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Re: Dromaeosaurid tails like rhamphorhynchid tails from flight use



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