Re: Amargasaurus...

["philidor11" <philidor11@snet.net>]

HP Keesey observed:
<Evolution doesn't always happen in the best way possible. Animals don't
plan out their evolutionary strategies. Variations occur randomly, and
natural selection weeds out the unfit variations. The ancestors of
_Amargasaurus_ may not have been "lucky" enough to have an armor-producing
mutation. Instead they had mutations leading to high cervical spines, and
these variations were, for some reason, naturally selected for...>

As HP Kinman observed (and HP Keesey recognized), sometimes an event comes
along which reduces survival to luck.
In addition, I'd point out that adaptations are selected within a particular
situation.  Armor has disadvantages as well as advantages, and an animal
slowed and made less flexible by the additional weight might sometimes not
have as good a chance of survival as one which can run away.
In this case, we are talking about the survival of whole populations as
opposed to just individuals.  Here's where I get to make a case for the
generalist:  a species which has less specialization in body plan or food
source may have an improved chance of survival  when situations change.
To me, one of the most remarkable things about dinosaurs is their
flexibility.  Consider, for example, the generally accepted scenario:  dinos
began as carnivores and outcompeted all others, and then, to me
surprisingly, some of them became herbivores and became dominant in that
category as well.
Now that's what I call being a successful generalist!