Re: Therizinosaur track from Denali National Park, Alaska

["Harris, Jerald" <jharris@dixie.edu>]

>Yes, I thought the mention of oviraptorids was a little incongruous.
A functionally tetradactyl foot (in a locomotory sense) is only found
in therizinosaurs among non-avian theropods.  In derived
therizinosaurs the pes was completely overhauled: metatarsals I-IV
became very short and very thick, and metatarsal I and its digit were
hypertrophied for weight support.  But metatarsal I still inserted
fairly high on metatarsal II - but the shortness of the metatarsus
meant that the hallux now contacted the substratum (ground) for weight
support.

      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). _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.

      Anyway, to answer another posed question, yes, the basic ichnological 
philosophy is to _not_ try and associate particular tracks to specific taxa 
(broader groups is OK), and instead study the tracks for the information they 
provide in and of themselves. Groups of track morphologies that are similar are 
valid even if the track makers were not particularly close relatives--they are 
morphotaxa, rather than phylogenetically related taxa--many different kinds of 
taxa can make very similar traces (moreso among invert traces than among 
vertebrates, but it happens in both groups).               
                
-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Dr. Jerry D. Harris
Director of Paleontology
Dixie State College
Science Building
225 South 700 East
St. George, UT  84770   USA
Phone: (435) 652-7758
Fax: (435) 656-4022
E-mail: jharris@dixie.edu
and     dinogami@gmail.com
http://cactus.dixie.edu/jharris/

"The fact that an opinion has been widely held is no
evidence whatever that it is not utterly absurd."

                                    -- Bertrand Russell