Re: Therizinosaur track from Denali National Park, Alaska

[Tim Williams <tijawi@gmail.com>]

Jerald Harris <jharris@dixie.edu> wrote:

>       But the fact remains that other than having a functional, 
> weight-bearing digit I,  _Saurexallopus_ tracks are wholly unlike Late 
> Cretaceous therizinosaur feet. Those
> feet have metatarsals that are widely splayed in an arcuate fashion (as might 
> be expected for such heavy animals) from which the toes extended in a nearly 
> parallel
> arrangement, but _Saurexallopus lovei_ tracks were clearly made by something 
> that had an extremely compact, closely bundled distal metatarsus from which 
> the toes
> extended in a radial fashion (in many, but not all, tracks--digit I was 
> apparently pretty mobile and could be extended across an impressive range 
> from cranial to medial).


Yes indeed; I was trying to make the point that there are other
possibilities for four-toed theropod tracks other than therizinosaurs.


> _Macropodosaurus_ tracks are much better matches for Late Cretaceous 
> therizinosaur feet 
> (http://theropoda.blogspot.com/2009/12/walking-with-therizinosaurs.html,
> though I'm not convinced of full, bear-style plantigrady). The original 
> _Saurexallopus_ description is freely available: 
> http://cactus.dixie.edu/jharris/Saurexallopus.pdf.


>From the paper:


"A bird large enough to make the _Exallopus_ tracks would rival the moas
(Order Dinornithiformes) in size, and no such birds are known until the
Paleocene diatrymas (Feduccia , 1980). Of the ratites, many of which have
attained large size, those that retain a hallux display the reduced and elevated
condition for that digit. In fact, we can find no reference to any
bird, living or
fossil, which possesses an unreversed, functional, weight-bearing
hallux; thus, it
is unlikely that any neornithine bird was responsible for the
_Exallopus_ tracks."


No arguments that the theropod which made the _Sauroexallopus
(_Exallopus_) tracks was probably *not* a neornithean.  Nevertheless,
I'd bet dollars to donuts that a very large non-neornithean avialan
was behind these tracks.  Little _Patagopteryx_ had a hallux that was
unreversed and presumably functional (given that this digit is quite
large and robust).  But the hallux was still too high on the foot to
be "weight-bearing"; and besides _Patagopteryx_ was too small
(~chicken-sized) to need an extra toe for this purpose.  However,
_Patagopteryx_ does represent a template from which a
forward-directed, weight-supporting hallux could have evolved in a
much larger flightless bird: either the hallux shifted to a lower
position on the foot, or (as in therizinosaurs) the other metatarsals
became much shorter.


There was a chicken-sized flightless basal ornithuromorph in the Late
Cretaceous of Argentina (_Patagopteryx_).  So why not a moa-sized
flightless basal ornithuromorph in the Late Cretaceous of Wyoming?









Cheers

Tim