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Well, a lot of non-maniraptoran theropods (e.g. spinosaurids, some carnosaurs, _Deinocheirus_ and other ornithomimosaurs) have large claws that may have been used for other tasks than seizing small vertebrates - maybe they were defensive or used for intraspecific interaction/agonistic behavior (and herbivorous foraging in the ornithomimids). Which leads me to another question, where exactly are ornithomimosaurs in the carpal evolution story? I know Ornithomimus has been shown to have pennate wing feathers.
Thomas Yazbeck From: dinosaur-l-request@usc.edu <dinosaur-l-request@usc.edu> on behalf of Tim Williams <tijawi@gmail.com>
Sent: Monday, March 27, 2017 11:06 PM To: dinosaur-l@usc.edu Subject: 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 locomotion. |