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Re: Dromaeosaurid tails like rhamphorhynchid tails from flight use

I'd be careful assigning such significance to Mahakala. It is at least
75 Ma removed from the common ancestor of all dromaeosaurs. So while
Mahakala may be among the most basal dromaeosaurs, that doesn't
necessarily mean it's the most primitive. Monotremes are the most
basal living mammals, but that doesn't mean the ancestral mammal was
platypus-like. It's always possible that such odd (for a dromaeosaur)
features as very small forelimbs and un-stiffened tails in Mahakala
were due to some novel ecological niche (burrowing?).


On Thu, Jan 17, 2013 at 1:12 AM, Tim Williams <tijawi@gmail.com> wrote:
> Ben Creisler <bcreisler@gmail.com> wrote:
>> W. Scott Persons IV & Philip J. Currie (2012)
>> Dragon Tails: Convergent Caudal Morphology in Winged Archosaurs
>> Acta Geologica Sinica - English Edition 86 (6): 1402–1412
>> DOI: 10.1111/1755-6724.12009
>> http://onlinelibrary.wiley.com/doi/10.1111/1755-6724.12009/abstract
> The paper seems to be saying that the chevron and zygapophyeal caudal
> rods of dromaeosaurids evolved in an aerial/arboreal context (as in
> rhamphorhynchoids).  In other words, the specialized stiffened tail of
> dromaeosaurids evolved "on the wing" to assist in gliding or flying.
> Therefore, the highly specialized tail of large, and presumably fully
> terrestrial dromaeosaurids (such as _Deinonychus_ and _Velociraptor_)
> would be a relict of an aerial/arboreal ancestry (represented by basal
> dromaeosaurids such as _Microraptor_)... according to this hypothesis.
> I'm not convinced.  Little _Mahakala_, which comes up as the most
> basal dromaeosaurid, lacks the elongate prezygapophyses and chevrons
> present in more derived dromaeosaurids; but it nevertheless has an
> underdeveloped fourth trochanter.  So unless _Mahakala_'s caudal
> features are a reversal (like the short forelimbs), then the presence
> of rod-like prezygapophyses and chevrons in more derived
> dromaeosaurids is not necessarily correlated with the proposed
> reduction in femoral retraction (i.e., a more bird-like locomotor
> style).
> Both "Groucho running" and caudal rods are regarded by this study as
> indicators of aerial behavior (or descended from ancestors that
> exhibited aerial behavior).  I'm still extremely skeptical that
> deinonychosaurs had a bent-legged locomotor style ("Groucho running").
>  I'm still extremely skeptical that deinonychosaurs had a bent-legged
> locomotor style ("Groucho running").  And as for the caudal rods:
> _Deinonychus_, _Velociraptor_ and _Achillobator_ had these specialized
> caudal rods, even though they were not likely to have been aerial
> gliders (or fliers!)  Presumably these highly specialized rods had a
> purpose in these large, derived dromaeosaurids that had nothing to do
> with aerial behavior.  If so, maybe the original purpose for these
> caudal rods also had nothing to do with aerial behavior.
> The authors aver: "A thin light-weight tail with reduced caudofemoral
> musculature that preferentially retains lateral flexibility while
> increasing dorsoventral rigidity is well suited for a flying or
> gliding animal."  I don't doubt that this is true.  But that doesn't
> mean such a tail couldn't be used for other behaviors.  The
> biomechanical pressures might have been similar between the tails of
> dromaeosaurids and rhamhorhynchoids; however, the behavioral pressures
> may have been quite different.  Ostrom's hypothesis that _Deinonychus_
> attacked prey much larger than itself has been criticized by some
> (e.g., Darren Naish), but I'm more supportive.  Perhaps the
> specialized tail of dromaeosaurids was associated with predation -
> more in line with what Ostrom proposed in 1969 for the tail of
> _Deinonychus_.
> Cheers
> Tim