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Re: [dinosaur] Dinosauria reclassification joins Ornithischia and Theropoda in Ornithoscelida



Mike Habib <biologyinmotion@gmail.com> wrote:

> That's a darn good point. Fair enough - then I suppose the pathway to 
> reduction merely *maintained* the supinated condition, which means far less. 
> No explanation
> needed, after all.


The pathway to forelimb reduction might have other side effects.  The
evolution of the highly derived semilunate carpal (including the
trochlear articulation) has been linked to all sorts of behaviors:
specialized prey-catching strategy ("predatory stroke"); specialized
trunk-climbing motion; or protection of the forelimb plumage when not
in use.  But it may just be that the semilunate carpal was simply a
means of getting the clawed hands out of the way.  Converting the
hand-wrist joint into a simple swivel allowed the hand to be
flexed/tucked against the forearm when they weren't being used.

After becoming obligate bipeds, dinosaurs were faced with the
question: So what do we do with the forelimbs?  The immediate priority
may simply be to make sure the forelimbs didn't interfere with other
functions.  Reduction/abbreviation of the forelimbs was one solution -
Persons & Currie (2017) suggest that drastic forelimb reduction was
originally associated with the shift to bipedality (reduction in
forward mass facilitated easier passive balance on the hindlimb...
though I'm not too sure about this particular aspect of their
scenario).

Later, when theropod forelimbs became specialized as
display/ornamental structures, or were used in terrestrial locomotory
behaviors (like braking or turning or leaping) - the forelimbs could
be compactly folded and kept out of the way when not in use.  This
folding mechanism may well have been used to help protect the plumage
in winged theropods (Sullivan et al. 2010;
doi:10.1098/rspb.2009.2281).  And this folding mechanism undoubtedly
went on to be an important component of the flight stroke of birds.
In the beginning though, the semilunate carpal might have just been a
spandrel.

I suspect that the highly disparate forelimb sizes seen across various
maniraptoran lineages might have very little to do with predation
strategies, and more to do with use of the forelimbs in display or
locomotion (not mutually exclusive), which varied between species.
Although I was once quite taken by the concept of a "predatory stroke"
(a rapid mantis-like strike deployed to capture large prey), I'm no
longer convinced this was a thing.  As for the idea that the
semilunate carpal facilitated some kind of trunk-climbing motion by
'arboreal' theropods, an idea credited to Chatterjee (IIRC)...
interesting idea, but a non-starter.  Arboreality typically requires
highly mobile limb joints across many directions, to negotiate
movement through a complex, three-dimensional substratum.  So the
semilunate carpal, which described an arcuate range of motion across a
single plane, would be very unhelpful to any arboreal quadruped.