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Re: Therizinosaur track from Denali National Park, Alaska



Ben Creisler <bcreisler@gmail.com> wrote:


> A bit more about the therizinosaur tracks found in Alaska (in English):
>
> http://www.thenewstribune.com/2012/08/21/2263087/6-ton-feathered-dinosaurs-once.html


>From the original paper by Fiorillo and Adams, 2012 (open access at:
http://www.bioone.org/doi/abs/10.2110/palo.2011.p11-083r):


"Two groups of theropods that are known to have four forward-facing
digits are therizinosaurs and oviraptorids."


There is at least one other group of Cretaceous theropods that have
four forward-facing toes.  The flightless bird _Patagopteryx_ has a
"pamprodactyl" foot, with four long, robust digits directed cranially
(e.g., Chiappe, 2002).  _Patagopteryx_ was only about the size of a
chicken, so it's hard to see how the hallux needed to be recruited for
weight support.  The big, pot-bellied therizinosaurs undoubtedly used
their enlarged, forward-pointing hallux for weight support.  Large,
flightless, and possibly graviportal birds like _Gargantuavis_ might
have done the same.


Among modern birds, pamprodactyly is seen in certain arboreal birds
that are specialized for clinging to vertical surfaces like tree
trunks.  The enantiornithean _Rapaxavis_ might also have had a
pamprodactyl pes  - the metatarsal I of this bird is apparently
straight (O'Connor et al., 2011), not J-shaped as originally described
(Morschhauser et al. (2009). This means that the large hallux was
probably directed forward, and argues against anisodactyl perching by
_Rapaxavis_.  Otherwise, the proportions of the foot strongly suggest
an arboreal lifestyle.  Although O'Connor &c don't discuss this, the
pedal morphology of _Rapaxavis_ makes me think of pamprodactyly, with
all four toes pointed forward to help support the bird against the
pull of gravity.






Cheers

Tim