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Re: Deinonychus claw use and origin of flapping
> Fowler, D.W., Freedman, E.A., Scannella, J.B.& Kambic, R.E. (2011)
> The Predatory Ecology of Deinonychus and the Origin of Flapping in Birds.
> PLoS ONE 6(12): e28964.
I think people would be disappointed if there wasn't any discussion of
this paper, in light of how revolutionary this Raptor Prey Restraint
(RPR or "ripper") hypothesis is.
So to get the ball rolling (so it can then slowly grind to a halt)...
(1) I'm enthralled by the idea that basal paravians had a grasping
pes. Unlike birds, in which an enlarged and fully reversed first
digit opposes the third digit to achieve an anisodactyl grasping pes
(such as for perching), _Deinonychus_ achieved a grasping pes by the
medially directed first digit effectively opposing the fourth digit
during flexion. The foot's grasping abilities were helped by
elongation of the pedal digits, and a metatarsus that was held close
to the substratum. Prey captured by the foot would be held against
the "plantar" surface, gripped by all four toes. This certainly
explains the descent of the hallux.
Troodontids had a more mobile first digit, and therefore might have
been even better at grasping small prey. The metatarsal I of the
troodontid specimen shown in Fig. 9 actually reminds me of the
inverted P-shaped metatarsal I of certain non-ornithurine birds -
although it is much shorter and higher on the metatarsus in
(2) This grasping/predatory function inferred for the paravian foot is
regarded by Fowler &c as a possible precursor to the perching foot of
birds, via exaptation. Thus, a "hallux reversal required for perching
could be exapted from a predatory function in a wholly terrestrial
predator, without invoking a hypothetical pre-flight arboreal or
scansorial stage for non-avian theropods." Although the authors are
skeptical of arboreality in basal paravians, I think it is possible
that critters like _Microraptor_ could have used their feet to help
climb trees and/or grasp branches. Fowler &c actually allow for this
possibility, so I'm not quibbling with their work here. But I think
one can reconstruct certain paravians such as microraptorines,
jeholornithids and long-armed troodontids (e.g., _Jinfengopteryx_)
venturing up trees without necessarily being arboreal.
IMHO, in many theropods the forelimbs were not that much use in prey
capture - either too short, or the range of motion was too limited -
and the manus of paravians was ill-equipped for grasping small
objects. So when it comes to grasping small prey, it made sense for
the feet to take up the slack.
However, if the tree presented in the _Xiaotingia_ paper is correct,
and archaeopterygids, dromaeosaurids and troodontids form a clade that
is the sister taxon to the bird clade... then predation might not have
been the reason behind the evolution of a grasping pes. This tree (Xu
et al., 2011) suggest that herbivory (and a robust skull) are
primitive for Paraves (or the bird clade, at least), and that
deinonychosaurs are exceptional in their carnivorous/predatory habits
(and a gracile skull).
(3) The "stability flapping" behavior suggested as part of RPR is
consistent with a "flapping-first" hypothesis of the origin of avian
flight. But isn't this at odds with Senter's (2006) work which
inferred that flapping came late to birds, because the scapular
orientation in non-ornithoracean paravians precluded a full stroke?