Mononykus, Titanis, and Ground Sloths

["Richard L. Dieterle" <Richard.L.Dieterle-1@tc.umn.edu>]

A news item in Discover Magazine for June 1997 (p. 14) relates to the giant 
ground sloth, Megatherium.  The 7 inch claws of Megatherium were conventionally 
thought to be used for stripping bark.  However, Richard Farina (there is a 
tilde over the ÔnÕ) of Uruguay analyses the olecranon process to which the 
triceps are attached to arrive at a different conclusion: ÒIf you have a long 
lever arm, you have a strong movement.  A digger, like an armadillo, has a very 
long olecranon process.  If the arm is short, you have fast movement.Ó  In 
predators the structure is always shorter.  "If you wish to captue prey, usually
your prey is not very cooperative, so you need to be fast."  According to 
Farina, for an animal MegatheriumÕs size to achieve the fastest stabbing speed, 
the olecranon process must be 3 to 5 inches long.  MegatheriumÕs is 4.75 inches.
He believes that the claws were used particularly for flipping over and stabbing
Glyptodonts, which resemble armadillos.  So we have an animal thought to be a 
bark-stripper that now appears, on the basis of its arms, to be a carnivore.  

Where does this leave Mononykus, which I argued is also a bark-stripper?  It may
well establish it as a carnivore, and now, at long last, we have a good 
convergent parallel to Mononykus and its closely related taxa: Titanis; for 
which see my other new posting entitled, ÒTitanis Article.Ó  The revised 
understanding of the presumed vestigial wing preserved in related birds, as well
as this genus, is that it had reevolved into an arm (and not by Òthrow-backÓ 
mutations either), an arm that was short and terminated in a single long digit 
and claw that has a striking resemblance to the Mononykus arms and claw.  There 
are two other digits to the manus and they too terminate in claws, but they are 
almost vestigial.  It has been recently discovered that Mononykus has two other 
nearly vestigial digits itself.  So it looks very likely that we have convergent
evolution in the arms in these two lineages.  The function of these odd manus 
and their claws appear to have been stabbing and post-insertion gripping.  
Mononykus may even have fed off the convergent counterpart of Glyptodonts, 
juvenile Ankylosaurs.  Claws like these could surely be used for prying as well 
as stabbing.  

I think we have here a sudden confluence of data and theory that bodes well for 
understanding Mononykus.  It needs some work, as the sloth material and the 
Titanis material are not perfectly congruent, although they are not really 
incompatible either.  Part of this can even be tested by a reexamination of the 
size of Mononykus' olecranon process in relation to the size of animal as a 
whole or the size of its arms.  

Richard Dieterle