From: swf@tools3teradata.com (Stan Friesen)
> Fossilization is a rare event. Not every animal that dies
> is turned into a fossil. It would be unusual if we *did*
> find examples of every creature that ever lived.

Yep.

In fact it goes even deeper than that. Fossilization is a strongly *biased*
process. The organisms that do get fossilized are not, by any means, a
uniform sample of those in the area.

Two very important factors contribute to this bias - presence or abscence of
persistant tissues (aka 'hard parts'), and proximity to sediment sinks.

Both of these biases can be easly demonstrated, simply be taking subfossil
assemblages and comparing them to the adjacent living assemblages. This has
been done, and the extent and magnitude of the biases have been estimated.

But, let's get more concrete. Take a river flood plain where sedimentation
is going on. What may get buried and preserved there? The plants that 
actually live in the flood plain, and the animals that spend time there, 
the more time they spend there the more likely it is that they will be
preserved.

Now, anyone who has studied ecology, or even simply gone out and observed
such things, knows that the plants that grow in river valleys are
*different* than those found outside of them. And, of course, different
animals have different preferences for spending their time, some avoiding
valleys, others never leaving them.

Now, what do we see when we look at flood plain deposits for fossils?
Guess what we see mostly (or entirely): flood plain plants, and a skewed
picture of the animals. Many animals show up frequently, and with nearly
complete skeletons. Others show up only in fragments, and mainly in river
lag deposits (the coarse stuff at the bottom of a river that has been
hauled down from the hills).

Let's take a real case. In the Hell Creek beds of Montana some animals are 
found mostly complete - Hadrosaurus, Anatotitan, Tyrannosaurus, and a few
others. Others show up mostly as isolated sturdy bones - Pachycephalosaurus,
Ankylosaurus, and to some extent Triceratops (which is found mostly as
skulls, few skeletons, but it is intermediate, since some skeletons are
found). In fact there are articles which estimate how far these animals
lived from the rivers that made the beds by using completeness statistics!

-- 
sarima@teradata.com (formerly tdatirv!sarima) or
Stanley.Friesen@ElSegundoCA.ncr.com