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Re: Oviraptorosaur tail forms and functions



> W. Scott Persons, IV, Philip J. Currie, and Mark A. Norell
> Oviraptorosaur tail forms and functions.
> Acta Palaeontologica Polonica (in press)
> doi: http://dx.doi.org/10.4202/app.2012.0093
> http://app.pan.pl/article/item/app20120093.html


It's certainly interesting how muscular and flexible the tails of
these oviraptorosaurs were.  It's tempting to reconstruct
oviraptorosaurs as having an elaborate terminal tail "fan" (e.g.
www.wired.com/wiredscience/2013/01/dinosaur-courtship-feathers), with
the impressive caudal musculature used to wave the fan around during
courtship or threat postures.  However, I wonder if the tail was a
little 'over-engineered' for display purposes alone.  Perhaps the tail
was used as a weapon instead, and swung side-to-side to clobber any
predators that came too close.  I'm not convinced that the presence of
a pygostyle necessarily indicates the attachment of large pennaceous
feathers at the end of the tail.  Although large terminal tail
feathers are present in caudipterygids, these were small animals with
extremely short (and presumably weak) tails by oviraptorosaurian
standards - as Persons et al. note.


The array of feathers inferred to have attached to the pygostyle of
oviraptorosaurs is referred to as a "fan".  I'm not sure this is the
best term.  Firstly, it recalls the tail fan of modern birds, which is
a highly specialized aerodynamic structure controlled by rectricial
bulbs, used to modulate lift forces during flight.  Secondly, it is
not known if the tail feathers attached to the oviraptorosaurian
pygostyle actually formed a radial array; perhaps it was just a
terminal "frond", as inferred for _Caudipteryx_ (Gatesy, 2001).  There
are many basal avialans that have a pygostyle, but no pennaceous
rectrices, and certainly not a true tail fan.


That's assuming that the larger pygostyle-bearing oviraptorosaurs
(like _Nomingia_, _Citipati_ and _Conchoraptor_) had large terminal
tail feathers at all.  Like I said, it wouldn't surprise me if the
hefty muscularized tail was used as a weapon (intraspecific combat?;
defense against approaching predators?), and so would likely lack a
tail-feather fan.


On another topic, from the paper:

"A large M. caudofemoralis is a characteristic of basal
theropods (Gatesy 1990a; Persons and Currie 2011a). In avialans and
deinonychosaurs, the M.
caudofemoralis is greatly reduced in size or altogether absent, and
the stroke mechanics of the
hind limbs emphasize knee-flexion, rather than femoral retraction
(Gatesy 1990a, 1990b; Persons
and Currie in press)."


I may be reading too much into this, but is it saying here that even
deinonychosaurs and basal birds had knee-based stride generation, not
just more derived birds (which was my impression)?  It's hard for me
to imagine _Velociraptor_ walking around like a ground bird, with the
femora tucked under the body and stride generation occurring at the
knee....





Cheers

Tim