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RE: Thou Shall Not Climb!
Kris (email@example.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
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
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???
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
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
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)....
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.
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