more on forams

>Why would a turbulent flood sort these creatures all
> over the world by the decorations on their shells. 

The problem for a global flood is much worse than just forams. The
various groups that I mentioned plus several more obscure ones
(ebridians, silicoflagellates, acritarchs, dinoflagellates,
tintinnids...) all show consistent zonations, not to mention
macrofossils and pollen. Hydrodynamic sorting can't do it. The
organisms have to be dispersed around the world and then disappear as
the next set appears.

> What environmental factors drive natural selection for plankton? 

Predation, competition (dissolved nutrient levels making a big
difference in what kind of organism dominates), staying afloat,
constraints of any symbioses (e.g., most planktic forams have symbiotic
algae), and prey capture (if applicable) are the main factors that come
to mind. It's not easy to study the biology of something microscopic
that lives in the open ocean, so details are often poorly known.

> Have oceans changed so much and so rapidly that over 66
> million years it drove such changes? 

There are some major changes in the oceans that correspond to major
extinctions and subsequent radiations. For example, the end-Paleocene
warm spike wiped out a lot of benthics, followed by gradual
rediversification.

> And why is the rate of change relatively constant?  (if it is, of
course.  I couldn't tell for sure from the snippets I read)<

Although the background rate of random variation probably doesn't vary
that much, much of the apparently constant rate is probably an artifact
of the fact that much of the study has been for the purpose of
identifying the age of the layer you are sampling and thus the chances
of finding oil. Therefore, people have looked for events happening at
small intervals of time. There has not been much effort to thoroughly
trace the evolution of a single lineage through time, whether or not
the changes are commercially useful. Instead, it's more "species X
first appears, then species Y appears, then X disappears by evolving
into Z, but Z isn't too common so we don't bother with it..."

>How are the differences in the sequential species affecting the
ability of plankton to survive?

Larger-probably harder to eat, more tendency to sink, more food
required to survive, higher reproductive capacity

Spinier-probably harder to eat, less tendency to sink

More globose-less effort to maintain skeleton, more tendency to sink

Flatter-more skeleton per body volume but higher surface area

Changes in water depth preferences (traceable in fossils with stable
isotopes)-different light, predator, prey, nutrient, and temperature
occurences

etc.

-- 
Dr. David Campbell
425 Scientific Collections Building
Department of Biological Sciences
Biodiversity and Systematics
University of Alabama, Box 870345
Tuscaloosa AL 35487-0345  USA