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

Thomas Yazbeck <yazbeckt@msu.edu> wrote:

> See Maniraptora - avian-style semi-lunate carpus allows for greater range of
> motion in the wrist. Presumably this evolved hand-in-hand (pun intended)
> with pennaceous wing feathers - having them and being unable to fold up the
> forelimb in the avian fashion would expose the feathers to damage. This is
> getting into speculative territory, but if pennate wing feathers evolved
> primarily for display and/or for purposes of brooding, then there is an
> imperative to protect them & hence evolve the semilunate carpus.

Your idea is very reasonable, and not so speculative at all.  Sullivan
et al. (2010; doi:10.1098/rspb.2009.2281) arrived at a similar
hypothesis. Yes, the maniraptoran-style semilunate carpal allows for
greater range of motion in the wrist, allowing the long forelimb
feathers to be protected by folding up the 'wings'.  As you know,
birds inherited this semilunate carpal, and absorbed it into their
fused carpometacarpus as the trochlear facet.  So long-feathered
non-avian theropods would have utilized of this feature, even if they
didn't fly.

But it's not clear if the evolution of long pennaceous forelimb
feathers led to the evolution of the semilunate carpal, or vice versa.
As Sullivan et al. put it: "However, it is an open question whether
the development of longer feathers originally necessitated an abducted
wrist to protect the pennibrachium, or the evolution of a deflected
wrist in response to some other functional need created an opportunity
for the subsequent development of longer pennibrachial feathers on a
longer arm."

(A 'pennibrachium' is a forelimb bearing long feathers that form a
planar, wing-like surface but are not necessarily used in aerial
locomotion.  These days, most people simply call the wing-like
pennaceous structure a 'wing'.)

The other point is that the maniraptoran-style 'semilunate' carpal did
not come out of the blue.  It evolved in a gradual/incremental
fashion, with individual carpal elements incorporated and modified in
stages (in some cases independently in different lineages).  This, and
the putative homologies of the various constituent elements, are
described in detail by Xu et al. (2014; DOI: 10.1038/srep06042). A
'semilunate' carpal of some description - varying in its relative
size, composition, and shape (especially 'convex-ness'), and
possessing some kind of trochlear groove - is known in various other
theropods, including _Allosaurus_ and non-maniraptoran coelurosaurs
(including basal tyrannosauroids).  It's possible that the primordial
'semilunate' carpal may have initially evolved to protect forelimb
plumage, which implies that allosaurids and basal coelurosaurs had
long feathers to protect.  Then again, the _Allosaurus_-style
'semilunate' carpal is not all that 'semilunate' (proximally convex),
despite having a clear 'trochlear' groove.  So this structure wouldn't
have allowed *that* much abductive/adductive motion, meaning the wrist
couldn't 'swivel' too much.  I'm tempted to suggest that the
_Allosaurus_-style 'semilunate' structure originally served to
*confine* rather than *expand* mobility at the wrist.  Why, I have no
idea.  In any case, in the course of tetanurine evolution this
structure became more crescent-shaped (semilunate) and then bigger
(capping two metacarpals), allowing increased mobility expressed as
extensive adduction/abduction along the radio-ulnar plane.

Long story short: The enlarged and highly mobile semilunate carpal of
maniraptorans might have served to protect the long forelimb plumage,
via wing-folding.  But the function of homologous (at least partially)
'semilunate' carpal structures in non-maniraptorans is unclear.  A
predatory function has been invoked, but without much concrete
evidence.  Because carnivorous theropods are assumed to have used
their forelimbs in predation (to capture, handle, or subdue prey), the
semilunate carpal is assumed to be involved in predation.  However,
because of the limited anterior reach of theropod forelimbs
(especially for short-armed theropods), I think this assumption is
questionable.  But I'm stuck for alternative hypotheses - unless the
changes within the hand/wrist articulation were originally adapted
toward keeping the forelimbs (including the claws, and/or any
feather-like integument) out of the way during cursorial/terrestrial