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Re: Deinonychus claw use and origin of flapping
K Kripchak <email@example.com> wrote:
>> So, we end up with arguments that boil down to saying;
>> "Birds/theropods were unlikely to have become arboreal without
>> cursorial basal birds/theropods possessing the capabilities, traits,
>> and behaviours for becoming arboreal. However, the capabilities,
>> traits, and behaviours for becoming arboreal wouldn't have been
>> selected for until cursorial basal birds/theropods became arboreal."
>> A Catch-22.
This is a good point. As you say, exaptation can be invoked for this
terrestrial-to-arboreal transition. For example, arboreal adaptations
might have begun as characters that originally evolved for
non-arboreal functions (such as predation), but could be co-opted for
climbing or perching.
The trouble with this hypothesis is that the overall morphology of
theropods (their bauplan, if you will) was so poorly adapted for
arboreality. Even if (and it is still an *if*) certain characters in
the hands and feet improved trunk-climbing or branch-holding ability,
these characters must be set against the rest of the skeleton, which
remained resolutely terrestrial. Theropod backbones and joints lack
even the most basic requirements for arboreality.
This is why comparisons between theropods and mammals are so inapt -
including goats. The much-vaunted tree-climbing abilities of goats
are a partly a product of their heritage (placentals began as small
arboreal mammals) and partly a consequence of their current ecology
(negotiating steep and uneven terrain). Theropods lacked the flexible
vertebral columns and range of motion at the joints which, by and
large, are present in mammals.
So the question becomes: Were the changes to the distal limb elements
(= the putative incipient scansorial/arboreal characters) adequate to
allow theropods to climb trunks or grasp branches, given the
constraints imposed by the rest of the body? This is a tricky
question, because it is difficult to frame and test hypotheses that
specifically address this issue.