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RE: Maniraptor arms?

John Scanlon wrote:

So I don't see any reason to
expect (as some have implied) that the synchronous downstroke was FORCED on
bird ancestors by their anatomy; it was one of a range of options for
maniraptors, and seems to work pretty well for flying.

It's possible that the situation in maniraptorans (or in certain maniraptorans, anyway) might have been more complicated. A synchronous ("mirror-image") extension of the forelimbs may have been forced upon these particular bipeds by the anatomy of the manus.

Non-avian maniraptorans tend to have very long and very stiff fingers. Based on the work of Gishlick and others, the manual digits of deinonychosaurs and oviraptorosaurs (and perhaps other non-avian maniraptorans too) have been inferred to have had decreased mobility. This was due to the inflexible ("locked") interphalangeal joints, as well as the close appression of the fingers at their base (proximal metacarpals). These changes to the anatomy of the manus meant that these maniraptorans would have had difficulty grasping and manipulating an object with only one hand.

The biomechanical studies of Gishlick indicate that deinonychosaurs could only grasp objects using both hands at once (= synchronously). This may have been true for other maniraptorans too.
For the big dromaeosaurids (like _Velociraptor_ and _Deinonychus_) there might have been a selective advantage to having a large and inflexible manus, and being compelled to seize objects with both hands at once - such as for grasping large prey. I'm sure Greg Paul would say that this long, stiff manus of dromaeosaurids is a relict of an ancestry among flying/winged theropods - and he might well be right. But if this synchronous motion of the forelimbs for prey capture is primitive for the Eumaniraptora/Paraves, then this is a behavior that was inherited by birds and modified for powered flight.



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