don't think so (but that's just my interpretation).  A reversed hallux is 
one character that aids in the ability of the pes to grasp a branch.

> Selection only works if there is a need... How did the need for a reversed 
> toe start if theropods were not up in the trees before they needed the 
> reversed toe to be up in the trees???

One could propose that a reversed hallux evolved to grap prey on the ground, 
and was subsequently co-opted toward perching.  So far, there is no evidence 
for this trajectory in theropods.  The hallux remained obstinately high on 
the theropod foot well into Aves.

> Maybe... just maybe... a certain group of theropods were scrambling up into 
> trees... using their feathered arms with their big'ole fingers and sharp 
> claws... to cling to branches while being assisted by their feet. But 
> eventually, as the feathered arms became wings, and the fingers faded 
> away... the feet completely absorbed the job of clinging to the branches... 
> leading to selection favoring the reversed 1st toe. Why would such a 
> scenario not be possible?

Sounds plausible to me.  We need (a) good biomechanical data to support the 
hypothesis that non-avian theropods could climb; and (b) phylogenetic data 
to support the hypothesis that such a sequence of character acquisition is 
well-supported.  I think we have (b), although I would quibble with the 
precise order in which fingers "faded away" and the feet became specialized 
for perching.

> Given how these animals are preserved, with the added complication of how 
> that toe attaches, isn't it possible that due to the nature of the fossil 
> record, we might never be able to tell that it was on its way to being 
> reversed? (especially if the toe was mobile, which depends on how firmly 
> the toe was attached to metatarsal 2 by the ligaments)....

Check out:

Middleton, K.M. (2001) The morphological basis of hallucal orientation in 
extant birds.  Journal of Morphology.  250: 51-60.

Middleton's study has relevance to fossil birds, in that certain characters 
associated (correlated) with a reversed hallux are osteological, and can be 
preserved in fossils.  Unfortunately, these characters have rarely been 
examined for Mesozoic maniraptorans (both avian and non-avian).  Too often, 
the halluces of fossil birds (especially _Archaeopteryx_) were interpreted 
as reversed because they were either preserved in such a way (taphonomic 
artefact), or because a reversed hallux "fitted" in with an a priori 
assumption that the bird could perch.

Cheers

Tim

_________________________________________________________________
PC Magazine?s 2007 editors? choice for best Web mail?award-winning Windows 
Live Hotmail. 
http://imagine-windowslive.com/hotmail/?locale=en-us&ocid=TXT_TAGHM_migration_HM_mini_pcmag_0507