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I wonder if we aren't being overly dramatic in our images of ankylosaur 
defensive behavior and capability--maybe for stegosaurs, too.  I don't 
think we need to find tyrannosaurs with broken shins to prove that 
ankylosaurs used their tail clubs for defense from predators (or _mainly_ 
from predators).  Again, living animals provide an analogy.  In the event 
a pronghorn, for example, must use its horns to defend itself from a 
predator, chances are the attacker will learn its lesson from a single 
strategically placed/timed head butt.  No serious injury need be 
inflicted for the predator to decide that some other victim might be 
easier prey.  One such encounter could last a lifetime.

In the case of ankylosaurs, they were probably slow, plodding animals.  
Their tails could have made an easy target for pursuing tyrannosaurs, 
which most certainly could have kept up with them.  But perhaps 
ankylosaurs weren't like modern lizards that can afford to sacrifice 
their tails.  Maybe their caudo-femoralis muscles (I think that's the 
right term) were too important for moving their great bulk, and it was 
better to have a whole tail than part of one for proper muscle action.  
So, tail clubs helped them discourage predators from ripping off 
otherwise vulnerable tails (maybe also the spiky tails of stegosaurs, 
although I know one can make a better case for aggressive capability 
there).  Then, once you have a neat appendage like that, you might find 
all sorts of other uses for it.  For example, maybe ankylosaurs whacked 
trees to announce their territory, their presence to prospective mates, 
and/or intimidate rivals.  Speculation could go on indefinitely.

The analogy might break down in the case of glyptodonts.  Who knows what 
special value glyptodont tails had that favored their reproductive 

Norman R. King                                       tel:  (812) 464-1794
Department of Geosciences                            fax:  (812) 464-1960
University of Southern Indiana
8600 University Blvd.
Evansville, IN 47712                      e-mail:  nking.ucs@smtp.usi.edu